SIP panels / 10.08.2023

The principle of operation and mode of operation of a bipolar transistor. Bipolar transistor: principle of operation. Types of bipolar transistors

The transistor is a ubiquitous and important component in modern microelectronics. Its purpose is simple: it allows you to control a much stronger one using a weak signal.

In particular, it can be used as a controlled “damper”: by the absence of a signal at the “gate”, block the flow of current, and by supplying it, allow it. In other words: this is a button that is pressed not by a finger, but by applying voltage. This is the most common application in digital electronics.

Transistors are available in different packages: the same transistor can look completely different in appearance. In prototyping, the most common cases are:

TO-92 - compact, for light loads

TO-220AB - massive, good heat dissipation, for heavy loads

The designation on the diagrams also varies depending on the type of transistor and the designation standard used in the compilation. But regardless of the variation, its symbol remains recognizable.

Bipolar transistors

Bipolar junction transistors (BJT, Bipolar Junction Transistors) have three contacts:

Collector - high voltage is applied to it, which you want to control

Base - a small amount is supplied through it current to unlock large; the base is grounded to block it

Emitter - current flows through it from the collector and base when the transistor is “open”

The main characteristic of a bipolar transistor is the indicator hfe also known as gain. It reflects how many times more current in the collector-emitter section the transistor can pass in relation to the base-emitter current.

For example, if hfe= 100, and 0.1 mA passes through the base, then the transistor will pass through itself a maximum of 10 mA. If in this case there is a component in the high current section that consumes, for example, 8 mA, it will be provided with 8 mA, and the transistor will have a “reserve”. If there is a component that draws 20 mA, it will only be provided with the maximum 10 mA.

Also, the documentation for each transistor indicates the maximum permissible voltages and currents at the contacts. Exceeding these values leads to excessive heating and reduced service life, and a strong excess can lead to destruction.

NPN and PNP

The transistor described above is a so-called NPN transistor. It is called that because it consists of three layers of silicon connected in the order: Negative-Positive-Negative. Where negative is a silicon alloy with an excess of negative charge carriers (n-doped), and positive is an alloy with an excess of positive charge carriers (p-doped).

NPNs are more effective and common in industry.

When designating PNP transistors, they differ in the direction of the arrow. The arrow always points from P to N. PNP transistors have an “inverted” behavior: current is not blocked when the base is grounded and blocked when current flows through it.

Field effect transistors

Field effect transistors (FET, Field Effect Transistor) have the same purpose, but differ in internal structure. A particular type of these components are MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) transistors. They allow you to operate with much greater power with the same dimensions. And the control of the “damper” itself is carried out exclusively using voltage: no current flows through the gate, unlike bipolar transistors.

Field effect transistors have three contacts:

Drain - high voltage is applied to it, which you want to control

Gate - voltage is applied to it to allow current to flow; the gate is grounded to block the current.

Source - current flows through it from the drain when the transistor is “open”

N-Channel and P-Channel

By analogy with bipolar transistors, field transistors differ in polarity. The N-Channel transistor was described above. They are the most common.

P-Channel when designated differs in the direction of the arrow and, again, has an “inverted” behavior.

Connecting transistors to drive high-power components

A typical task of a microcontroller is to turn a specific circuit component on and off. The microcontroller itself usually has modest power handling characteristics. So Arduino, with 5 V output per pin, can withstand a current of 40 mA. Powerful motors or ultra-bright LEDs can draw hundreds of milliamps. When connecting such loads directly, the chip can quickly fail. In addition, for the operation of some components, a voltage greater than 5 V is required, and Arduino cannot produce more than 5 V from the digital output pin.

But it is easily enough to control a transistor, which in turn will control a large current. Let's say we need to connect a long LED strip that requires 12 V and consumes 100 mA:

Now, when the output is set to logical one (high), the 5 V entering the base will open the transistor and current will flow through the tape - it will glow. When the output is set to logic zero (low), the base will be grounded through the microcontroller and current flow will be blocked.

Pay attention to the current limiting resistor R. It is necessary so that when control voltage is applied, a short circuit does not form along the route microcontroller - transistor - ground. The main thing is not to exceed the permissible current through the Arduino contact of 40 mA, so you need to use a resistor with a value of at least:

Here U d- this is the voltage drop across the transistor itself. It depends on the material from which it is made and is usually 0.3 – 0.6 V.

But it is absolutely not necessary to keep the current at the permissible limit. It is only necessary that the gain of the transistor allows you to control the required current. In our case it is 100 mA. Acceptable for the transistor used hfe= 100, then a control current of 1 mA will be enough for us

A resistor with a value from 118 Ohm to 4.7 kOhm is suitable for us. For stable operation on one side and light load on the chip on the other, 2.2 kOhm is a good choice.

If you use a field-effect transistor instead of a bipolar transistor, you can do without a resistor:

This is due to the fact that the gate in such transistors is controlled solely by voltage: there is no current in the microcontroller - gate - source section. And due to its high characteristics, a circuit using MOSFETs allows you to drive very powerful components.

In this article we will try to describe principle of operation The most common type of transistor is bipolar. Bipolar transistor is one of the main active elements of radio-electronic devices. Its purpose is to work to amplify the power of the electrical signal arriving at its input. Power amplification is carried out using an external energy source. A transistor is a radio-electronic component with three terminals

Design feature of a bipolar transistor

To produce a bipolar transistor, you need a semiconductor of hole or electronic conductivity type, which is obtained by diffusion or alloying with acceptor impurities. As a result, regions with polar conductivities are formed on both sides of the base.

