the main Accounts What are the main parameters of a field-effect transistor. What is a field-effect transistor and how to check it. Physical foundations of the field-effect transistor

What are the main parameters of a field-effect transistor. What is a field-effect transistor and how to check it. Physical foundations of the field-effect transistor

TOPIC 5. FIELD TRANSISTORS

A field-effect transistor is an electrical conversion device in which the current flowing through a channel is controlled by an electric field that occurs when a voltage is applied between the gate and the source, and which is designed to amplify the power of electromagnetic oscillations.

The field-effect class includes transistors, the principle of operation of which is based on the use of charge carriers of only one sign (electrons or holes). The current in field-effect transistors is controlled by changing the conductivity of the channel through which the transistor current flows under the influence of an electric field. As a result, transistors are called field-effect.

According to the method of creating the channel, field-effect transistors are distinguished with a gate in the form of a control pn-junction and with an insulated gate (MOS - or MOS - transistors): a built-in channel and an induced channel.

Depending on the conductivity of the channel, field-effect transistors are divided into: field-effect transistors with a p-type and n-type channel. The p-type channel has hole conductivity, and the n-type channel has electron conductivity.

5.1 Field-effect transistors with control p-n-transition

5.1.1 Design and principle of operation

A field-effect transistor with a control pn-junction is a field-effect transistor, the gate of which is electrically separated from the channel by a pn-junction biased in the opposite direction.

Figure 5.1 - Device of a field-effect transistor with a control pn-junction (channel n-type)

Figure 5.2 - Conventional designation of a field-effect transistor with a p-n-junction and an n-type channel (a), a p-type channel (b)

The channel of a field-effect transistor is a region in a semiconductor in which the current of the main charge carriers is regulated by changing its cross section.

The electrode (lead) through which the main charge carriers enter the channel is called the source. The electrode through which the main charge carriers leave the channel is called a drain. The electrode used to regulate the channel cross-section by means of a control voltage is called a gate.

Typically, silicon field effect transistors are available. Silicon is used because the gate current, i.e. the reverse current of the pn junction is many times less than that of germanium.

Symbols of field-effect transistors with channel n- and p-types are shown in Fig. 5.2.

The polarity of external voltages supplied to the transistor is shown in Fig. 5.1. The control (input) voltage is applied between the gate and the source. The voltage Uzi is the opposite for both pn-junctions. The width of the pn junctions, and, consequently, the effective cross-sectional area of the channel, its resistance and the current in the channel depend on this voltage. With its growth, the pn-junctions expand, the cross-sectional area of the conductive channel decreases, its resistance increases, and, consequently, the current in the channel decreases. Therefore, if a voltage source Uсi is connected between the source and drain, then the strength of the drain current Iс flowing through the channel can be controlled by changing the resistance (section) of the channel using the voltage applied to the gate. The operation of a field-effect transistor with a control pn junction is based on this principle.

At a voltage Uzi = 0, the channel cross-section is the largest, its resistance is the smallest, and the current Ic is the largest.

The drain current Iс beginning at Uzi = 0 is called the initial drain current.

The voltage Uzi, at which the channel is completely closed, and the drain current Ic becomes very small (tenths of microamperes), is called the cutoff voltage Uziotc.

5.1.2 Static characteristics of a field-effect transistor with a control p-n-transition

Consider the volt-ampere characteristics of field-effect transistors with a pn-junction. For these transistors, two types of volt-ampere characteristics are of interest: drain and drain-gate.

The stock (output) characteristics of a field-effect transistor with a pn-junction and an n-type channel are shown in Fig. 5.3, a. They reflect the dependence of the drain current on the voltage Usi at a fixed voltage Uzi: Ic = f (Usi) at Uzi = const.

a) b)

Figure 5.3 - Volt-ampere characteristics of a field-effect transistor with a pn-junction and a n-type channel: a - drain (output); b - stock - bolt

A feature of the field-effect transistor is that the channel conductivity is influenced by both the control voltage Uzi and the voltage Usi. At Ussi = 0, the output current Ic = 0. At Ussi> 0 (Uzi = 0), a current Ic flows through the channel, as a result of which a voltage drop is generated that increases in the direction of the drain. The total voltage drop of the source-drain section is equal to Us. An increase in the voltage Usi causes an increase in the voltage drop in the channel and a decrease in its cross section, and, consequently, a decrease in the channel conductivity. At a certain voltage Usi, the channel narrows, at which the boundaries of both pn junctions close and the channel resistance becomes high. Such a voltage Usi is called the overlap voltage or saturation voltage Usinas. When the reverse voltage Uzi is applied to the gate, an additional narrowing of the channel occurs, and its overlap occurs at a lower value of the voltage Usinas. In the operating mode, flat (linear) sections of the output characteristics are used.

The drain - the gate characteristic of the field-effect transistor shows the dependence of the current Ic on the voltage Uzi at a fixed voltage Usi: Ic = f (Usi) with Usi = const (Fig.5.3, b).

5.1.3 Basic parameters

· Maximum drain current Iсmax (at Uzi = 0);

· Maximum drain-source voltage Usymax;

· Cut-off voltage Uziots;

Internal (output) resistance ri - is the resistance of the transistor between the drain and the source (channel resistance) for alternating current:

at Uzi = const;

The steepness of the drain-gate characteristics:

at Usi = const,

displays the effect of gate voltage on the output current of the transistor;

Input impedance

at Usi = const of the transistor is determined by the resistance of the pn-junctions biased in the opposite direction. The input resistance of field-effect transistors with a pn-junction is quite high (reaches units and tens of megohms), which distinguishes them favorably from bipolar transistors.

5.2 Insulated Gate Field Effect Transistors

5.2.1 Design and principle of operation

A field-effect transistor with an insulated gate (MIS - transistor) is a field-effect transistor, the gate of which is electrically separated from the channel by a dielectric layer.

MIS - transistors (structure: metal-dielectric-semiconductor) are made of silicon. Silicon oxide SiO2 is used as a dielectric. hence the other name for these transistors - MOS - transistors (structure: metal-oxide-semiconductor). The presence of a dielectric provides a high input resistance of the considered transistors (1012 ... 1014 Ohm).

The principle of operation of MIS - transistors is based on the effect of changing the conductivity of the near-surface layer of a semiconductor at the interface with a dielectric under the influence of a transverse electric field. The surface layer of the semiconductor is the conductive channel of these transistors. МДП - transistors are of two types - with built-in and with induced channel.

