2. TRANSISTOR CHARACTERISTICS AND BIASING

2.1. Bipolar Junction Transistor

The bipolar junction transistor consists of two back-to-back P-N junctions manufactured in a single piece of a semiconductor crystal as shown in figure (a) below. That is, the BJT is constructed with three doped semiconductor regions separated by two pn junctions. These two junctions give rise to three regions called emitter, base and collector. Physical representations of the two types of BJTs are shown in Figures (b) and (c). One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp). The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure.

 

The pn junction joining the base region and the emitter region is called the base-emitter junction. The pn junction joining the base region and the collector region is called the base-collector junction as shown.

The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions. The emitter is heavily doped because its main function is to supply majority charge carries (either electrons or holes) to the base. The main function of the collector (as indicated by its name) is to collect majority charge carriers coming from the emitter and passing through the base. The standard schematic symbols for the npn and pnp bipolar junction transistors are shown below.

 

The arrowhead is always at the emitter (not at the collector) and in each case, its direction indicates the conventional direction of current flow. For a PNP transistor, arrowhead points from emitter to base meaning that emitter is positive with respect to base (and also with respect to collector) For NPN transistor, it points from base to emitter meaning that base (and collector as well) is positive with respect to the emitter. Note that in a transistor, for normal operation, the collector and base have the same polarity with respect to the emitter. 

In most transistors, collector region is made physically larger than the emitter region because it has to dissipate much greater power. Because of this difference, there is no possibility of inverting the transistor i.e. making its collector the emitter and its emitter the collector.

1.1.1 Basic BJT Operation

In order for a BJT to operate properly as an amplifier, the two pn junctions must be correctly biased with external dc voltages.

1.1.1.1  Transistor Biasing

For proper working of a transistor, it is essential to apply voltages of correct polarity across its two junctions. It is worthwhile to remember that for normal operation; emitter-base junction is always forward biased and collector-base junction is always reverse- biased. This type of biasing is known as forward-reverse biasing (One p–n junction of a transistor is reverse-biased, whereas the other is forward-biased.).

In the figure below, two batteries respectively provide the dc emitter supply voltage VEE and collector supply voltage VCC for properly biasing the two junctions of the transistor.

 

or

 

NB: For a PNP transistor, both collector and base are negative with respect to the emitter (the letter N of Negative being the same as the middle letter of PNP). Of course, collector is more negative than base. Similarly, for NPN transistor, both collector and base are positive with respect to the emitter (the letter P of Positive being the same as the middle letter of NPN). Again, collector is more positive than the base. All these are shown below.

 

It may be noted that different potentials have been designated by double subscripts. The first subscript always represents the point or terminal which is more positive (or less negative) than the point or terminal represented by the second subscript. For example, in Fig (a) above, the potential difference between emitter and base is written as VEB (and not VBE) because the emitter is positive with respect to the base. Now, between the base and collector themselves, collector is more negative than base. Hence, their potential difference is written as VBC and not as VCB. Single subscripted voltages such as VB, VC, and VE are dc voltages from the transistor terminals to ground while IB, IC, and IE are the dc transistor currents. Similarly, single subscripted voltages such as Vb, Vc, and Ve are ac voltages from the transistor terminals to ground.

1.1.1.2  Transistor action

For a silicon p-n-p transistor, biased as shown in Figure 12.2(a) below, if the base-emitter junction is considered on its own, it is forward biased and a current flows. This is depicted in Figure 12.3(a). For example, if RE is 1000Ω, the battery is 4.5V and the voltage drop across the junction is taken as 0.7 V, the current flowing is given by (4.5 - 0.7) V /1000 Ω = 3.8 mA.

 

 

When the base-collector junction is considered on its own, as shown in Figure 12.3(b), it is reverse biased and the collector current is something less than 1 µA. However, when both external circuits are connected to the transistor, most of the 3.8 mA of current flowing in the emitter, which previously flowed from the base connection, now flows out through the collector connection due to transistor action.

In a p-n-p transistor, connected as shown in Figure 12.2(a), transistor action is accounted for as follows:

a). The majority carriers in the emitter p-type material are holes

b). The base-emitter junction is forward biased to the majority carriers and the holes cross the junction from the emitter region into the base region.

c). The base region is very thin and is only lightly doped with electrons so although some electron-hole pairs (electron-hole combinations) are formed, many holes are left in the base region

d). The base-collector junction is reverse biased to electrons in the base region and holes in the collector region, but forward biased to holes in the base region; these holes are attracted by the negative potential at the collector terminal.

e). A large proportion of the holes in the base region cross the base-collector junction into the collector region, creating a collector current; conventional current flow is in the direction of hole movement.

The transistor action is shown diagrammatically below. For transistors having very thin base regions, up to 99.5% of the holes leaving the emitter cross the base collector junction. 

 

In an n-p-n transistor, connected as shown in Figure 12.2(b), transistor action is accounted for as follows:

a). The majority carriers in the n-type emitter material are electrons.

b). The base-emitter junction is forward biased to these majority carriers and electrons cross the junction and appear in the base region.

c). The base-emitter junction is forward biased to these majority carriers and electrons cross the junction and appear in the base region.

d). The base-collector junction is reverse biased to holes in the base region and electrons in the collector region, but is forward biased to electrons in the base region; these electrons are attracted by the positive potential at the collector terminal.

e). A large proportion of the electrons in the base region cross the base collector junction into the collector region, creating a collector current.