Bipolar transistors are of two types based on conductivity: n-p-n and p-n-p. The operating rules that govern a bipolar transistor having n-p-n conductivity (for p-n-p it is necessary to change the polarity of the applied voltage):

The positive potential at the collector is more important compared to the emitter.
Any transistor has its own maximum permissible parameters Ib, Ik and Uke, exceeding which is in principle unacceptable, since this can lead to destruction of the semiconductor.
The base-emitter and base-collector terminals function like diodes. As a rule, the diode in the base-emitter direction is open, and in the base-collector direction it is biased in the opposite direction, that is, the incoming voltage interferes with the flow of electric current through it.
If steps 1 to 3 are completed, then the current Ik is directly proportional to the current Ib and has the form: Ik = he21*Ib, where he21 is the current gain. This rule characterizes the main quality of the transistor, namely that the low base current controls the powerful collector current.

For different bipolar transistors of the same series, the he21 indicator can fundamentally vary from 50 to 250. Its value also depends on the flowing collector current, the voltage between the emitter and the collector, and on the ambient temperature.

Let's study rule No. 3. It follows from this that the voltage applied between the emitter and the base should not be significantly increased, since if the base voltage is 0.6...0.8 V greater than the emitter (forward voltage of the diode), then an extremely large current will appear. Thus, in a working transistor, the voltages at the emitter and base are interconnected according to the formula: Ub = Ue + 0.6V (Ub = Ue + Ube)

Let us remind you once again that all these points apply to transistors with n-p-n conductivity. For the p-n-p type everything should be reversed.

You should also pay attention to the fact that the collector current has no connection with the conductivity of the diode, since, as a rule, reverse voltage is supplied to the collector-base diode. In addition, the current flowing through the collector depends very little on the potential on the collector (this diode is similar to a small current source)

When the transistor is turned on in amplification mode, the emitter junction is open and the collector junction is closed. This is achieved by connecting power supplies.

Since the emitter junction is open, the emitter current will pass through it, arising due to the transition of holes from the base to the emitter, as well as electrons from the emitter to the base. Thus, the emitter current contains two components - hole and electron. The injection ratio determines the efficiency of the emitter. Charge injection is the transfer of charge carriers from the zone where they were the majority to the zone where they become minority.

In the base, electrons recombine, and their concentration in the base is replenished from the plus of the EE source. As a result, a rather weak current will flow in the base electrical circuit. The remaining electrons that did not have time to recombine in the base, under the accelerating influence of the field of the locked collector junction, as minority carriers, will move into the collector, creating a collector current. The transfer of charge carriers from the zone where they were minority to the zone where they become majority is called extraction of electrical charges.

TOPIC 4. BIPOLAR TRANSISTORS

4.1 Design and principle of operation

A bipolar transistor is a semiconductor device consisting of three regions with alternating types of electrical conductivity and is suitable for power amplification.

Currently produced bipolar transistors can be classified according to the following criteria:

By material: germanium and silicon;

According to the type of conductivity of the areas: p-n-p and n-p-n types;

By power: low (Pmax £ 0.3 W), medium (Pmax £ 1.5 W) and high power (Pmax > 1.5 W);

By frequency: low-frequency, mid-frequency, high-frequency and microwave.

In bipolar transistors, the current is determined by the movement of charge carriers of two types: electrons and holes (or majority and minority). Hence their name - bipolar.

Currently, only transistors with planar p-n junctions are manufactured and used.

The structure of a planar bipolar transistor is shown schematically in Fig. 4.1.

It is a plate of germanium or silicon in which three regions with different electrical conductivities are created. In an n-p-n transistor, the middle region has hole, and the outer regions have electronic conductivity.

Transistors of the pnp type have a middle region with electronic conductivity, and outer regions with hole electrical conductivity.

The middle region of the transistor is called the base, one extreme region is the emitter, and the other is the collector. Thus, the transistor has two p-n junctions: the emitter - between the emitter and the base and the collector - between the base and the collector. The area of the emitter junction is smaller than the area of the collector junction.

The emitter is the region of the transistor whose purpose is to inject charge carriers into the base. A collector is a region whose purpose is to extract charge carriers from the base. The base is the region into which the emitter injects charge carriers that are non-majority for this region.

The concentration of the main charge carriers in the emitter is many times greater than the concentration of the main charge carriers in the base, and their concentration in the collector is somewhat less than the concentration in the emitter. Therefore, the emitter conductivity is several orders of magnitude higher than the base conductivity, and the collector conductivity is somewhat less than the emitter conductivity.

Conclusions are drawn from the base, emitter and collector. Depending on which of the terminals is common to the input and output circuits, there are three circuits for connecting the transistor: with a common base (CB), a common emitter (CE), and a common collector (CC).

The input, or control, circuit serves to control the operation of the transistor. In the output, or controlled, circuit, amplified oscillations are obtained. The source of amplified oscillations is included in the input circuit, and the load is connected to the output circuit.

Let's consider the principle of operation of a transistor using the example of a pnp type transistor connected according to a circuit with a common base (Fig. 4.2).

Figure 4.2 – Operating principle of a bipolar transistor (pnp type)

The external voltages of two power sources EE and Ek are connected to the transistor in such a way that the emitter junction P1 is biased in the forward direction (forward voltage), and the collector junction P2 is biased in the reverse direction (reverse voltage).