Let us consider the features of MIS - transistors with a built-in channel. The design of such an n-channel transistor is shown in Fig. 5.4, a. In the original p-type silicon plate with a relatively high resistivity, which is called a substrate, using diffusion technology, two heavily doped regions with the opposite type of electrical conductivity, n, are created. These areas are covered with metal electrodes - source and drain. There is a thin near-surface channel with n-type electrical conductivity between the source and drain. The surface of the semiconductor crystal between the source and drain is covered with a thin layer (about 0.1 μm) of the dielectric. A metal electrode - a gate - is applied to the dielectric layer. The presence of a dielectric layer makes it possible in such a field-effect transistor to supply a control voltage of both polarities to the gate.

Figure 5.4 - Design of MDP - transistor with built-in n-type channel (a); family of its runoff characteristics (b); drain-gate characteristic (v)

When a positive voltage is applied to the gate, by the electric field that is created, holes from the channel will be pushed out into the substrate, and electrons will be pulled from the substrate into the channel. The channel is enriched by the main charge carriers - electrons, its conductivity increases and the drain current increases. This mode is called the enrichment mode.

When a voltage negative with respect to the source is applied to the gate, an electric field is created in the channel, under the influence of which electrons are pushed out of the channel into the substrate, and holes are drawn from the substrate into the channel. The channel is depleted in the majority of charge carriers, its conductivity decreases and the drain current decreases. This mode of the transistor is called the depletion mode.

In such transistors at Uzi = 0, if a voltage is applied between the drain and the source (Usi> 0), the drain current Istart flows, which is called the initial and, which is the flow of electrons.

The design of an MIS transistor with an induced n-type channel is shown in Fig. 5.5, a

In semiconductor electronics, along with bipolar transistors, transistors controlled electric field, one of the positive features of which is high input impedance(is 1-10 MOhm and more). Such transistors are called field(unipolar).

Device and principle of operation

Field effect transistorsare called semiconductor devices in which the creation of an electric current is due to the movement of charge carriers of the same sign under the action longitudinal electric field and the output current control is based on modulation resistance semiconductor material transverse electric field.

The principle of operation of field-effect transistors can be based on:

On the dependence of the resistance of a semiconductor on the cross-section of its conducting region (the smaller the cross-section, the lower the current; implemented in field-effect transistors with the manager p-p- transition);

On the dependence of the semiconductor conductivity on the concentration of the majority carriers (implemented in field-effect transistors with insulated shutter structures metal-dielectric-semiconductor(MIS transistors)).

Field-effect transistor with the manager p-p- junction (PTUP) is a thin semiconductor wafer with one pn-transition and with non-rectifying contacts at the edges. The electrical conductivity of the plate material can be P-type or R-type. As an example, consider a transistor in which the main plate consists of a semiconductor n-type (Figure 1.32).

Figure 1.32 - Structure of a field-effect transistor with a control pn-transition

The main areas in the structure of a field effect transistor with the manager p-p- transition are:

Region source- the area from which the charge carriers begin to move;

Region runoff- the area to which the media are moved;

Region shutter- the area through which the media flow is controlled;

Region channel- the area through which the media move.

The leads from the corresponding areas of the transistor have similar names: source(AND), runoff(C) and gate(3) (Figure 1.32).

Figure 1.33 shows the conventional graphic designations of field-effect transistors with a control p-p- transition: with channel P-type (Figure 1.33, but) and channel R-type (Figure 1.33, b).

a b

Figure 1.33 - UGO field-effect transistors with a control pn-transition

Consider the principle of the functioning of the TPUP. Voltage sources are connected to the transistor in such a way that an electric current flows between the drain and source electrodes, and the voltage applied to the gate biased the electron-hole junction in the opposite direction.

Figure 1.34 shows a method of connecting voltage sources to the terminals of a PTUP with a channel P-type.

Figure 1.34 - Connecting voltage sources to the terminals of the PTUP

Source voltage E SI electrons will move from source to drain, providing a drain current in the external circuit I C.

Concentrations of charge carriers in the semiconductor material of the channel and the gate are chosen in such a way that when a reverse bias voltage is applied between the gate and the source pn-the transition will expand into the channel area. This leads to a decrease in the cross-sectional area of the conductive part of the channel and, therefore, to a decrease in the drain current I C.

The resistance of the region located under the electrical junction, in the general case, depends gate voltage... This is due to the fact that the size of the junction increases with an increase in the reverse voltage applied to it, and an increase in the region depleted of charge carriers leads to an increase in the electrical resistance of the channel (and, accordingly, to a decrease in the current flowing in the channel).

In this way, the operation of a field-effect transistor with a control p-n-junction is based on a change in the channel resistance due to a change in the size of the region depleted of the main charge carriers, which occurs under the action of the applied to the gate reverse voltage.

The voltage between the gate and the source at which the channel is completely blocked and the drain current reaches its minimum value ( I C»0) are called cutoff voltage(U out) field-effect transistor.

Unlike PTUP, in which the gate has an electrical contact with the channel, in field-effect transistors with insulated shutter The (PTIZ) gate is a thin film of metal isolated from the semiconductor. Depending on the type of insulation, MOS and MOS transistors are distinguished (respectively, metal - dielectric - semiconductor and metal - oxide - semiconductor, for example, silicon dioxide SiO 2).

In the initial state, the PTIZ channel can be impoverished charge carriers or enriched by them. Depending on this, two types of IGBTs are distinguished: MIS transistors with built-in channel(Figure 1.35, but) (the channel is created during manufacture) and MIS transistors with induced channel(Figure 1.35, b) (the channel arises under the action of a voltage applied to the control electrodes). The PTIZ has an additional output from the crystal on which the device is made (Figure 1.35), called substrate.

a b

Figure 1.35 - The device of field-effect transistors with an insulated gate

In the PTIZ, the drain and source electrodes are located on both sides of the gate and have direct contact with the semiconductor channel.

The channel is called built-in if it was originally enriched charge carriers. In this case, the control electric field will lead to impoverishment channel by charge carriers. If the channel was originally impoverished carriers of electric charges, then it is called induced... In this case, the control electric field (between the gate and the source) will enrich the channel with carriers of electric charges (that is, increase its conductivity).

The channel conductivity can be e or hole... If the channel has electronic conductivity, then it is called P-channel. Hole conduction channels are called R-channels. As a result, they distinguish four types field-effect transistors with insulated gate: with channel P- either R-types, each of which can have induced or built-in channel. Graphic symbols of the named types of field-effect transistors are shown in Figure 1.36.

Control voltage in PTIZ can be submitted as between the gate and backing and independently on substrate and gate... Consider, as an example, the principle of current control in field-effect transistors, the structures of which are shown in Figure 1.35.

Figure 1.36 - UGO field-effect transistors with an insulated gate

If a positive voltage is applied to the gate, then under the influence of the resulting electric field at the semiconductor surface (Figure 1.35, b) channel appears P -type due to the repulsion of holes from the surface into the interior of the semiconductor. In a transistor with built-in channel (Figure 1.35, but), the existing channel expands when a positive voltage is applied, or it narrows when a negative voltage is applied. Changing the control voltage changes the channel width and, accordingly, resistance and transistor current.

