Bipolar transistors: device, principle of operation, switching circuits

The word "transistor" is made up of the words TRANSfer and resISTOR - resistance converter. It replaced lamps in the early 1950s. It is a three-lead device used for amplification and switching in electronic circuits. The adjective "bipolar" (bipolar junction transistor) is used to distinguish it from field effect transistors (FETs). The principle of operation of a bipolar transistor is to use two p-n junctions, forming a barrier layer, which allows a small current to control bOmore current. The bipolar transistor is used both as a controlled resistance and as a switch. There are two types of transistors: pnp and npn.

P-N transition

Germanium (Ge) and silicon (Si) are semiconductors. Silicon is now mainly used. The valences of Si and Ge are four. Therefore, if we add pentavalent arsenic (As) to the crystal lattice of silicon, we get an “extra” electron, and if we add trivalent boron (B), we get a vacant place for an electron. In the first case, one speaks of a “donor” material that gives electrons, in the second case, of an “acceptor” material that accepts electrons. Also, the first type of material is called N (negative), and the second - P (positive).

If materials of P and N types are brought into contact, then a current will arise between them and a dynamic equilibrium with the depletion region, where the concentration of charge carriers - electrons and vacancies (“holes”) - small. This layer is unilaterally conductive and forms the basis of a device called a diode. Direct contact of materials will not create a high-quality transition; fusion (diffusion) or “plugging” into the crystal of dopant ions in a vacuum is necessary.

PNP transistor

For the first time, a bipolar transistor was made by fusing indium drops into a germanium crystal (n-type material). Indium (In) is a trivalent metal, p-type material. Therefore, such a transistor was called diffuse (fusion), having a p-n-p (or pnp) structure. The bipolar transistor in the figure below was manufactured in 1965. Its body is cut off for clarity.

The germanium crystal in the center is called the base, and the indium drops fused into it are called the emitter and collector. You can consider the transitions EB (emitter) and KB (collector) as ordinary diodes, but the transition FE (collector-emitter) has a special property. Therefore, it is not possible to make a bipolar transistor from two separate diodes.

If a voltage of several volts is applied in a pnp transistor between the collector (-) and the emitter (+), a very weak current, a few μA, will flow in the circuit. If you then apply a small (opening) voltage between the base (-) and emitter (+) - for germanium it is about 0.3 V (and for silicon 0.6 V) - then a current of a certain magnitude will flow from the emitter to base. But since the base is made very thin, it will quickly become saturated with holes (it will “lose” its excess of electrons, which will go to the emitter). Since the emitter is heavily doped with hole conduction, and in a lightly doped base, the electron recombination is slightly delayed, then significantly bOMost of the current will go from the emitter to the collector. The collector is made larger than the emitter and is lightly doped, which allows it to have bOhigher breakdown voltage (U_{samples. CE} > U_{samples. EB}). Also, since the main part of the holes recombines in the collector, it heats up more than the rest of the electrodes of the device.

There is a ratio between the collector and emitter currents:

Usually α lies in the range of 0.85-0.999 and inversely depends on the thickness of the base. This value is called the emitter current transfer coefficient. In practice, the reciprocal is more often used (also denoted as h_21e):

This is the base current transfer ratio, one of the most important parameters of a bipolar transistor. He often determines the amplifying properties in practice.

A pnp transistor is called a forward conduction transistor. But there is also another type of transistor, the structure of which perfectly complements pnp in circuitry.

NPN transistor

The bipolar transistor may have an N-type material emitter collector. Then the base is made of P-type material. And in this case, the npn transistor works exactly like the pnp transistor, except for the polarity - it is a reverse conduction transistor.

Silicon-based transistors overwhelm all other types of bipolar transistors. The donor material for the collector and emitter can be As, which has an “extra” electron. The technology of manufacturing transistors has also changed. Now they are planar, which makes it possible to use lithography and make integrated circuits. The picture below shows a planar bipolar transistor (as part of an integrated circuit at high magnification). Both pnp and npn transistors, including powerful ones, are manufactured using planar technology. Rafting has already been discontinued.

Sectional planar bipolar transistor in the following picture (simplified diagram).

The picture shows how well the design of the planar transistor is - the collector is effectively cooled by the crystal substrate. A planar pnp transistor was also manufactured.