If a reverse voltage is applied to the collector junction and the emitter circuit is open, then a small reverse current Iko (units of microamps) flows in the collector circuit. This current arises under the influence of reverse voltage and is created by the directional movement of minority charge carriers, base holes and collector electrons through the collector junction. The reverse current flows through the circuit: +Ek, base-collector, -Ek. The magnitude of the reverse collector current does not depend on the collector voltage, but depends on the temperature of the semiconductor.

When a constant voltage EE is connected to the emitter circuit in the forward direction, the potential barrier of the emitter junction decreases. The injection of holes into the base begins.

The external voltage applied to the transistor turns out to be applied mainly to the transitions P1 and P2, because they have high resistance compared to the resistance of the base, emitter and collector regions. Therefore, holes injected into the base move through it through diffusion. In this case, the holes recombine with the electrons of the base. Since the carrier concentration in the base is much lower than in the emitter, very few holes recombine. With a small base thickness, almost all holes will reach the collector junction P2. In place of the recombined electrons, electrons from the power source Ek enter the base. Holes that recombine with electrons in the base create a base current IB.

Under the influence of reverse voltage Ek, the potential barrier of the collector junction increases, and the thickness of the junction P2 increases. But the potential barrier of the collector junction does not prevent holes from passing through it. The holes entering the region of the collector junction fall into a strong accelerating field created at the junction by the collector voltage, and are extracted (retracted) by the collector, creating a collector current Ik. The collector current flows through the circuit: +Ek, base-collector, -Ek.

Thus, three currents flow in the transistor: emitter, collector and base current.

In the wire, which is the base terminal, the emitter and collector currents are directed in opposite directions. Therefore, the base current is equal to the difference between the emitter and collector currents: IB = IE - IK.

Physical processes in an n-p-n transistor proceed similarly to the processes in a p-n-p transistor.

The total emitter current IE is determined by the number of main charge carriers injected by the emitter. The main part of these charge carriers reaching the collector creates a collector current Ik. A small part of the charge carriers injected into the base recombine in the base, creating a base current IB. Consequently, the emitter current will be divided into base and collector currents, i.e. IE = IB + Ik.

The emitter current is the input current, the collector current is the output current. The output current is part of the input current, i.e.

(4.1)

where a is the current transfer coefficient for the OB circuit;

Since the output current is less than the input current, the coefficient a<1. Он показывает, какая часть инжектированных в базу носителей заряда достигает коллектора. Обычно величина a составляет 0,95¸0,995.

In a common emitter circuit, the output current is the collector current and the input current is the base current. Current gain for the OE circuit:

(4.2)

(4.3)

Consequently, the current gain for the OE circuit is tens of units.

The output current of the transistor depends on the input current. Therefore, a transistor is a current-controlled device.

Changes in emitter current caused by changes in emitter junction voltage are completely transmitted to the collector circuit, causing a change in collector current. And because The voltage of the collector power source Ek is significantly greater than the emitter Ee, then the power consumed in the collector circuit Pk will be significantly greater than the power in the emitter circuit Re. Thus, it is possible to control high power in the collector circuit of the transistor with low power spent in the emitter circuit, i.e. there is an increase in power.

4.2 Circuits for connecting bipolar transistors

The transistor is connected to the electrical circuit in such a way that one of its terminals (electrode) is the input, the second is the output, and the third is common to the input and output circuits. Depending on which electrode is common, there are three transistor switching circuits: OB, OE and OK. These circuits for a pnp transistor are shown in Fig. 4.3. For an n-p-n transistor in the switching circuits, only the polarity of the voltages and the direction of the currents change. For any transistor switching circuit (in active mode), the polarity of the power supplies must be selected so that the emitter junction is switched on in the forward direction, and the collector junction in the reverse direction.

Figure 4.3 – Connection circuits for bipolar transistors: a) OB; b) OE; c) OK

4.3 Static characteristics of bipolar transistors

The static mode of operation of the transistor is the mode when there is no load in the output circuit.

The static characteristics of transistors are the graphically expressed dependences of the voltage and current of the input circuit (input current-voltage characteristics) and the output circuit (output current-voltage characteristics). The type of characteristics depends on the method of switching on the transistor.

4.3.1 Characteristics of a transistor connected according to the OB circuit

The input characteristic is the dependence:

IE = f(UEB) with UKB = const (Fig. 4.4, a).

The output characteristic is the dependence:

IK = f(UKB) with IE = const (Fig. 4.4, b).

Figure 4.4 – Static characteristics of a bipolar transistor connected according to the OB circuit

The output current-voltage characteristics have three characteristic regions: 1 – strong dependence of Ik on UKB (nonlinear initial region); 2 – weak dependence of Ik on UKB (linear region); 3 – breakdown of the collector junction.

Transistors are divided into bipolar and field-effect. Each of these types has its own operating principle and design, however, what they have in common is the presence of semiconductor p-n structures.

Symbols of transistors are given in the table:

Device type
Bipolar	Bipolar pnp type
Bipolar	Bipolar n-p-n type
Field	With the manager p-n junction	With p-type channel
	With the manager p-n junction	With n-type channel
	With isolated shutter MOSFET transistors	With built-in channel	Built-in channel p-type
		With built-in channel	Built-in channel n-type
		With induced channel	Induced channel p-type
		With induced channel	Induced channel n-type

Bipolar transistors

The definition of “bipolar” indicates that the operation of a transistor is associated with processes in which charge carriers of two types take part - electrons and holes.

A transistor is a semiconductor device with two electron-hole junctions, designed to amplify and generate electrical signals. A transistor uses both types of carriers - major and minor, which is why it is called bipolar.