Essential advantage PTIZ before PTUP is reaching values of 10 10 - 10 14 Ohm (for transistors with a control pn-transition - 10 7 - 10 9 Ohm).

An important advantage of field-effect transistors over bipolar transistors is low voltage drop across them when switching weak signals.

In addition, it should be noted such advantages as:

- high input impedance;

- low noise;

- ease of manufacture;

- no residual voltage in the open state between the source and drain of the open transistor.

Current-voltage characteristics and basic parameters of field-effect transistors

From what was considered earlier, it follows that there are six types of field-effect transistors in total. Their typical transfer characteristics are shown in Figure 1.37. Using these characteristics, you can set the polarity of the control voltage, the direction of the current in the channel and the range of the control voltage. Of all the above types of transistors, only PTIZ with an integrated channel are currently not produced. R-type.

Figure 1.37 - Transfer characteristics of field-effect transistors

Let's consider some of the features of these characteristics. All characteristics of field effect transistors with a channel P-types are located in the upper half of the graph and, therefore, have a positive current, which corresponds to a positive voltage at the drain. On the contrary, all characteristics of devices with a channel R-types are located in the lower half of the graph and therefore have a negative current value and a negative drain voltage. The characteristics of the PTUP at zero gate voltage have a maximum current value, which is called the initial I From the beginning... With an increase in the blocking voltage, the drain current decreases and with a cut-off voltage U out becomes close to zero.

PTIZ characteristics with induced channel at zero gate voltage have zero current. The appearance of a drain current in such transistors occurs when the gate voltage is greater than the threshold value U then... Increasing the gate voltage increases the drain current.

PTIZ characteristics with built-in channel at zero gate voltage have an initial current value I C. early... Such transistors can operate in both enrichment and depletion modes. With an increase in the gate voltage, the channel is enriched and the drain current increases, and with a decrease in the gate voltage, the channel is depleted and the drain current decreases.

Figure 1.38 shows the output volt-ampere characteristics of the STUP with the channel n-type. The characteristics of other types of transistors are similar, but differ in gate voltage and the polarity of the applied voltages.

Figure 1.38 - Output I - V characteristics of PTUP

Two areas can be distinguished on the I - V characteristic of a field effect transistor: linear and saturation.

In the linear region, the I – V characteristics up to the inflection point are straight lines, the slope of which depends on the gate voltage. In the saturation region, the current-voltage characteristics run almost horizontally, which allows us to speak about the independence of the drain current from the voltage at the drain. In this area, the output characteristics of all types of field-effect transistors are similar to those of electric vacuum pentodes. The features of these characteristics determine the use of field-effect transistors. In the linear region, the field-effect transistor is used as resistance, gate voltage controlled, and in the saturation region - as reinforcing element.

The maximum voltage applied between the drain and the source of the FET is different for each type of transistor. But in the general case, as shown in Figure 1.39, when a certain value is exceeded U SI samples the drain current increases sharply, which can lead to failure of the transistor as a result of breakdown.

Figure 1.39 - Family of output current-voltage characteristics of a field-effect transistor

The main parameters of field-effect transistors include:

The steepness of the stock gate characteristic

Typical values: S= 0.1-500 mA / V;

Slope over the substrate

Typical values: S p= 0.1-1 mA / V;

Initial drain current I From the beginning- drain current at zero voltage U ZI.

For transistors with a control R-P-transition I C start= 0.2-600 mA, with built-in channel - I From the beginning= 0.1-100 mA, with induced channel - I From the beginning= 0.01-0.5 μA;

Cut-off voltage U ZI ot(typical values U ZI ot= 0.2-10 V);

Resistance drain - source in open state R SI open(typical values R SI open= 2-300 Ohm);

Residual drain current I C ost- drain current at voltage U ZI ot (I C ost= 0.001-10 mA);

Maximum gain frequency f p is the frequency at which the power gain is equal to unity (typical values f p- tens - hundreds of MHz).

Field-effect transistors are active semiconductor devices in which the output current is controlled by an electric field (in bipolar transistors, the output current is controlled by the input current). Field-effect transistors are also called unipolar, since only one type of carrier is involved in the flow of electric current.

There are two types of field-effect transistors: with a control junction and with an insulated gate. They all have three electrodes: a source (source of current carriers), a gate (control electrode), and a drain (an electrode where carriers flow).

Controlled transistorp - n-transition . Its schematic representation is shown in Fig. 1.21, but The conventional graphic designation of this transistor is shown in Fig. 1.22, but, b (p- and n-types, respectively). The arrow indicates the direction from the layer R to layer P(like the arrow in the emitter image of a bipolar transistor). In integrated circuits, the linear dimensions of transistors can be significantly less than 1 micron.

Fig. 1.22 Transistor device

Fig. 1.23 Graphical representation:a - p-type channel; b - channel n -type

Layer resistivity n(gate) much less layer resistivity R(channel), so the area R-n-junction, depleted in mobile charge carriers and having a very high resistivity, is located mainly in the layer R.

If the types of conductivity of the semiconductor layers in the considered transistor are changed to the opposite, then we get a field-effect transistor with a control
R-n-transition and channel n-type. If you apply a positive voltage between the gate and the source of the p-channel transistor: and zi> 0, then it will displace p—n-transition in the opposite direction.

With an increase in the reverse voltage at the junction, it expands mainly due to the channel (due to the above difference in resistivity). Increasing the width of the junction decreases the thickness of the channel and, therefore, increases its resistance. This leads to a decrease in the current between the source and drain. It is this phenomenon that makes it possible to control the current using voltage and the corresponding electric field. If the voltage and zi is large enough, the channel is completely covered by the area p—n-transition (cutoff voltage).

In working mode R—n- the junction must be under reverse or zero voltage. Therefore, in the operating mode, the gate current is approximately zero ( i s? 0 ), and the drain current is practically equal to the source current.

Width R—n-junction and channel thickness are also directly influenced by the voltage between the source and drain. Let be usi= 0 and positive voltage applied uis(fig. 1.24). This voltage will also be applied to the gate - drain gap, i.e. it turns out that uss= uis and R—n- the junction is under reverse voltage.

Reverse voltage in various areas R—n-transition is different. In areas near the source, this voltage is practically zero, and in areas near the drain, this voltage is approximately equal to uis. therefore p—n- the junction will be wider in those areas that are closer to the drain. It can be assumed that the voltage in the channel from source to drain increases linearly.

When uis =Usifrom the channel will completely close off near the drain (Fig. 1.25). With a further increase in voltage uis this area of the channel in which it is overlapped will expand.

