Bipolar transistors. For dummies

Foreword

Since the topic of transistors is very, very extensive, there will be two articles devoted to them: separately on bipolar and separately on field-effect transistors.

The transistor, like a diode, is based on the pn junction phenomenon. Those who wish can refresh the memory of the physics of the processes taking place in it here or here .

The necessary explanations are given, we pass to the point.

Transistors Definition and History

A transistor is an electronic semiconductor device in which the current in a circuit of two electrodes is controlled by a third electrode. (tranzistors.ru)

Field transistors were first invented (1928), and bipolar appeared in 1947 at the Bell Labs laboratory. And it was, without exaggeration, a revolution in electronics.

Very quickly transistors replaced vacuum tubes in various electronic devices. In this regard, the reliability of such devices has increased and their size has greatly decreased. And to this day, no matter how “sophisticated” the microcircuit is, it still contains a lot of transistors (as well as diodes, capacitors, resistors, etc.). Only very small.

By the way, initially “transistors” called resistors, the resistance of which could be changed using the magnitude of the applied voltage. If we distract from the physics of processes, then a modern transistor can also be represented as a resistance, depending on the signal supplied to it.

What is the difference between field effect and bipolar transistors? The answer lies in their names themselves. In a bipolar transistor, both electrons and holes participate in charge transfer (bis — twice). And in the field (aka unipolar) - either electrons or holes.

Also, these types of transistors vary in applications. Bipolar are used mainly in analog technology, and field ones - in digital.

And finally:The main field of application of any transistors is amplification of a weak signal due to an additional power source.

Bipolar transistor. Principle of operation. Main characteristics

The bipolar transistor consists of three areas: emitter, base and collector, each of which is supplied with voltage. Depending on the type of conductivity of these areas, npn and pnp transistors are distinguished. Typically, the collector area is wider than the emitter. The base is made of a lightly doped semiconductor (because of which it has great resistance) and is made very thin. Since the emitter-base contact area is much smaller than the base-collector contact area, it is not possible to interchange the emitter and collector by changing the polarity of the connection. Thus, the transistor refers to asymmetric devices.

Before considering the physics of the operation of the transistor, we outline the general problem.

It consists in the following: a strong current flows between the emitter and the collector ( collector current), and between the emitter and the base there is a weak control current ( base current ). The collector current will vary with the base current. Why?
Consider the pn junctions of a transistor. There are two of them: emitter-base (EB) and collector-base (BC). In the active mode of the transistor, the first one is connected with direct, and the second with reverse bias. What happens at the pn junctions? For more definiteness, we will consider an npn transistor. For pnp, everything is similar, only the word "electrons" needs to be replaced by "holes".

Since the transition of the EB is open, the electrons easily “run over” to the base. There they partially recombine with holes, but used forMost of them, due to the small thickness of the base and its weak alloying, manage to run to the base-collector transition. Which, as we recall, is included with reverse bias. And since in the base electrons are minority charge carriers, the electric field of the transition helps them overcome it. Thus, the collector current is only slightly less than the emitter current. Now watch your hands. If you increase the base current, the EB transition will open more strongly, and more electrons can slip between the emitter and the collector. And since the collector current is initially greater than the base current, this change will be very, very noticeable. Thus, amplification of the weak signal arriving at the base will occur . Once again: a strong change in collector current is a proportional reflection of a weak change in base current.

I remember that my classmate explained the principle of operation of a bipolar transistor using an example of a water tap. The water in it is the collector current, and the control current of the base is how much we turn the knob. A little effort (control action) is enough to increase the flow of water from the tap.

In addition to the processes considered, a number of phenomena can occur at the pn junctions of the transistor. For example, with a strong increase in voltage at the base-collector junction, avalanche multiplication of the charge due to impact ionization can begin. And coupled with the tunneling effect, this will give first electric, and then (with increasing current) and thermal breakdown. However, thermal breakdown in the transistor can occur without electrical (i.e., without increasing the collector voltage to breakdown). For this, one excessive current through the collector will suffice.