A bipolar transistor consists of three regions of a monocrystalline semiconductor with different types of conductivity: emitter, base and collector.

E - emitter,
B - base,
K - collector,
EP - emitter junction,
KP - collector junction,
W - base thickness.

Each of the transitions of the transistor can be turned on either in the forward or reverse direction. Depending on this, there are three operating modes of the transistor:

Cut-off mode - both p-n junctions are closed, while a relatively small current usually flows through the transistor
Saturation mode - both p-n junctions are open
Active mode - one of the p-n junctions is open and the other is closed

In cutoff mode and saturation mode, the transistor cannot be controlled. Effective control of the transistor is carried out only in active mode. This mode is the main one. If the voltage at the emitter junction is direct, and at the collector junction it is reverse, then the switching on of the transistor is considered normal; if the polarity is opposite, it is inverse.

In normal mode, the collector p-n junction is closed, the emitter junction is open. The collector current is proportional to the base current.

The movement of charge carriers in an n-p-n transistor is shown in the figure:

When the emitter is connected to the negative terminal of the power source, an emitter current Ie occurs. Since an external voltage is applied to the emitter junction in the forward direction, the electrons cross the junction and enter the base region. The base is made of a p-semiconductor, so electrons are minority charge carriers for it.

Electrons that enter the base region partially recombine with holes in the base. However, the base is usually made of a very thin p-conductor with a high resistivity (low impurity content), so the concentration of holes in the base is low and only a few electrons entering the base recombine with its holes, forming a base current Ib. Most electrons, due to thermal motion (diffusion) and under the influence of the collector field (drift), reach the collector, forming a component of the collector current Ik.

The relationship between the increments of the emitter and collector currents is characterized by the current transfer coefficient

As follows from a qualitative examination of the processes occurring in a bipolar transistor, the current transfer coefficient is always less than unity. For modern bipolar transistors α = 0.9 ÷ 0.95

When Ie ≠ 0, the transistor collector current is equal to:

In the considered connection circuit, the base electrode is common to the emitter and collector circuits. This circuit for connecting a bipolar transistor is called a circuit with a common base, while the emitter circuit is called the input circuit, and the collector circuit is called the output circuit. However, such a circuit for switching on a bipolar transistor is used very rarely.

Three circuits for switching on a bipolar transistor

There are switching circuits with a common base, a common emitter, and a common collector. Circuits for a pnp transistor are shown in figures a, b, c:

In a circuit with a common base (Fig. a), the base electrode is common to the input and output circuits; in a circuit with a common emitter (Fig. b), the emitter is common; in a circuit with a common collector (Fig. c), the collector is common.

The figure shows: E1 – power supply of the input circuit, E2 – power supply of the output circuit, Uin – source of the amplified signal.

The main switching circuit is one in which the common electrode for the input and output circuits is the emitter (switching circuit for a bipolar transistor with a common emitter). For such a circuit, the input circuit passes through the base-emitter junction and a base current arises in it:

The low value of the base current in the input circuit has led to the widespread use of a common emitter circuit.

Bipolar transistor in a common emitter (CE) circuit

In a transistor connected according to the OE circuit, the relationship between current and voltage in the input circuit of the transistor Ib = f1 (Ube) is called the input or basic current-voltage characteristic (VC) of the transistor. The dependence of the collector current on the voltage between the collector and the emitter at fixed values of the base current Iк = f2 (Uke), Ib – const is called the family of output (collector) characteristics of the transistor.

The input and output current-voltage characteristics of a medium-power bipolar transistor of the n-p-n type are shown in the figure:

As can be seen from the figure, the input characteristic is practically independent of the voltage Uke. The output characteristics are approximately equidistant from each other and almost linear over a wide range of voltage changes Uke.

The dependence Ib = f(Ube) is an exponential dependence characteristic of the current of a forward-biased p-n junction. Since the base current is recombination, its value Ib is β times less than the injected emitter current Ie. As the collector voltage Uк increases, the input characteristic shifts to the region of higher voltages Ub. This is due to the fact that due to modulation of the base width (Early effect), the proportion of recombination current in the base of the bipolar transistor decreases. The voltage Ube does not exceed 0.6...0.8 V. Exceeding this value will lead to a sharp increase in the current flowing through the open emitter junction.

The dependence Ik = f(Uke) shows that the collector current is directly proportional to the base current: Ik = B Ib

Bipolar transistor parameters

Representation of a transistor in a small-signal mode of operation as a four-terminal network

In a small-signal operating mode, the transistor can be represented by a four-terminal network. When voltages u1, u2 and currents i1, i2 change according to a sinusoidal law, the connection between voltages and currents is established using Z, Y, h parameters.

Potentials 1", 2", 3 are the same. It is convenient to describe a transistor using h-parameters.

The electrical state of a transistor connected according to a circuit with a common emitter is characterized by four quantities: Ib, Ube, Ik and Uke. Two of these quantities can be considered independent, and the other two can be expressed in terms of them. For practical reasons, it is convenient to choose the quantities Ib and Uke as independent ones. Then Ube = f1 (Ib, Uke) and Ik = f2 (Ib, Uke).

In amplifying devices, the input signals are increments of input voltages and currents. Within the linear part of the characteristics, the following equalities are true for the increments Ube and Ik:

Physical meaning of the parameters:

For a circuit with OE, the coefficients are written with the index E: h11e, h12e, h21e, h22e.

The passport data indicates h21е = β, h21b = α. These parameters characterize the quality of the transistor. To increase the value of h21, you need to either reduce the base width W or increase the diffusion length, which is quite difficult.

