Wednesday, December 12, 2012

Bipolar Transistors

Bipolar Transistors

The transistor is a semiconductor device that can either amplify an electrical signal or act as an electronic switch. Basically a transistor consists of a germanium or silicon crystal which contains three separate regions. The three regions may consist of either two p-type regions separated by an n-type region (Fig .l(a(( or two n-type regions separated by a p-type region (Fig. 1(b)). The first type of transistor is known as a p-n-p transistor and the second type as an n-p-n transistor. Both types of transistor are employed, sometimes together in the same circuit, but the discussion throughout this chapter will be in terms of the n-p-n transistor since it is the more commonly employed; However, for the corresponding operation of a p-n-p transistor it is merely necessary to read electron for hole, hole for electron, negative for positive, and positive for negative.

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The middle of the three regions in a transistor is known as the base and the two outer regions are known as the emitter and the collector. In most transistors the collector region is made physically larger than the emitter region because it will be expected to dissipate a greater power. The symbol for a p-n-p transistor is given in Fig . 2(a) and the symbol for an n-p-n transistor in Fig . 2(b). Note that the emitter lead arrowhead is pointing in different directions in the two figures, pointing inwards for the p-n-p transistor and outwards for the n-p-n transistor. The arrowhead indicates the direction in which holes travel in the emitter.

Both p-n-p and n-p-n transistors are generally classified into one of the following groups:

(a) small-signal low-frequency

(b) low-power and medium-power low-frequency

(c) high-power low-frequency

(d) small-signal high-frequency

(e) medium- and high-power high-frequency

(f) switching.

The majority of the transistors listed in manufacturers' and distrib­utors' catalogues are p-n-p silicon types.

An n-p-n transistor contains two p-n junctions and is normally operated so that one junction, the emmiter-base junction, is forward-biased and the other, the collector-base junction, is reverse-biased. This is shown in Fig. 3 together with the directions of the various charge carriers and currents in the transistor. The usual convention whereby the direction of current flow is opposite to the direction of electron movement has been ernployed . d.c. values of current and voltage are indicated by CAPITAL subscripts; a.c. values by lower-case subscripts .

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Consider that, initially, the emitter-base bias voltage VEB zero. Then the majority charge carrier current crossing the emitter-base junction is equal to the minority charge carrier current that is flowing in the opposite direction and the net junction current is zero. The collector-base junction is reverse-biased by the bias voltage VCB and so a small minority charge carrier current flows in the collector lead . This current is the reverse saturation current discussed in the previous chapter but now it is known as the collector leakage current and is given the symbol ICBO .

If the emitter-base bias voltage is increased in the negative direction by a few tenths of a volt, the emitter-base junction will be forward-biased and then a majority charge carrier current flows. This current consists of electrons travelling from the emitter to the base and holes passing from the case to the emitter. Only the electron current is useful to the action of the transistor, as will soon be evident, and it is therefore made much larger than the hole current by doping die emitter more heavily than the base. The ratio of the electron current to the total emitter current consists of holes passing from the base to the emitter .

Immediately the electrons cross the emitter-base junction, and are said to have been emitted or injected into the base, they become minority charge carriers and start to diffuse across the base towards the collector-base junction. Because the base is fairly narrow and is also lightly doped, most of the emitted electrons reach the collector

base junction and do not recombine with a free hole on the way. On reaching the junction, the emitted electrons augment the minority , charge carrier current crossing the junction and cause an increase in the collector current. The ratio of the number of electrons arriving at the collector to the number of emitted electrons is known as the base transmission factor, symbol β . Typically β = 0.995.

(1) The collector current is less than the , emitter current because (a ) part of the emitter current consists of holes that do not contribute to the collector current and (b) not all of the electrons injected into the base are successful reaching the collector . Factor(a) is represented by the emitter injection ratio and factor (b) by the base transmission factor hence the ratio of collector current to emitter current is equal to βɤ. Substituting the typical values quoted for ɤ and β shows that, typically, the collector current is about 0.99 times the emitter current.

(2) The base current is small and has three components: (a ) a current entering the base to replace the holes lost by recombination with the diffusing electrons, (b) the majority charge carrier hole current flowing from base to emitter and (c) the collector leakage current ICBO. The first two of these components are currents that flow into flows out of the base , and so the total base current flows into the base. The total current flowing into the transistor must be equal to the total current flowing out of it and hence the emitter current IE is equal to the sum of the collector and base currents, Ic and 1B respectively, that is

Ie = Ic + Ib (1.1)

Typically, IC is equal to 0.99 IE so that IB is equal to 0.01 IE.

(3) If the emitter current is varied by some means, the number of electrons arriving at the collector, and hence the collector current. will vary accordingly. The magnitude of the collector-base voltage Vcb has relatively little effect on the collector current as will be seen shortly. Control of the output (collector) current can thus be obtained by means of the input (emitter) current and this, in turn, can be controlled by variation of the bias voltage applied to the emitter-base junction. An increase in the forward-bias voltage of the potential barrier and allows an increased emitter current to flow ؛ conversely, a decrease in the forward-bias voltage reduces the emitter current .

(4) The ratio of the output current of a transistor to its input current in the absence of an a.c. signal is known as the d.c. current gain of the transistor. The output current is the collector current input current is the emitter current IE. Thus,

d.c. current gain. -hFB = IC/IE (1.2)

The minus sign indicates that the input and output, currents are flowing in opposite directions. By convention, a current flowing into a transistor is taken to be positive and current flowing out is taken to be negative. Since the operation of the transistor depends on the movement of both holes and electrons , the device is known as a bipolar transistor .

(5) A transistor may be connected in a circuit in one of three ways and in each case one of its three terminals is common to both input and output. The connection is then described in terms of the common terminal; for example, the common-emitter connection has the emitter common to both input and output, the input signal is fed between the base and the emitter, and the output signal is developed between the collector and the emitter. In all connections, the base-emitter junction is always forward-biased and the collector-base junction is always reverse-biased.