Transistor switching circuits . For a field-effect transistor, as well as for a bipolar transistor, there are three switching circuits: circuits with a common gate (03), a common source (OI) and a common drain (OS). The most commonly used circuits with a common source (Fig. 1.26).

Since in working mode i c? 0, then the input characteristics are usually not considered.

Output (stock) characteristics . The output characteristic is a dependence of the form

Where f- some function.

Output characteristics for a transistor with R—n-transition and channel n -type are shown in Fig. 1.27.

Let us turn to the characteristic corresponding to the condition u zi= 0. In the linear domain ( u is < 4 В) характеристика почти линейна (все характеристики этой области представляют собой почти прямые линии, веерообразно выходящие из начала координат). Она определяется сопротивлением канала. Транзистор, работающий в линейной области, можно использовать в качестве линейного управляемого сопротивления.

When u is> 4 V the channel in the drain area is overlapped. A further increase in the voltage leads to a very insignificant increase in the current, since with an increase in the voltage, the region in which the channel is blocked expands. In this case, the resistance of the source-drain gap increases, and the current i c practically does not change. This is the saturation area. Drain current in the saturation region u si = 0 and at a given voltage and si is called the initial drain current and is denoted by i c start... For the characteristics under consideration i c start= 5 mA at and si= 10 V.

The parameters characterizing the properties of the transistor to amplify the voltage are:

1) The steepness of the stock gate characteristic S(the slope of the characteristics of the field-effect transistor):

2) Internal differential resistance Ris diff

3) Gain

You can see that

Insulated Gate Transistors. An IGBT is a transistor whose gate is electrically separated from the channel by a dielectric layer. The physical basis for the operation of such transistors is the field effect, which consists in a change in the concentration of free charge carriers in the near-surface region of a semiconductor under the action of an external electric field. In accordance with their structure, such transistors are called MIS transistors (metal-dielectric-semiconductor) or MOS transistors (metal oxide semiconductor). There are two types of MOS transistors: with induced and with built-in channels.

In fig. 1.28 shows the principle of a transistor with an embedded channel.

The base (substrate) is a silicon plate with electrical conductivity p-type. Two areas with electrical conductivity are created in it. n+ -type with increased conductivity. These areas are the source and drain, and conclusions are drawn from them. There is an n-type near-surface channel between the drain and the source. The shaded area is a dielectric layer of silicon dioxide (its thickness is usually 0.1 - 0.2 μm). On top of the dielectric layer there is a shutter in the form of a thin metal film. The crystal of such a transistor is usually connected to the source, and its potential is taken as zero. Sometimes a separate conclusion is drawn from the crystal.

If zero voltage is applied to the gate, then when a voltage is applied between the drain and the source, a current will flow through the channel, which is a stream of electrons. No current will flow through the crystal, since one of the p—n- The junction is under reverse voltage. When a voltage of negative polarity is applied to the gate with respect to the source (hence, to the crystal), a transverse electric field is formed in the channel, which pushes electrons out of the channel in the region of the source, drain, and crystal. The channel is depleted in electrons, its resistance increases, and the current decreases. The higher the gate voltage, the lower the current. This mode is called impoverishment ... If a positive voltage is applied to the gate, then under the action of the field, electrons will come from the regions of the drain, source, and crystal into the channel. The channel resistance drops, the current increases. This mode is called enrichment regime ... If the crystal n-type, then the channel must be p-type and the polarity of the voltage is reversed.

Another type is transistor with induced (inverse) channel (fig. 1.29). It differs from the previous one in that the channel appears only when a voltage of a certain polarity is applied to the gate.

In the absence of voltage at the gate, there is no channel, between the source and drain
n+ -type located only crystal p-type and on one of p-n+ -transitions, a reverse voltage is obtained. In this state, the resistance between drain and source is high and the transistor is off. When a voltage of positive polarity is applied to the gate, under the influence of the gate field, conduction electrons will move from the drain and source regions and p- the area towards the shutter. When the voltage at the gate reaches its unlocking (threshold) value (units of volts), the concentration of electrons in the near-surface layer increases so much that it exceeds the concentration of holes, and the so-called inversion type of electrical conductivity, i.e. a thin canal is formed n-type, and the transistor will begin to conduct current. The higher the gate voltage, the higher the drain current. It is obvious that such a transistor can only operate in the enrichment mode. If the substrate n-type, then we get the induced channel p-type. Induced channel transistors are often found in switching devices. The switching circuits of field-effect transistors are similar to the switching circuits of bipolar ones. It should be noted that a field-effect transistor allows you to get a much higher gain than a bipolar one. With their high input impedance (and low output impedance), field-effect transistors are gradually replacing bipolar ones.

The electrical conductivity of the channel is distinguished p-channel and n- channel MOS transistors. The conventional designation of these devices on the electrical circuits is shown in Fig. 1.30 . There is a classification of MIS transistors according to their structural and technological characteristics (more often by the type of gate material).

Fig. 1.30 Graphical symbols of field-effect transistors
with an insulated gate: a - with a built-in p-channel; b - with built-in
n-channel; c - with an induced p-channel; d - with induced n-channel

Integrated circuits containing simultaneously p — channel and n-channel MIS transistors are called complementary (abbreviated CMDP-IC). KMDP-IMS are distinguished by high noise immunity, low power consumption, high speed.

Frequency properties field-effect transistors are determined by the time constant RC- shutter chains. Since the input capacitance FROMsi for transistors with R—n-the transition is large (tens of picofarads), their use in amplifying stages with a large input impedance is possible in the frequency range not exceeding hundreds of kilohertz - megahertz units.

When operating in switching circuits, the switching speed is completely determined by the time constant of the RC gate circuit. The input capacitance of insulated gate field-effect transistors is much lower, therefore their frequency properties are much better than that of pn-junction field-effect transistors.

The history of the creation and implementation of field-effect transistors

The first field-effect transistor was invented by Julius Edgar Lilienfeld, an Austro-Hungarian physicist who devoted most of his life to the study of the transistor effect. It happened in 1928, but the first transistor manufacturing technology did not allow the physical implementation of this radioelement in industry. The first working field-effect transistor with an insulated gate, according to the writings of Lilienfeld, was produced in the United States only in 1960. 7 years earlier, another technology for manufacturing a field-effect transistor based on a control p-n junction (MOS transistor) was proposed. Based on the works of Walter Schottky in 1966, American engineer Carver Andress Mead proposed a new type of transistor using a Schottky barrier. In 1977, it was found that the use of field-effect transistors in computer technology significantly increases the calculated power of electronic devices, which marked the beginning of the development of computer processors and logic microcircuits based on a field-effect transistor. A more correct name for a field-effect transistor is a unipolar transistor (controlled by one electric field), but this name did not take root among the people.