Another phenomenon is due to the fact that when the voltages at the collector and emitter junctions change, their thickness changes. And if the base is too thin, then a closing effect (the so-called “puncture” of the base) may occur - the connection of the collector junction with the emitter junction. In this case, the base area disappears, and the transistor stops working normally.

The collector current of the transistor in the normal active mode of the transistor is more than the base current a certain number of times. This number is called the current gain and is one of the main parameters of the transistor. It is designated h21 . If the transistor turns on without a load on the collector, then at a constant voltage the collector-emitter, the ratio of the collector current to the base current will givestatic current gain . It can be tens or hundreds of units, but it is worth considering the fact that in real schemes this coefficient is lower due to the fact that when the load is turned on, the collector current naturally decreases.

The second important parameter is the input resistance of the transistor . According to Ohm's law, it is the ratio of the voltage between the base and the emitter to the control current of the base. The larger it is, the lower the base current and the higher the gain.

The third parameter of a bipolar transistor is the voltage gain. It is equal to the ratio of the amplitude or current values ​​of the output (emitter-collector) and input (base-emitter) variable voltages. Since the first value is usually very large (units and tens of volts), and the second is very small (tenths of a volt), this coefficient can reach tens of thousands of units. It is worth noting that each base control signal has its own voltage gain.

Also, transistors have a frequency response, which characterizes the ability of a transistor to amplify a signal whose frequency approaches the boundary frequency of amplification. The fact is that with increasing frequency of the input signal, the gain decreases. This is due to the fact that the time course of the main physical processes (the time of carrier transfer from the emitter to the collector, the charge and discharge of the barrier capacitive transitions) becomes comparable with the period of the input signal. Those. the transistor simply does not have time to respond to changes in the input signal and at some point simply ceases to amplify it. The frequency at which this occurs is called the boundary .

Also, the parameters of a bipolar transistor are:

• reverse collector-emitter current
• on time
• collector reverse current
• maximum current

Symbols of npn and pnp transistors differ only in the direction of the arrow designating the emitter. It shows how current flows in a given transistor.

Bipolar Transistor Modes

The option considered above is the normal active mode of operation of the transistor. However, there are several more open / closed combinations of pn junctions, each of which represents a separate transistor operation mode.

1. Inverse active mode . The BC transition is open here, and the EB is closed. The amplifying properties in this mode, of course, have nowhere worse, therefore, transistors in this mode are used very rarely.
2. Saturation mode . Both transitions are open. Accordingly, the main charge carriers of the collector and emitter “run” to the base, where they actively recombine with its main carriers. Due to the arising redundancy of charge carriers, the resistance of the base and pn junctions decreases. Therefore, a circuit containing a transistor in saturation mode can be considered short-circuited, and this radio element itself can be represented as an equipotential point.
3. Cutoff mode . Both transistor junctions are closed, i.e. the current of the main charge carriers between the emitter and the collector ceases. The flows of minority charge carriers create only small and uncontrolled thermal currents of transitions. Due to the poverty of the base and transitions by charge carriers, their resistance increases significantly. Therefore, it is often believed that a transistor operating in a cutoff mode is an open circuit.
4. Barrier mode In this mode, the base is closed directly or through low resistance to the collector. Also in the collector or emitter circuit include a resistor that sets the current through the transistor. Thus, an equivalent diode circuit with a series resistance is obtained. This mode is very useful, because it allows the circuit to work at almost any frequency, in a large temperature range and is undemanding to the parameters of transistors.

Bipolar Transistor Switching Circuits

Since the transistor has three contacts, in the general case it is necessary to supply power to it from two sources, which together have four outputs. Therefore, one of the contacts of the transistor must be supplied with voltage of the same sign from both sources. And depending on what kind of contact this is, there are three schemes for switching on bipolar transistors: with a common emitter (OE), a common collector (OK) and a common base (OB). Each of them has both advantages and disadvantages. The choice between them is made depending on which parameters are important to us and which can be compromised.