The graphic symbols of the bipolar transistor are shown in the following picture.

These UGOs are international and are also valid in accordance with GOST 2.730-73.

Transistor switching circuits

Usually, a bipolar transistor is always used in direct connection - the reverse polarity on the FE junction does not give anything interesting. For a direct connection diagram, there are three connection schemes: a common emitter (OE), a common collector (OK), and a common base (OB). All three inclusions are shown below. They explain only the principle of operation itself - if we assume that the operating point is somehow, with the help of an additional power source or an auxiliary circuit, is established. To open a silicon transistor (Si), it is necessary to have a potential of ~ 0.6 V between the emitter and the base, and ~ 0.3 V is enough for germanium.

Common emitter

The voltage U1 causes the current Ib, the collector current Ic is equal to the base current multiplied by β. In this case, the + E voltage should be large enough: 5V-15V. This circuit amplifies current and voltage well, hence power. The output signal is opposite in phase to the input signal (inverted). This is used in digital technology as a NOT function.

If the transistor does not operate in the key mode, but as an amplifier of small signals (active or linear mode), then by selecting the base current, the voltage U is set₂ equal to E / 2 so that the output signal is not distorted. This application is used, for example, when amplifying audio signals in high-end amplifiers, with low distortion and, as a result, low efficiency.

Common collector

In terms of voltage, the OK circuit does not amplify, here the gain is α ~ 1. Therefore, this circuit is called an emitter follower. The current in the emitter circuit is β + 1 times greater than in the base circuit. This circuit amplifies the current well and has a low output and very high input impedance. (It's time to remember that a transistor is called a resistance transformer.)

The emitter follower has properties and performance characteristics very suitable for oscilloscope probes. It uses its huge input impedance and low output impedance, which is good for matching with a low impedance cable.

Common base

This circuit has the lowest input impedance, but its current gain is α. A common base circuit amplifies well in voltage, but not in power. Its feature is the elimination of the effect of capacitance feedback (eff. Miller). The OB stages are ideal as input stages of amplifiers in RF paths matched at low impedances of 50 and 75 ohms.

Cascades with a common base are very widely used in microwave technology and their application in radio electronics with an emitter follower cascade is very common.

Two main modes of operation

Distinguish between modes of operation using "small" and "large" signal. In the first case, a bipolar transistor operates on a small portion of its characteristics, and this is used in analog technology. In such cases, linear amplification of signals and low noise are important. This is a linear mode.

In the second case (key mode), the bipolar transistor operates in the full range - from saturation to cutoff, like a key. This means that if you look at the I-V characteristic of the p-n junction, you should apply a small reverse between the base and the emitter to completely block the transistor voltage, and for full opening, when the transistor goes into saturation mode, slightly increase the base current, compared to the small-signal mode. Then the transistor works like a pulse switch. This mode is used in switching and power devices, it is used for switching power supplies. In such cases, they try to achieve a short switching time of transistors.

Digital logic is characterized by an intermediate position between “large” and “small” signals. A low logic level is limited to 10% of the supply voltage, and a high logic level is limited to 90%. Time delays and switching tends to be reduced to the limit. This mode of operation is key, but the power is sought to be minimized here. Any logical element is a key.

Other types of transistors

The main types of transistors already described do not limit their design. Composite transistors are produced (Darlington circuit). Their β is very large and is equal to the product of the coefficients of both transistors, therefore they are also called “superbeta” transistors.

Electrical engineering has already mastered the insulated gate bipolar transistor (IGBT), with an insulated gate. The gate of the field-effect transistor is indeed isolated from its channel. True, there is a question of recharging its input capacitance during switching, so it cannot do without current.

Such transistors are used in powerful power switches: pulse converters, inverters, etc. At the input, IGBTs are very sensitive, due to the high resistance of the gates of the field-effect transistors. On the way out - they make it possible to receive huge currents and can be manufactured for high voltage. For example, in the United States there is a new solar power plant, where such transistors in a bridge circuit are loaded onto powerful transformers that give energy to the industrial grid.

In conclusion, we note that transistors, in simple terms, are the "workhorse" of all modern electronics. They are used everywhere: from electric locomotives to mobile phones. Any modern computer consists of practically only transistors. The physical foundations of transistor operation are well understood and promise many more new advances.

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