Physical foundations of the field-effect transistor

A field-effect (unipolar) transistor is an electronic device based on the principle of using charges of only one sign, i.e. electrons or holes. Current control in field-effect transistors is carried out by changing the channel conductivity under the influence of an electric field, and not a voltage potential, which is the main difference between a field-effect transistor and a bipolar one. According to the method of creating a channel, field-effect transistors with a p-n junction, a built-in channel and an induced channel are distinguished. Transistors with built-in and induced channel also belong to a variety of MIS transistors.

Field effect transistor device

a - with p-n junction; b - with an insulated gate and a built-in channel; c - with an isolated gate and an induced channel.

The work of field-effect transistors is based on the movement of the main carriers in a semiconductor.

Field-effect transistor with pn junction.

This transistor consists of an n-type semiconductor main channel made from a silicon wafer with ohmic leads at each end. The channel is formed by the diffusion method (the introduction of a doped material) and forms the thinnest layer with hole conductivity. The channel is enclosed between two p-type electrodes connected to each other. Thus, the n-channel forms two p-n junctions parallel to the direction of the current. The output through which the charge carrier enters is called the source (I), and the electrode from where the charge flows out is called the drain (C). Both p-layers are electrically connected to each other and have an external electrode called a gate (G). There are two types of channel. A positive charge flows through the p channel, while a negative charge flows through the n channel. The figure below shows a negative conduction field channel driven by a positive polarity field. In this case, electrons move through the channel from the source to the drain. P-channel field-effect transistors have a similar design.

The control or input voltage (Uzi) is applied between the gate and the source. This voltage for both pn junctions is reversed. In the output circuit, which also includes the transistor channel, the voltage Usi is connected with a positive pole to the drain.

The ability to control the transistor is explained by the fact that when the voltage Uzi changes, the width of the p-n junctions will change, which are sections in the semiconductor that are depleted in charge carriers. Since the p-layer with a lower resistance has a higher concentration of impurities in comparison with the n-layer, the channel width change is controlled due to the higher-resistance n-layer. In this case, the cross section and the conductivity of the conductive channel (Ic - drain current) change from source to drain.

The peculiarity of the field-effect transistor is in the influence of the voltage Uzi and Usi on the channel conductivity. The influence of applied voltages is shown in the figure below.

On the image:

A) voltage is applied only to the input control circuit. Changing Uzi controls the channel cross-section over the entire width, however, the output current Ic = 0 due to the absence of voltage Usi.

B) Only the channel voltage is present, the control voltage is absent and the current Ic begins to flow. A voltage drop is created at the drain electrode, as a result, the channel capacity is narrowed and at a certain value of the boundaries of the p-n junctions are closed. The internal resistance of the channel rises and the current Ic is no longer able to pass.

C) In this version, the figure shows the total value of the voltages when the voltage channel Usi is locked by a small control voltage Uzi. When this voltage is applied, the n region expands and the current Ic begins to flow.

Insulated Gate Field Effect Transistor (MOSFET & MOSFET)

In these transistors, the gate electrode is separated from the channel by a thin insulating layer of silicon oxide. Hence the other name for these transistors - MOS transistors (metal-oxide-semiconductor structure). The presence of a dielectric provides a high input impedance of the considered transistors. The penetration of the control field into the channel is not difficult, but the gate current is greatly reduced and does not depend on the polarity of the voltage applied to the gate. MIS transistors (metal - dielectric - semiconductor structure) are made of silicon. The principle of operation of MIS transistors is based on the effect of changing the conductivity of the near-surface layer of a semiconductor at the interface with a dielectric under the influence of a transverse electric field.

The channels of the field-effect MOS transistors can be depleted (b - built-in channel) and enriched type (c - induced channel), (see the figure of the field-effect transistor device).

The current Ic flows through the built-in channel in the absence of voltage Uzi. Its value can be controlled downward by applying a positive voltage Uzi, if the transistor is with a p-channel, and a negative voltage, if the transistor is with an n-channel. In other words, close the transistor with a reverse voltage control.

In the induced channel, if there is no voltage Uzi, the current between the drain and the source is very small. When the control voltage is applied, the current Ici increases.

So, when the control voltage is applied to the gate of a transistor with a built-in channel, it closes the transistor, in the induction channel, it opens the transistor.

Volt - ampere and drain - gate characteristics of the field-effect transistor

The I – V characteristic of a field-effect transistor determines its output (drain) characteristics, and also contains information about its properties in various operating modes. In addition, the I - V characteristic displays the relationship of the parameters to each other. According to the graph, you can determine some parameters that are not documented in the description of the transistor, calculate the voltage level of the bias circuits (Uzi), stabilize the mode, and also evaluate the operation of the field-effect transistor in a wide range of currents and voltages.

The figure on the left shows an example of the drain characteristic of a field-effect transistor with a p-n junction and a p-type channel at various fixed control voltages Uzi. The graphs show the dependence of the drain current (Ic) on the drain-source voltage (Usi). Each of these curves has 3 characteristic areas:

1. Strong dependence of the current Ic on the voltage Usi (section up to the dash - dotted line). This part determines the saturation period of the channel up to the voltage Usi us, at which the transistor goes into the closed (open) state. The higher the control bias voltage Uzi, the earlier the field-effect transistor will close (open).

2. Weak dependence of the current Ic, when the channel is saturated to its maximum value and goes into a permanently closed (open) state.

3. At the moment when the voltage Usi exceeds the maximum allowable for a field-effect transistor, an irreversible electrical breakdown of the p-n junction occurs. In this case, the field-effect transistor fails.

The drain-gate characteristic shows the dependence of Ic on the voltage between the gate and the source.

The gate voltage at which the drain current goes to zero is a very important characteristic of a field effect transistor. It corresponds to the blocking voltage of the device along the gate circuit and is called the blocking voltage or cutoff voltage.

Conditional graphic images of field-effect transistors in electrical circuits are as follows.

Where is the field-effect transistor:

a - with p-n junction and p-channel;

b - with p-n junction and n-channel;

c - with a built-in p-channel of the depleted type;

d - with built-in depleted type n-channel;

e - with the induced p-channel of the enriched type;

e - with the induced n-channel of the enriched type;

g - p-type (c) and output from the substrate;

h - p-type (e) and output from the substrate

European designation of contacts: gate - gate, drain - drain, source - source, tab - substrate (often in non-isolated transistors it is a drain).

