Kypshakov Sartay

BIPOLAR TRANSISTORS

Introduction In American scientists J. Bardeen and V. Brattein created a semiconductor triode, or a transistor. This event was of tremendous importance for the development of semiconductor electronics. Transistors can operate at much lower voltages than tube transistors, and they are not simple substitutes for the latter: they can be used not only to amplify and generate alternating current, but also as key elements. The definition of "bipolar" indicates that the work of the transistor is associated with processes in which two types of charge carriers (electrons and holes) participate. The word "transistor" comes from the English word combination "transfer resistor" - a resistance converter. The appearance of the first samples of bipolar transistors stimulated research and development work on the creation of various types of such devices in many countries of the world, including in the Soviet Union, whose scientists made a significant contribution to the development of this problem. At present, the bipolar transistor is one of the most important semiconductor devices. It is used in radio electronics as a discrete active element, and in planar design is the basis for creating integrated solid-state circuits. In turn, solid-state circuits are the main elements of the modern generation of computers and other complex radioelectronic devices

General information Bipolar transistor is a semiconductor device consisting of three regions with alternating types of electrical conductivity, suitable for power amplification. These regions are separated by electron-hole transitions (e-d transitions). The peculiarity of the transistor is that between its e-transitions there is interaction - the current of one of the electrodes can control the current of the other. Such control is possible, because charge carriers injected through one of the e-d junctions can be up to another transition under reverse voltage and change its current. Each of the transistor transitions can be turned on either in the forward or reverse direction. Depending on this, three modes of operation of the transistor are distinguished:

Principle of operation of the transistor When the switch is open, there is no current in the emitter circuit (hereinafter E). In this case, there is a small current in the collector circuit (K), called the reverse current K and denoted by Ikb. This current is very small, since with a reverse bias of the K transition, the potential barrier is large and insurmountable for the main carriers-reservoir holes and free base electrons. To doped with an impurity is much stronger than the base. Because of this, the minority carriers in the collector are much smaller than in the base, and the reverse current K is produced mainly by minority carriers: holes generated in the base as a result of thermal vibrations and electrons generated in K.

For the p-n-p transistor under consideration, a negative voltage K-B is assumed to be plotted to the right along the axis of the abscissa. The output characteristics corresponding to the negative values of the K-B voltage in the upper right quadrant go almost horizontally, but with a slight rise. To explain this we consider the potential diagram of a transistor (color is the depleted layers): Since E and K are more doped with an impurity than the base, the depleted layers are concentrated mainly in the base. The effective thickness of the base is Wf, that is, the distance between the boundaries of the depleted layers is less than the thickness of the base W. An increase in the negative voltage on the collector widens the depleted layer of the collector junction and, therefore, causes a decrease in the effective thickness of the base. This phenomenon is called the Earley effect. The modulation of the thickness of the base explains a certain increase in the output characteristics with increasing negative voltage KB. The collector current thus increases, since a smaller part of the holes is lost in the base due to recombination with electrons.

The principle of the transistor as an amplifier A transistor is a semiconductor device having two pn junctions located in a single semiconductor single crystal at a distance much smaller than the diffusion length of the minority charge carriers. In Fig. 1 shows the inclusion of a transistor of the p-pn type in a circuit with a common base. The left pn junction is called the emitter junction, and its p-region is called the emitter. The right pn junction is called the collector junction, and its p-region is called the collector. The n-region between the emitter and the collector is called the base of the transistor. A transistor with an n-type emitter and a p-type base is called an n-p-n type transistor. When a p-n-p-type transistor operates in the amplification mode, the emitter junction is turned on in the transmission direction and injects the holes into the base, from where they enter the reverse-biased collector junction circuit. Since the thickness of the base of the transistor W is much smaller than the diffusion length of the holes Lp, the concentration of holes injected by the emitter during flight through the base remains almost unchanged. Thus, the strength of the hole current in the collector circuit Irk is approximately equal to the strength of the hole current in the emitter circuit Ip. The saturation current of the collector junction is small, and it can be neglected in the first approximation in comparison with Ipk. Since the collector junction is biased in the opposite direction, its resistance is large, which makes it possible to include a large load resistance Rn in the collector circuit without an appreciable change in the collector current. In this case, of course, RH should be significantly less than the collector resistance. In connection with the above circumstances, a relatively small change in the voltage drop across the emitter junction, whose resistance is small, will cause a large change in the voltage drop across the load resistance (V = Ipk * Rn) with an almost identical change in the current in the emitter and collector circuits. As a result of a sharp difference in the input and output resistances, the transistor performs a power amplification.

Energy diagram of the transistor and distribution of carrier concentration The energy diagram of the transistor can be constructed on the basis of the energy diagram of the p-n structure, each transition having its potential barrier preventing the transition of the main carriers to the neighboring region : The state of the transistor, in which there is no voltage at the p-n junction between the emitter and the base (E-B), is called the equilibrium state (Fig. A). In the equilibrium state, both transitions establish a dynamic equilibrium between the fluxes of holes and electrons flowing in both directions. Each p-n junction of a transistor can be considered separately, provided that the distance between junctions is much larger than the diffusion length of the minority carriers in the middle region. Because of the presence of potential barriers in the equilibrium state, a "potential well" is formed at the pn junctions, from which only electrons that have a sufficient thermal energy can escape, and in the equilibrium state, a dynamic equilibrium between the electron fluxes is established in both transitions. An analogous equilibrium is established between the fluxes of holes that are on the "potential ridges" and freely move to adjacent layers. In the equilibrium state, the resulting currents through the two chambers are zero.