Composite transistors

To increase the value of h21, bipolar transistors are connected using a Darlington circuit:

In a composite transistor that has the same characteristics as one, the base VT1 is connected to the emitter VT2 and ΔIе2 = ΔIb1. The collectors of both transistors are connected and this terminal is the terminal of the composite transistor. The base VT2 plays the role of the base of the composite transistor ΔIb = ΔIb2, and the emitter VT1 plays the role of the emitter of the composite transistor ΔIe = ΔI1.

Let us obtain an expression for the current gain β for the Darlington circuit. Let us express the relationship between the change in the base current dIb and the resulting change in the collector current dIk of the composite transistor as follows:

Since for bipolar transistors the current gain is usually several tens (β1, β2 >> 1), the total gain of the composite transistor will be determined by the product of the gains of each transistor βΣ = β1 · β2 and can be quite large in value.

Let us note the features of the operating mode of such transistors. Since the emitter current VT2 Ie2 is the base current VT1 dIb1, then, therefore, transistor VT2 should operate in micro-power mode, and transistor VT1 - in high-injection mode, their emitter currents differ by 1-2 orders of magnitude. With such a suboptimal choice of operating characteristics of bipolar transistors VT1 and VT2, it is not possible to achieve high current gain values in each of them. Nevertheless, even with gain values β1, β2 ≈ 30, the total gain βΣ will be βΣ ≈ 1000.

High gain values in composite transistors are realized only in statistical mode, so composite transistors are widely used in the input stages of operational amplifiers. In circuits at high frequencies, composite transistors no longer have such advantages; on the contrary, both the limiting current amplification frequency and the operating speed of the composite transistors are less than the same parameters for each of the transistors VT1, VT2 separately.

Frequency properties of bipolar transistors

The process of propagation of minority charge carriers injected into the base from the emitter to the collector junction proceeds by diffusion. This process is quite slow, and the carriers injected from the emitter will reach the collector no earlier than during the diffusion of carriers through the base. Such a delay will lead to a phase shift between the current Ie and the current Ik. At low frequencies, the phases of the currents Ie, Ik and Ib coincide.

The frequency of the input signal at which the modulus of the gain decreases by a factor of compared to the static value β0 is called the limiting frequency of current amplification of a bipolar transistor in a common-emitter circuit

Fβ – limiting frequency (cutoff frequency)
fgr – cut-off frequency (unity gain frequency)

Field effect transistors

Field-effect, or unipolar, transistors use the field effect as the main physical principle. Unlike bipolar transistors, in which both types of carriers, both major and minor, are responsible for the transistor effect, field-effect transistors use only one type of carrier to realize the transistor effect. For this reason, field-effect transistors are called unipolar. Depending on the conditions for implementing the field effect, field-effect transistors are divided into two classes: field-effect transistors with an insulated gate and field-effect transistors with a control p-n junction.

Field-effect transistors with control p-n junction

Schematically, a field-effect transistor with a control pn junction can be represented as a plate, to the ends of which electrodes, a source and a drain are connected. In Fig. shows the structure and connection diagram of a field-effect transistor with an n-type channel:

In an n-channel transistor, the majority charge carriers in the channel are electrons, which move along the channel from a low-potential source to a higher-potential drain, producing a drain current Ic. A voltage is applied between the gate and the source, blocking the p-n junction formed by the n-region of the channel and the p-region of the gate.

When a blocking voltage is applied to the p-n junction Uzi, a uniform layer appears at the channel boundaries, depleted of charge carriers and having a high resistivity. This leads to a decrease in the conductive width of the channel.

By changing the value of this voltage, it is possible to change the cross-section of the channel and, consequently, change the value of the electrical resistance of the channel. For an n-channel field effect transistor, the drain potential is positive with respect to the source potential. When the gate is grounded, current flows from drain to source. Therefore, to stop the current, a reverse voltage of several volts must be applied to the gate.

The voltage value Uzi, at which the current through the channel becomes almost equal to zero, is called the cut-off voltage Uzap

Thus, a field-effect transistor with a gate in the form of a p-n junction represents a resistance, the value of which is regulated by an external voltage.

The field-effect transistor is characterized by the following current-voltage characteristic:

Here, the dependence of the drain current Ic on the voltage at a constant voltage at the gate Uzi determines the output, or drain, characteristics of the field-effect transistor. At the initial section of the characteristics Usi + |Uzi |< Uзап ток стока Iс возрастает с увеличением Uси . При повышении напряжения сток - исток до Uси = Uзап - |Uзи | происходит перекрытие канала и дальнейший рост тока Iс прекращается (участок насыщения). Отрицательное напряжение Uзи между затвором и истоком смещает момент перекрытия канала в сторону меньших значений напряжения Uси и тока стока Iс . Участок насыщения является рабочей областью выходных характеристик полевого транзистора. Дальнейшее увеличение напряжения Uси приводит к пробою р-n-перехода между затвором и каналом и выводит транзистор из строя.

The current-voltage characteristic Ic = f(Uzi) shows the voltage Uzap. Since Uzi ≤ 0 the p-n junction is closed and the gate current is very small, about 10 -8…10-9 A, therefore, the main advantages of a field-effect transistor, compared to a bipolar transistor, include a high input resistance, about 10 10…1013 Ohm. In addition, they are distinguished by low noise and manufacturability.