The main technical characteristics of the field-effect transistor

Modern field-effect transistors are characterized by basic characteristics, temperature characteristics and electrical characteristics at temperatures up to +25 degrees on the substrate (source). In addition, there are static and dynamic characteristics of field-effect transistors, which determine the maximum performance when used in frequency signals. Particular attention should be paid to frequency characteristics when using transistors in generators, modulators, switching power supplies, modern digital amplifiers of class D and higher. Frequency properties are determined by the time constant of the RC gate circuit, which determines the rate of on / off the channel. In field-effect transistors with an insulated gate (MOS and MOS), the input capacitance is significantly less than field-effect transistors with a p-n junction, which makes it possible to use them in high-frequency equipment.

The main characteristics of field-effect transistors include:

Vds (Vdss) or Usi max - determines the maximum allowable voltage between the source and drain;

Id or Ic - the maximum allowable drain current passing through the open channel of the transistor;

Rdc (on) - resistance of the channel between the gate and the source (usually indicated together with the control voltage Uzi or Vgs).

Iz ut or Igss - gate leakage current at a given voltage between the gate and the rest of the terminals, closed among themselves.

Pd or Pmax is the maximum power dissipation of the transistor at a temperature, usually +25 degrees.

The thermal parameters of a field-effect transistor determine the stability of its characteristics when operating in a temperature range, since when the temperature changes, the properties of semiconductor materials change. Temperature is highly dependent on the value of Ic, the slope and the leakage current of the gate.

Tj or Tmax - the temperature of destruction of the substrate crystal corresponding to the maximum allowable operating temperature

Tstg or Tmin - the minimum negative temperature at which the main passport parameters of the transistor are observed

A distinctive feature of the operation of field-effect transistors in comparison with bipolar ones is a very low noise figure or Ksh. This coefficient has little effect on the drain - source voltages, drain current, as well as the operating temperature of the transistor (up to +50 degrees).

1. It is not recommended to lower the temperature of field-effect transistors during their operation below -5 degrees, as well as go beyond the operating temperature of +60 +70 degrees (popularly - the temperature of holding a finger).
2. During operation, it is necessary to select operating voltages and currents that will not exceed 70% of the maximum permissible parameters according to the passport (datasheet).
3. It is impossible to use transistors in maximum modes in two parameters at the same time.
4. Do not allow the transistor to work with the shutter off.
5. The gate of field-effect transistors with pn junction must not be supplied with a voltage that bias the junction in the forward direction. For p-channel it will be negative voltage, for n-channel it will be positive.
6. It is desirable to store field MOS and MOS transistors with short-circuited terminals. Low-power transistors, frequency transistors of this structure fail from static voltage.
7. You can check the serviceability of the field-effect transistor with an electronic tester by analogy with this video http://www.youtube.com/watch?v=jQ6l6C8LMSw

Semiconductor devices, the operation of which is based on the modulation of the resistance of a semiconductor material by a transverse electric field, are called field-effect transistors. They have only one type of charge carriers (electrons or holes) involved in the creation of an electric current.

Field-effect transistors are of two types: with a control pn-junction and with a metal-dielectric-semiconductor structure (MIS transistors).

Fig. 2.37. Simplified structure of a field-effect transistor with a control (a); legend of a transistor having an n-type (b) and p-type (c) channel; typical structures (d, e): high-speed transistor structure (f)

A transistor with a control p-n-junction (Fig. 2.37) is a plate (section) made of a semiconductor material with a certain type of conductivity, from the ends of which two leads are made - the drain and source electrodes. An electrical junction (pn junction or Schottky barrier) is made along the plate, from which a third terminal is made - the gate.

External voltages are applied so that an electric current flows between the drain and source electrodes, and the voltage applied to the gate biases the electrical junction in the opposite direction. The resistance of the area located under the electrical junction, which is called the channel, depends on the voltage at the gate. This is due to the fact that the size of the junction increases with an increase in the reverse voltage applied to it, and an increase in the region depleted of charge carriers leads to an increase in the electrical resistance of the channel.

Thus, the operation of a field-effect transistor with a control pn junction is based on a change in the channel resistance due to a change in the size of the region depleted of the main charge carriers, which occurs under the action of a reverse voltage applied to the gate.

The electrode from which the main charge carriers begin to move in the channel is called the source, and the electrode to which the main charge carriers move is called the drain. A simplified structure of a field-effect transistor with a control p-n-junction is shown in Fig. 2.37, a. Symbols are given in Fig. 2.37, b, c, and the structures of industrially produced field-effect transistors are shown in Fig. 2.37, d - f.

If in a semiconductor plate, for example n-type, zones with p-type conductivity are created, then when a voltage is applied to the p-n-junction, biasing it in the opposite direction, regions depleted in the main charge carriers are formed (Fig. 2.37, a). The semiconductor resistance between the source and drain electrodes increases, since the current only flows through the narrow channel between the junctions. A change in the gate - source voltage leads to a change in the dimensions of the space-charge zone (dimensions), i.e., to a change in the resistance of the channel. The channel can be almost completely blocked and then the resistance between the source and drain will be very high (several - tens).

The voltage between gate and source at which the drain current reaches a predetermined low value is called the cutoff voltage of the FET. Strictly speaking, at the cut-off voltage, the transistor should close completely, but the presence of leaks and the difficulty of measuring especially low currents force us to consider the cut-off voltage as the voltage at which the current reaches a certain small value. Therefore, in the technical conditions for the transistor, they indicate at what drain current the measurement was made.

The width of the pn junction also depends on the current flowing through the channel. If, for example (Fig. 2.37, a), then the current flowing through the transistor will create a voltage drop along the length of the latter, which turns out to be blocking for the gate-channel transition.

Fig. 2.38. Output characteristics of a field-effect transistor with its control input characteristic (6) and transmission characteristic (drain gate) (c): I - steep region; II - flat area, or saturation area; III - breakout area

This leads to an increase in the width and, accordingly, to a decrease in the cross-section and conductivity of the channel, and the width of the p-n-junction increases as it approaches the drain region, where the largest voltage drop will occur, caused by the current on the channel resistance. So, if we assume that the resistance of the transistor is determined only by the resistance of the channel, then the voltage will act at the edge of the pn junction facing the source, and the voltage will act at the edge facing the drain. At low voltage values and small, the transistor behaves like a linear resistance. An increase leads to an almost linear increase, and a decrease leads to a corresponding decrease. As it grows, the characteristic deviates more and more from linear, which is associated with the narrowing of the channel at the drain end. At a certain value of the current, the so-called saturation mode occurs (section II in Fig. 2.38, a), which is characterized by that. that with increasing current changes insignificantly. This is because at high voltage, the channel at the drain contracts into a narrow throat. A kind of dynamic equilibrium sets in, in which an increase and increase in the current cause a further narrowing of the channel and, accordingly, a decrease in the current. As a result, the latter remains almost constant. The voltage at which saturation occurs is called the saturation voltage. It, as can be seen from Fig. , changes when the voltage changes. Since the effect on the channel width at the drain outlet is almost the same, then

So, the cutoff voltage, determined at low voltage, is numerically equal to the saturation voltage at, and the saturation voltage at a certain gate voltage is equal to the difference between the cutoff voltage and the gate - source voltage.