Common emitter switching circuit

This circuit gives the greatest gain in voltage and current (and hence in power - up to tens of thousands of units), and therefore is the most common. Here, the emitter-base transition is switched on directly, and the base-collector transition is switched back on. And since the voltage of the same sign is applied to both the base and the collector, the circuit can be powered from a single source. In this circuit, the phase of the output AC voltage changes 180 degrees relative to the phase of the input AC voltage.

But to all the goodies, the OE scheme has a significant drawback. It lies in the fact that the increase in frequency and temperature leads to a significant deterioration in the amplifying properties of the transistor. Thus, if the transistor should work at high frequencies, then it is better to use a different switching circuit. For example, with a common base.

Common base switching circuit

This circuit does not give significant signal amplification, but it is good at high frequencies, since it allows you to more fully use the frequency response of the transistor. If you turn on the same transistor first according to the scheme with a common emitter, and then with a common base, then in the second case there will be a significant increase in its limiting gain frequency. Since with this connection the input impedance is low and the output impedance is not very large, the transistor cascades assembled according to the circuit with OB are used in antenna amplifiers, where the wave impedance of the cables usually does not exceed 100 Ohms.

In a circuit with a common base, the phase of the signal is not inverted, and the noise level at high frequencies is reduced. But, as already mentioned, her current gain is always slightly less than unity. True, the voltage gain here is the same as in the circuit with a common emitter. The disadvantages of a common base scheme include the need to use two power supplies.

Common collector switching circuit

The peculiarity of this circuit is that the input voltage is completely transmitted back to the input, i.e., negative feedback is very strong.

Let me remind you that a feedback is called negative in which the output signal is fed back to the input, which reduces the level of the input signal. In this way, an automatic correction occurs when the input signal parameters are randomly changed.

The current gain is almost the same as in the common emitter circuit. But the voltage gain is small (the main drawback of this circuit). He approaches unity, but always less than it. Thus, the power gain is only a few tens of units.

In a common collector circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, the output voltage in phase and amplitude coincides with the input, i.e., repeats it. That is why such a circuit is called an emitter follower. Emitter - because the output voltage is removed from the emitter relative to the common wire.

This inclusion is used to match transistor stages or when the input source has a high input impedance (for example, a piezoelectric pickup or a condenser microphone).

Two words about cascades

It happens that you need to increase the output power (i.e. increase the collector current). In this case, parallel connection of the required number of transistors is used.

Naturally, they should be approximately the same in characteristics. But it must be remembered that the maximum total collector current should not exceed 1.6-1.7 of the maximum collector current of any of the transistors in the cascade.
However (thanks wrewolfper remark), in the case of bipolar transistors, this is not recommended. Because two transistors of even one type rating are at least a little, but they differ from each other. Accordingly, when connected in parallel, currents of different sizes will flow through them. To equalize these currents, balanced resistors are placed in the emitter circuits of the transistors. The value of their resistance is calculated so that the voltage drop across them in the range of operating currents is at least 0.7 V. It is clear that this leads to a significant deterioration in the efficiency of the circuit.

There may also be a need for a transistor with good sensitivity and at the same time with a good gain. In such cases, a cascade is used from a sensitive but low-power transistor (VT1 in the figure), which controls the power energy of a more powerful counterpart (VT2 in the figure).

Other applications of bipolar transistors

Transistors can be used not only in signal amplification schemes. For example, due to the fact that they can work in saturation and cutoff modes, they are used as electronic keys. It is also possible to use transistors in signal generator circuits. If they work in the key mode, a rectangular signal will be generated, and if in the amplification mode, then a signal of arbitrary shape, depending on the control action.

Marking

Since the article has already grown to an indecently large volume, then in this paragraph I will simply give two good links that detail the basic marking systems for semiconductor devices (including transistors) in detail: http://kazus.ru/guide/transistors/mark_all .html and .xls file (35 kb) .

List of sources:
http://ru.wikipedia.org
http://www.physics.ru
http://radiocon-net.narod.ru
http://radio.cybernet.name
http://dvo.sut.ru

Useful comments:
http://habrahabr.ru/blogs/easyelectronics/133136/#comment_4419173