There are two main switching schemes that have practical application. A circuit with a common source (Fig. a) and a circuit with a common drain (Fig. b), which are shown in the figure:

Insulated gate field effect transistors
(MOS transistors)

The term "MOS transistor" is used to refer to field-effect transistors in which the control electrode - the gate - is separated from the active region of the field-effect transistor by a dielectric layer - an insulator. The basic element for these transistors is the metal-insulator-semiconductor (M-D-S) structure.

The technology of an MOS transistor with a built-in gate is shown in the figure:

The original semiconductor on which the MOS transistor is made is called the substrate (pin P). The two heavily doped n+ regions are called source (I) and drain (C). The area of the substrate under the gate (3) is called the embedded channel (n-channel).

The physical basis for the operation of a field-effect transistor with a metal-insulator-semiconductor structure is the field effect. The field effect is that under the influence of an external electric field the concentration of free charge carriers in the near-surface region of the semiconductor changes. In field devices with an MIS structure, the external field is caused by the applied voltage to the metal gate electrode. Depending on the sign and magnitude of the applied voltage, there can be two states of the space charge region (SCR) in the channel - enrichment, depletion.

The depletion mode corresponds to a negative voltage Uzi, at which the electron concentration in the channel decreases, which leads to a decrease in the drain current. The enrichment mode corresponds to a positive voltage Uzi and an increase in drain current.

The current-voltage characteristic is shown in the figure:

The topology of an MOS transistor with an induced (induced) p-type channel is shown in the figure:

When Uzi = 0 there is no channel and Ic = 0. The transistor can only operate in Uzi enrichment mode< 0. Если отрицательное напряжение Uзи превысит пороговое Uзи.пор , то происходит формирование инверсионного канала. Изменяя величину напряжения на затворе Uзи в области выше порогового Uзи.пор , можно менять концентрацию свободных носителей в инверсионном канале и сопротивление канала. Источник напряжения в стоковой цепи Uси вызовет ток стока Iс .

The current-voltage characteristic is shown in the figure:

In MOS transistors, the gate is separated from the semiconductor by a layer of SiO2 oxide. Therefore, the input resistance of such transistors is about 1013 ... 1015 Ohms.

The main parameters of field-effect transistors include:

The slope of the characteristic at Usp = const, Upi = const. Typical parameter values are (0.1...500) mA/V;
The slope of the characteristic along the substrate at Usp = const, Uzi = const. Typical parameter values (0.1...1) mA/V;
Initial drain current Is.init. – drain current at zero voltage value Uzi. Typical parameter values: (0.2...600) mA – for transistors with a control channel p-n junction; (0.1...100) mA – for transistors with a built-in channel; (0.01...0.5) µA – for transistors with an induced channel;
Cut-off voltage Uzi.ots. . Typical values (0.2...10) V; threshold voltage Up. Typical values (1...6) V;
Drain-source resistance in open state. Typical values (2..300) Ohm
Differential resistance (internal): at Uzi = const;
Statistical gain: μ = S ri

Thyristors

A thyristor is a semiconductor device with three or more electron-hole p-n junctions. They are mainly used as electronic keys. Depending on the number of external terminals, they are divided into thyristors with two external terminals - dinistors and thyristors with three terminals - thyristors. The letter symbol VS is used to designate thyristors.

Design and principle of operation of the dinistor

The structure, UGO and current-voltage characteristics of the dinistor are shown in the figure:

The outer p-region is called the anode (A), the outer n-region is called the cathode (K). Three p-n junctions are designated by numbers 1, 2, 3. The structure of the dinistor is 4-layer - p-n-p-n.

The supply voltage E is supplied to the dinistor in such a way that 1 of the 3 junctions is open and their resistance is insignificant, and transition 2 is closed and all the supply voltage Upr is applied to it. A small reverse current flows through the dinistor, the load R is disconnected from the power source E.

When a critical voltage is reached equal to the switch-on voltage Uon, transition 2 opens, while all three transitions 1, 2, 3 will be in the open (on) state. The resistance of the dinistor drops to tenths of an ohm.

The turn-on voltage is several hundred volts. The dinistor opens and significant currents flow through it. The voltage drop across the dinistor in the open state is 1-2 volts and depends little on the magnitude of the flowing current, the value of which is τa ≈ E / R, and UR ≈ E, i.e. the load is connected to the power source E. The voltage across the dinistor, corresponding to the maximum permissible point Iopen.max, is called the open state voltage Uokr. The maximum permissible current ranges from hundreds of mA to hundreds of A. The dinistor is in the open state until the current flowing through it becomes less than the holding current Iud. The dinistor closes when the external voltage decreases to a value of the order of 1V or when the polarity of the external source changes. Therefore, such a device is used in transient current circuits. Points B and D correspond to the limit values of dinistor currents and voltages. The recovery time of transition 2 resistance after removing the supply voltage is about 10-30 μs.

By their principle, dinistors are key action devices. In the on state (BV section) it is similar to a closed key, and in the off state (EG section) it is like an open key.

The design and principle of operation of a thyristor (thyristor)

The thyristor is a controlled device. It contains a control electrode (CE) connected to a p-type semiconductor or an n-type semiconductor of the middle junction 2.

The structure, UGO and current-voltage characteristics of a trinistor (usually called a thyristor) are shown in the figure:

The voltage Uoff, at which an avalanche-like increase in current begins, can be reduced by introducing minority charge carriers into any of the layers adjacent to junction 2. The extent to which Uon decreases is shown on the current-voltage characteristic. An important parameter is the unlocking control current Iу.оt, which ensures that the thyristor switches to the open state at voltages lower than the voltage Uon. The figure shows three values of switching voltage UI on< Un вкл < Um вкл соответствует трем значениям управляющего тока UI у.от >Un u.ot > Um u.ot .