With a significant increase in the voltage of the drain end, a breakdown of the pn junction is observed.

In the output characteristics of the field-effect transistor, two working areas OA and OV can be distinguished. The OA region is called the steep region of the characteristic, the AB region is called the flat or saturation region. In the steep region, the transistor can be used as an ohmic controlled resistance. In the amplifier stages, the transistor operates on a flat section of the characteristic. A breakdown of the electrical transition occurs behind point B.

The input characteristic of a field-effect transistor with a control junction (Fig. 2.38, b) is the reverse branch of the current-voltage characteristic of the junction. Although the gate current changes slightly with voltage and reaches its highest value under the condition of a short circuit of the source and drain terminals (gate leakage current), it can be neglected in most cases. A change in voltage does not cause significant changes in the gate current, which is typical for a reverse current -junction.

When operating in a flat region of the current-voltage characteristic, the drain current at a given voltage 11w is determined from the expression

where is the initial drain current, under which the current at and the drain voltage exceeding the saturation voltage:.

Since the field-effect transistor is controlled by the gate voltage, the slope of the characteristic is used to quantify the gate control action

The slope of the characteristic reaches its maximum value at. To determine the value of S at any voltage, we differentiate the expression

When expression (2.73) takes the form

Substituting (1.74) into expression (1.73), we obtain.

Thus, the slope of the FET response decreases with increasing voltage applied to its gate.

The initial value of the slope of the characteristic can be determined graphically and analytically. To do this, draw a tangent line from a point to a stock gate characteristic (Fig. 2.38. C). It will cut off a segment on the stress axis, and its slope will determine the value.

The amplifying properties of field-effect transistors are characterized by the gain

which is related to the slope of the characteristic and the internal resistance by the equation, where is the differential internal resistance of the transistor.

Indeed, in the general case.

If, with a simultaneous change in and, then whence

As with bipolar transistors, large and small signal modes are distinguished in field-effect transistors. The large signal mode is most often calculated using the input and output characteristics of the transistor and the equivalent circuit in Fig. 2.39, a. To analyze the small signal mode, small-signal equivalent circuits are widely used in Fig. 2.39, b-d (transistor with a p-type channel). Since the resistances of closed junctions in silicon field-effect transistors are large (tens - hundreds of megohms), in most cases they can be ignored. For practical calculations, the equivalent circuit in Fig. 2.39, d, although it reflects much worse the actual physical processes occurring in the transistors under consideration. All gate capacitances in the circuit are replaced by one equivalent capacitance C „, which is charged through the average equivalent resistance.

Fig. 2.39. Simplified equivalent circuit of a field-effect transistor with a control p-n-junction for direct current (a); small-signal equivalent circuits: complete (b), simplified (c), modified (d).

It can be considered that it is equal to the static resistance in the steep range of characteristics - the resistance between the drain and the source in the open state of the transistor at a given drain-source voltage, which is less than the saturation voltage. The gate resistance (ohmic) is reflected by the equivalent resistance, which, due to its large value (tens to hundreds), can be ignored.

Typical values of the parameters of silicon transistors included in the equivalent circuit:.

The capacitances of the field-effect transistor, as well as the final velocity of the charge carriers in the channel, determine its inertial properties. The inertia of the transistor in the first approximation is taken into account by introducing the operator slope of the characteristic

where is the limiting frequency, determined at the level of 0.7 of the static value of the slope of the characteristic.

When the temperature changes, the parameters and characteristics of field-effect transistors with a control change due to the influence of the following factors: changes in the reverse current of the closed p-n-junction; changes in the contact potential difference, changes in the resistivity of the channel.

The reverse current at the closed one increases exponentially with increasing temperature. Roughly, we can assume that it doubles with an increase in temperature by 6-8 C. If there is a large external resistance in the gate circuit of the transistor, then the voltage drop across it, caused by the changed current, can significantly change the voltage at the gate.

The contact potential difference decreases with an increase in temperature by approximately. With a constant gate voltage, this leads to an increase in the drain current. For transistors with low cut-off voltage, this effect is predominant and changes in drain current will be positive.

Since the temperature coefficient characterizing the change in the resistivity of the channel is positive, the drain current decreases with increasing temperature. This opens up the possibility of a correct choice of the position of the operating point of the transistor to mutually compensate for changes in current caused by a change in the contact potential difference and the resistivity of the channel. As a result, the drain current will be nearly constant over a wide temperature range.

The operating point at which the change in the flow rate with the change in temperature has a minimum value is called the thermostable point. Its approximate position can be found from the equation

From (2.78) it can be seen that with a significant slope of the characteristic at the thermostable point is small and a much lower gain can be obtained from the transistor than when operating with low voltage.

Fig. 2.40. The inclusion of a field-effect transistor in the circuit: a - with a common source; b - with a common drain

Modern field-effect transistors, made on the basis of silicon, are operational up to a temperature of 120-150 C. Their inclusion in the circuits of amplifying stages with a common source and common drain is shown in Fig. 2.40, a, b. The constant voltage provides a certain value of the channel resistance and a certain drain current. When the input amplified voltage is applied, the gate potential changes, and, accordingly, the drain and source currents change, as well as the voltage drop across the resistor R.

The increment in the voltage drop across the resistor R at a large value is much larger than the increment in the input voltage. Due to this, the signal is amplified. Due to its low prevalence, the inclusion with a common gate is not shown. When the type of electrical conductivity of the channel changes, only the polarity of the applied voltages and the direction of the currents change, including in equivalent circuits.

The main advantages of field-effect transistors with a control p-n-junction over bipolar ones are high input resistance, low noise, ease of manufacture, the absence of residual voltage in the open state between the source and drain of the open transistor.

МДП - transistors can be of two types: transistors with built-in channels (the channel is created during manufacture) and transistors with induced channels (the channel appears under the action of a voltage applied to the control electrodes).

Transistors of the first type can operate both in the channel depletion mode with charge carriers and in the enrichment mode. Transistors of the second type can only be used in enrichment mode. In MIS transistors, unlike transistors with a control pn junction, the metal gate is insulated from the semiconductor by a dielectric layer and there is an additional output from the crystal on which the device is made (Fig. 2.41), called a substrate.

Fig. 2.41. MIS transistor structures: a - planar transistor with an induced channel. b - planar transistor with a built-in channel; , transistor - and.