In the alloy transistor the base is homogeneous, therefore the mechanism of carrier displacement has a diffusion character and such transistors are called diffusion ones. In the operating mode, the transistor transitions are fed with constant voltages Ueb and Ukb, which are generated by sources of emf. Ee and Ek in the emitter and collector circuits:

Токи в транзисторе As a result of reducing the potential barrier on the emitter junction from the emitter to the base, the diffusion motion of the main carriers begins. Since the holes (electrons) in the emitter (base) are much larger than in the base (emitter), the injection coefficient is very high. The conduction of holes in the base increases The volumetric positive charge that appeared near the emitter junction is nearly instantly compensated by the charge of the electrons entering the base from the Ue source. The Emitter-Base current loop is closed. Electrons striving to the base create a volume negative charge near the emitter junction. A region of increased concentration of holes and electrons forms near the junction. They begin to diffuse towards the collector. Since the base is narrow, holes (minority carriers) do not have time to recombine and, falling into the accelerating field of the collector Transition, are drawn into the collector. This process is called extraction. Electrons, the number of which is equal to the number of holes that leave the collector, rush to the basic conclusion. The collector-base circuit is closed.

Ie = Ik + Ib Ie is the current in the emitter circuit, Iк - current in the collector circuit, Ib - current at the base terminal. In the active mode, a direct voltage is applied to the emitter and a current flows through the junction Ie = Ier + Ien + Ier, Where Ier is the current of injection of holes from the emitter into the base, Ien is the electron injection current from the base to the emitter, I er is the recombination current in the emitter junction. Iob = Io + Ig + Iy, Where Io is the thermal current, Ig is the generation current, and Iy is the leakage current. Ic = Ier - Ibr.

Static characteristics and current transfer coefficient in various switching circuits. Scheme with a common base When the transistor is switched on according to the scheme with a common base (Fig. 1), the input current is the emitter current, and the output one is the collector. The current transfer coefficient in this case is determined by the formula (1): A = (dIk / dIe) Vkb = const = g * b * Mk (1) The family of input static characteristics, that is, the dependence of Ie on Veb for fixed Vkb, is described by the expression (2): J = jps / sh (W / Lp) * {[exp (q * veb / k * T) -1] * ch (W / Lp) - [exp (q * Vkb / k * T) -1] + Jns * [exp (qVeb / k * T) - 1]. (2)

If Vkb = 0, then Ie ~ [exp (q * Veb / k * T) - 1] (Fig. 2, curve 1). For Vkb 2.3k * T / q, but then p (W) = -pn, and p = pn. Thus, in the situation under consideration, there is a gradient of hole concentration in the base of the transistor and Ie is not equal to 0. To compensate for this current, it is necessary to apply a bias in the stopping direction to the emitter junction (Fig. 2, curve 2). The family of output characteristics (the dependence of Ik on Vk for fixed values of Ie) is described by formula (3): Jk = jps / sh (W / Lp) * {[exp (q * Veb / k * T) -1] - - [exp (q * Vkb / k * T) -1] * ch (W / Lp)} - jns * [exp (q * Vkb / k * T) - 1] (3) In the case where Ie = 0 and veb = 0, and Vkb <0, the output characteristic is similar to the volt- ampere characteristic of the reverse-shifted pn junction, that is, Ik0 = - [Ipsk * cth (W / Lp) + Insk] [exp (-q * | Vkb | / k * T) -1], (4)

Where Ipsk = jps * Sk, Insk = jns * Sk, and Sk is the area of the collector junction. In Fig. 3 this curve corresponds to curve 1. Since se >> snb, then jps >> jns and the last term in (2) can be neglected. In addition, usually exp (q * Veb / k * * T) - 1 >> exp (-q * | Vkb | / k * T) - 1. Taking these circumstances into account, it follows from (2) that Ie Ips * cth (W / Lp) * [exp (q * | Ve | / k * T) -1], where Ips = jps * Se, and Se is the area of the emitter junction. Then, assuming that Sk = Se, we can rewrite (3) in the form: Ik ~ a0 * Ie + Ik0. (5) Here it is assumed that g0 = 1 and a0 = sch (W / Lp).

Common emitter circuit In practice, rather often used transistors, included in the scheme with a common emitter (Figure 4). In this scheme, the input current is the base current, and the output current, as in the previous case, is the collector current. In accordance with the definition of the current transfer coefficient for a circuit with a common emitter, we have h21e * B0 = dIk / dIb, but Ib = Ie * Ik, and therefore, B0 = dIk / (dIe - dIk) = a0 / (1 - a0). (6)

The scheme with a common collector In this circuit of inclusion, just like in the previous case, the base current is the control (or input), but the emitter current plays the role of the output (Fig. 7). Current transmission ratio B0 * = dIe / dIb = 1 / (1 - a0). (9)