Let's consider the simplest circuit with a thyristor loaded onto a resistor load Rн

Ia – anode current (power current in the anode-cathode circuit of the thyristor);
Uak – voltage between anode and cathode;
Iу – control electrode current (in real circuits current pulses are used);
Uuk is the voltage between the control electrode and the cathode;
Upit – supply voltage.

To transfer the thyristor to the open state, the non-control electrode is supplied from the pulse generation circuit with a short-term (on the order of several microseconds) control pulse.

A characteristic feature of the non-lockable thyristor in question, which is very widely used in practice, is that it cannot be turned off using the control current.

To turn off the thyristor in practice, reverse voltage Uac is applied to it< 0 и поддерживают это напряжение в течении времени, большего так называемого времени выключения tвыкл . Оно обычно составляет единицы или десятки микросекунд.

The design and principle of operation of a triac

So-called symmetrical thyristors (triacs, triacs) are widely used. Each triac is similar to a pair of the considered thyristors, connected back-to-back. Symmetrical thyristors are a controlled device with a symmetrical current-voltage characteristic. To obtain a symmetrical characteristic, double-sided p-n-p-n-p semiconductor structures are used.

The structure of the triac, its UGO and current-voltage characteristics are shown in the figure:

A triac (triac) contains two thyristors p1-n1-p2-n2 and p2-n2-p1-n4 connected back-to-back. The triac contains 5 transitions P1-P2-P3-P4-P5. In the absence of a control electron, the UE triac is called a diac.

With positive polarity on electrode E1, a thyristor effect occurs in p1-n1-p2-n2, and with opposite polarity in p2-n1-p1-n4.

When a control voltage is applied to the UE, depending on its polarity and value, the switch voltage Uon changes

Thyristors (dinistors, thyristors, triacs) are the main elements in power electronics devices. There are thyristors for which the switching voltage is greater than 1 kV, and the maximum permissible current is greater than 1 kA

Electronic keys

To increase the efficiency of power electronics devices, the pulsed operating mode of diodes, transistors and thyristors is widely used. The pulse mode is characterized by sudden changes in currents and voltages. In pulse mode, diodes, transistors and thyristors are used as switches.

Using electronic keys, electronic circuits are switched: connecting/disconnecting a circuit to/from sources(s) of electrical energy or signal, connecting or disconnecting circuit elements, changing the parameters of circuit elements, changing the type of the influencing signal source.

UGO ideal keys are shown in the figure:

Keys that operate to open and close, respectively.

The key mode is characterized by two states: “on”/“off”.

Ideal keys are characterized by an instantaneous change in resistance, which can take the value 0 or ∞. The voltage drop across an ideal closed switch is 0. When the switch is open, the current is 0.

Real keys are also characterized by two extreme resistance values Rmax and Rmin. The transition from one resistance value to another in real switches occurs in a finite time. The voltage drop across a real closed switch is not zero.

The switches are divided into keys used in low-power circuits and keys used in high-power circuits. Each of these classes has its own characteristics.

The keys used in low-power circuits are characterized by:

Key resistances in open and closed states;
Performance – the time it takes for a key to transition from one state to another;
Voltage drop on a closed switch and leakage current on an open switch;
Noise immunity – the ability of a key to remain in one of the states when exposed to interference;
The sensitivity of the key is the magnitude of the control signal that transfers the key from one state to another;
Threshold voltage - the value of the control voltage, in the vicinity of which there is a sharp change in the resistance of the electronic key.

Diode electronic keys

The simplest type of electronic keys is diode switches. The diode switch circuit, static transfer characteristic, current-voltage characteristic and the dependence of the differential resistance on the diode voltage are shown in the figure:

The principle of operation of a diode electronic switch is based on changing the value of the differential resistance of a semiconductor diode in the vicinity of the threshold voltage value on the diode Uthr. Figure “c” shows the current-voltage characteristic of a semiconductor diode, which shows the value of Uthr. This value is located at the intersection of the voltage axis with the tangent drawn to the ascending member of the current-voltage characteristic.

Figure "d" shows the dependence of the differential resistance on the voltage across the diode. It follows from the figure that in the vicinity of the threshold voltage of 0.3 V there is a sharp change in the differential resistance of the diode with extreme values of 900 and 35 Ohms (Rmin = 35 Ohms, Rmax = 900 Ohms).

In the “on” state, the diode is open and Uout ≈ Uin.

In the “off” state, the diode is closed and , Uout ≈ Uin · Rн / Rmax<

In order to reduce the switching time, diodes with a low transition capacitance of the order of 0.5-2 pF are used, while providing a turn-off time of the order of 0.5-0.05 μs.

Diode switches do not allow electrical separation of the control and controlled circuits, which is often required in practical circuits.

Transistor switches

The majority of circuits used in computers, remote control devices, automatic control systems, etc. are based on transistor switches.

The switch circuits on the bipolar transistor and the current-voltage characteristics are shown in the figure:

The first state “off” (transistor closed) is determined by point A1 on the output characteristics of the transistor; it is called cutoff mode. In the cutoff mode, the base current Ib = 0, the collector current Ik1 is equal to the initial collector current, and the collector voltage Uk = Uk1 ≈ Ek. The cutoff mode is implemented at Uin = 0 or at negative base potentials. In this state, the switch resistance reaches its maximum value: Rmax = , where RT is the transistor resistance in the closed state, more than 1 MOhm.