Fig. 2.42. Distribution of charge carriers in the surface layer

The control voltage can be applied both between the gate and the substrate and independently to the substrate and the gate. Under the influence of the generated electric field, a -type channel appears at the semiconductor surface due to the repulsion of electrons from the surface into the interior of the semiconductor in a transistor with an induced channel. In a transistor with an embedded channel, the existing channel is expanded or narrowed. Changing the control voltage changes the channel width and, accordingly, the resistance and current of the transistor.

A significant advantage of MOS transistors is a high input resistance, reaching values of Ohm (for transistors with a control -junction Ohm).

Let us consider in somewhat more detail the operation of an MOS transistor with an induced -channel. Suppose that silicon with an electrical conductivity of the -type is used as the initial material of the transistor. The role of the dielectric film is played by silicon dioxide. In the absence of bias, the near-surface layer of the semiconductor is usually enriched with electrons (Fig. 2.42, a). This is explained by the presence of positively charged ions in the dielectric film, which is a consequence of the previous oxidation of silicon and its photolithographic processing, as well as the presence of traps at the boundary. Recall that traps are a set of energy levels located deep in the forbidden zone, close to its middle.

When a negative voltage is applied to the gate, the electrons of the near-surface layer are repulsed deep into the semiconductor, and the holes move to the surface. The near-surface layer acquires hole electrical conductivity (Fig. 2.42, b). A thin inverse layer appears in it, connecting the drain to the source. This layer acts as a channel. If a voltage is applied between the source and drain, the holes moving along the channel create a drain current. By varying the gate voltage, you can expand or narrow the channel and thereby increase or decrease the drain current.

The gate voltage at which the channel is induced is called the threshold voltage. Since the channel appears gradually, as the gate voltage increases, in order to avoid ambiguity in its definition, a certain drain current value is usually set, above which it is considered that the gate potential has reached the threshold voltage.

With increasing distance from the semiconductor surface, the concentration of induced holes decreases. At a distance approximately equal to the channel thickness, the electrical conductivity becomes intrinsic. Then there is a section depleted of the main charge carriers (-junction). Thanks to it, the drain, source and channel are isolated from the substrate; - the junction is biased by the applied voltage in the opposite direction. Obviously, its width and channel width can be changed by supplying an additional voltage to the substrate relative to the drain and source electrodes of the transistor. Therefore, the drain current can be controlled not only by changing the gate voltage, but also by changing the voltage on the substrate. In this case, the control of the MOS transistor is similar to the control of a field-effect transistor with a control junction. To form a channel, a higher voltage must be applied to the gate.

The thickness of the inverse layer is much less than the thickness of the depletion layer. If the latter is hundreds - thousands of nm, then the thickness of the induced channel is only 1-5 nm. In other words, the holes of the induced channel are “pressed” to the semiconductor surface; therefore, the structure and properties of the semiconductor – insulator interface play a very important role in MIS transistors.

The holes forming the channel enter it not only from the substrate -type, where there are few of them and they are generated relatively slowly, but also from the layers -type of the source and drain, where their concentration is practically unlimited, and the field strength near these electrodes is quite high.

In transistors with a built-in channel, the current in the drain circuit will flow even at zero gate voltage. To stop it, it is necessary to apply a positive voltage to the gate (for a structure with an -type channel) equal to or greater than the cutoff voltage. In this case, the holes from the inverse layer will be almost completely displaced into the interior of the semiconductor and the channel will disappear. When a negative voltage is applied, the channel expands and the current increases. In this way. MIS transistors with built-in channels operate in both depletion and enrichment modes.

Fig. 2.43. The structure of the MOS transistor with a changed channel width during current flow (a); its output characteristics with induced (b) and built-in (c) channels: I steep region; II - flat area, or saturation area; III - breakdown area; 1 - dining layer

Like field-effect transistors with a control junction, MOS transistors at low voltages (in the area of Fig. 2.43, b, c) behave like a linearized controlled resistance. As the voltage increases, the channel width decreases due to a drop in voltage across it and a change in the resulting electric field. This is especially pronounced in that part of the channel, which is located near the drain (Fig. 2.43, a). The voltage drops created by the current lead to an uneven distribution of the electric field strength along the channel, and it increases as it approaches the drain. With voltage, the channel near the drain becomes so narrow that dynamic equilibrium occurs when an increase in voltage causes a decrease in the channel width and an increase in its resistance. As a result, the current changes little with a further increase in voltage. These processes of changing the channel width depending on the voltage are the same as in field-effect transistors with a control pn junction.

The output characteristics of MOS transistors are similar to those of field-effect transistors with a control one (Fig. 2.43, b, c). They can be divided into steep and shallow areas, as well as a breakdown area. In the steep region, the MOS transistor can act as an electrically controlled resistance. The flat II region is usually used in the construction of amplifier stages. Analytical approximations of the current-voltage characteristics of MOS transistors are not very convenient and are little used in engineering practice. For rough estimates of the drain current in the saturation region, one can use the equation

For transistors with a built-in channel, you can use equations (2.79), if you replace and take into account the signs of voltages and .. They characterize the parameters of the field-effect transistor, which for a given measurement mode is represented by the equivalent circuit in Fig. 2.44, d. It reflects the characteristics of the transistor worse, but its parameters are known or can be easily measured (input capacitance, throughput capacitance, output capacitance).

The operator equation for the slope of the characteristic of MOS transistors has the same form as for field-effect transistors with a control. In this case, the time constant. In a typical case, with a channel length of 5 μm, the limiting frequency, at which the slope of the characteristic decreases by a factor of 0.7, lies within a few hundred megahertz.

The temperature dependence of the threshold voltage and cutoff voltage is due to a change in the position of the Fermi level, a change in the space charge in the depletion region, and the effect of temperature on the value of the charge in the dielectric. In MOS transistors, you can also find a thermostable operating point, in which the drain current is little dependent on temperature. For different transistors, the value of the drain current at the thermostable point is within. An important advantage of MOS transistors over bipolar ones is the low voltage drop across them when switching small signals. So, if in bipolar transistors in saturation mode, the voltage

When decreasing, it can be reduced to a value tending to zero. Since MIS transistors with a silicon dioxide dielectric have become widespread, we will further call them MOS transistors.

Nowadays the industry is also producing double insulated gate (tetrode) MOSFETs, for example. The presence of the second gate makes it possible to simultaneously control the transistor current using two control voltages, which facilitates the construction of various amplifying and multiplying devices. Their characteristics are similar to the characteristics of single-gate field-effect transistors, only their number is greater, since they are built for the voltage of each gate with a constant voltage on the other gate. Accordingly, the slope of the characteristics for the first and second gates, the cutoff voltage of the first and second gates, etc. are distinguished. The voltage supply to the gates is no different from the voltage supply to the gate of a single-gate MOS transistor.

Must exceed the threshold. Otherwise, the channel will not appear and the transistor will be locked.