The second state “on” (the transistor is open) is determined by point A2 on the current-voltage characteristic and is called the saturation mode. From the cutoff mode (A1) to the saturation mode (A2), the transistor is switched by a positive input voltage Uin. In this case, the voltage Uout takes a minimum value Uk2 = Uk.e.us of the order of 0.2-1.0 V, the collector current Ik2 = Ik.us ≈ Ek / Rk. The base current in saturation mode is determined from the condition: Ib > Ib.us = Ik.us / h21.

The input voltage required to switch the transistor to the open state is determined from the condition: U in > Ib.us · Rb + Uk.e.us

Good noise immunity and low power dissipation in the transistor are explained by the fact that most of the time the transistor is either saturated (A2) or closed (A1), and the transition time from one state to another is a small part of the duration of these states. The switching time of switches on bipolar transistors is determined by the barrier capacitances of the p-n junctions and the processes of accumulation and resorption of minority charge carriers in the base.

To increase speed and input resistance, field-effect transistor switches are used.

Switch circuits on field-effect transistors with a control pn junction and with an induced channel with a common source and common drain are shown in the figure:

For any switch on a field-effect transistor Rн > 10-100 kOhm.

The control signal Uin at the gate is about 10-15 V. The resistance of the field-effect transistor in the closed state is high, about 108 -109 Ohms.

The resistance of the field-effect transistor in the open state can be 7-30 Ohms. The resistance of the field-effect transistor along the control circuit can be 108 -109 Ohms. (circuits “a” and “b”) and 1012 -1014 Ohms (circuits “c” and “d”).

Power (power) semiconductor devices

Power semiconductor devices are used in energy electronics, the most rapidly developing and promising field of technology. They are designed to control currents of tens and hundreds of amperes, voltages of tens and hundreds of volts.

Power semiconductor devices include thyristors (dinistors, thyristors, triacs), transistors (bipolar and field-effect) and statically induced bipolar transistors (IGBT). They are used as electronic keys that switch electronic circuits. They try to bring their characteristics closer to the characteristics of ideal keys.

According to the operating principle, characteristics and parameters, high-power transistors are similar to low-power transistors, but there are certain features.

Power field effect transistors

Currently, the field-effect transistor is one of the most promising power devices. The most widely used transistors are insulated gate and induced channel transistors. To reduce the resistance of the channel, its length is reduced. To increase the drain current, hundreds and thousands of channels are made in the transistor, and the channels are connected in parallel. The probability of self-heating of the field-effect transistor is small, because The channel resistance increases with increasing temperature.

Power field-effect transistors have a vertical structure. Channels can be located both vertically and horizontally.

DMOS transistor

This MOS transistor, manufactured by the double diffusion method, has a horizontal channel. The figure shows a structure element containing a channel.

VMOS transistor

This V-shaped MOS transistor has a vertical channel. The figure shows one structure element containing two channels.

It is easy to see that the structures of a VMOS transistor and a DMIS transistor are similar.

IGBT transistor

IGBT is a hybrid semiconductor device. It combines two methods of controlling electric current, one of which is typical for field-effect transistors (control of the electric field), and the second for bipolar ones (control of the injection of electrical carriers).

Typically, IGBTs use an n-type induced channel MOS transistor structure. The structure of this transistor differs from the structure of a DMIS transistor by an additional layer of p-type semiconductor.

Please note that the terms “emitter”, “collector” and “gate” are commonly used to refer to IGBT electrodes.

Adding a p-type layer results in the formation of a second bipolar transistor structure (pnp type). Thus, IGBT has two bipolar structures - n-p-n type and p-n-p type.

The UGO and the IGBT switch-off circuit are shown in the figure:

A typical type of output characteristics is shown in the figure:

SIT transistor

SIT is a field-effect transistor with a control p-n junction with static induction. It is multi-channel and has a vertical structure. The schematic representation of the SIT and the common source circuit are shown in the figure:

The regions of a p-type semiconductor have the shape of cylinders, the diameter of which is a few micrometers or more. This cylinder system acts as a shutter. Each cylinder is connected to a gate electrode (in figure “a” the gate electrode is not shown).

The dotted line indicates the areas of p-n junctions. The actual number of channels can be thousands. Typically SIT is used in common source circuits.

Each of the devices considered has its own area of application. Thyristor switches are used in devices operating at low frequencies (kilohertz and below). The main disadvantage of such keys is their low performance.

The main area of application of thyristors are low-frequency devices with high switching power up to several megawatts, which do not impose serious performance requirements.

Powerful bipolar transistors are used as high-voltage switches in devices with a switching or conversion frequency in the range of 10-100 kHz, with an output power level from a few W to several kW. The optimal range of switching voltages is 200-2000 V.

Field-effect transistors (MOSFETs) are used as electronic switches for switching low-voltage, high-frequency devices. The optimal values of switching voltages do not exceed 200 V (maximum value up to 1000 V), while the switching frequency can range from a few kHz to 105 kHz. The range of switched currents is 1.5-100 A. The positive properties of this device are controllability by voltage, not current, and less dependence on temperature compared to other devices.

Insulated gate bipolar transistors (IGBTs) are used at frequencies below 20 kHz (some types of devices are used at frequencies above 100 kHz) with switching powers above 1 kW. Switched voltages are not lower than 300-400 V. Optimal values of switched voltages are above 2000 V. IGBT and MOSFET require a voltage of no higher than 12-15 V for full switching on; negative voltage is not required to close the devices. They are characterized by high switching speeds.

Material for preparation for certification