The transistor action is shown diagrammatically in Figure 12.5. Conventional current flow is taken to be in the direction of hole flow, that is, in the opposite direction to electron flow, hence the directions of the conventional current flow are as shown in Figure 12.5.

 

For a p-n-p transistor, the base-collector junction is reverse biased for majority carriers. However, a small leakage current, ICBO flows from the base to the collector due to thermally generated minority carriers (electrons in the collector and holes in the base), being present.  The base-collector junction is forward biased to these minority carriers. If a proportion, α, (having a value of up to 0.995 in modern transistors), of the holes passing into the base from the emitter, pass through the base-collector junction, then the various currents flowing in a p-n-p transistor are as shown in Figure 12.6(a).

 

Similarly, for an n-p-n transistor, the base-collector junction is reversed biased for majority carriers, but a small leakage current, ICBO flows from the collector to the base due to thermally generated minority carriers (holes in the collector and electrons in the base), being present. The base-collector junction is forward biased to these minority carriers. If a proportion, α, of the electrons passing through the base-emitter junction also pass through the base-collector junction then the currents flowing in an n-p-n transistor are as shown in Figure 12.6(b).

For the transistor as depicted in Figure 12.4, the emitter is relatively heavily doped with acceptor atoms (holes). When the emitter terminal is made sufficiently positive with respect to the base, the base-emitter junction is forward biased to the majority carriers. The majority carriers are holes in the emitter and these drift from the emitter to the base. The base region is relatively lightly doped with donor atoms (electrons) and although some electron-hole recombination’s take place, perhaps 0.5%, most of the holes entering the base, do not combine with electrons.

The base-collector junction is reverse biased to electrons in the base region, but forward biased to holes in the base region. Since the base is very thin and now is packed with holes, these holes pass the base-emitter junction towards the negative potential of the collector terminal. The control of current from emitter to collector is largely independent of the collector-base voltage and almost wholly governed by the emitter-base voltage. The essence of transistor action is this current control by means of the base-emitter voltage.

In a p-n-p transistor, holes in the emitter and collector regions are majority carriers, but are minority carriers in the base region. Also thermally generated electrons in the emitter and collector regions are minority carriers as are holes the base region. However, both majority and minority carriers contribute towards the total current flow (see Figure 12.6(a)). It is because a transistor makes use of both types of charge carriers (holes and electrons) that they are called bipolar. The transistor also comprises two p-n junctions and for this reason it is a junction transistor. Hence the name - bipolar junction transistor.

1.1.1.3  Transistor Currents

Consider the figures shown below:

 

Applying Kirchhoff's Current Law to the above figures, we have

 

In general, a small part (about 1 - 2%) of emitter current goes to supply base current and the remaining major part (98 - 99%) goes to supply collector current. This statement is true regardless of transistor type or transistor configuration.

 

1.1.2 Transistor Circuit Configurations

There are three types of circuit connections (called configurations) for operating a transistor:

a) Common-base (CB).

b) Common-emitter (CE).

c) Common-collector (CC).

The term ‘common’ is used to denote the electrode that is common to the input and output circuits. Because the common electrode is generally grounded, these modes of operation are frequently referred to as grounded-base, grounded-emitter and grounded-collector configurations as shown below for a PNP – transistor. Since a transistor is a 3-terminal (and not a 4-terminal) device, one of its terminals has to be common to the input and output circuits. 

 

1.1.2.1  CB Configuration

In this configuration, emitter current IE is the input current and collector current IC is the output current. The input signal is applied between the emitter and base whereas output is taken out from the collector and base as shown below (for n-p-n transistor):

 

Static characteristics for a common-base circuit

Static characteristics are curves which represent relationships between different d.c. currents and voltages of a transistor. They are helpful in studying the operation of a transistor when connected in a circuit.

(i) Input characteristic.

The figure below shows an arrangement for determining the static characteristics of an n–p–n transistor used in a common-base circuit.

 

OR

 

The input characteristic can be obtained by varying R1, which varies VEB, and noting the corresponding values of IE. This is repeated for various values of VCB. It will be found that the input characteristic is almost independent of VCB and it is usual to give only one characteristic as below.

The input to a common-base transistor is the emitter current, IE, and can be varied by altering the base emitter voltage VEB. The base-emitter junction is essentially a forward biased junction diode, so as VEB is varied, the current flowing is similar to that for a junction diode, as shown below for a silicon transistor.

  

The figure above is called the input characteristic for an n-p-n transistor having common-base configuration. The variation of the collector-base voltage VCB has little effect on the characteristic. A similar characteristic can be obtained for a p-n-p transistor, these having reversed polarities.

(ii) Output characteristics.

The value of the collector current IC is very largely determined by the emitter current, IE. IE is set to a suitable value by adjusting R1. For various values of VCB, set by adjusting R2, IC is noted. This procedure is repeated for various values of IE. To obtain the full characteristics, the polarity of battery V2 has to be reversed to reduce IC to zero. This must be done very carefully or else values of IC will rapidly increase in the reverse direction and burn out the transistor.

For a given value of IE the collector-base voltage, VCB, can be varied and has little effect on the value of IC. If VCB is made slightly negative, the collector no longer attracts the majority carriers leaving the emitter and IC falls rapidly to zero. That is, IE is kept constant while VCB is varied and IC is observed. The test is repeated for various values of the emitter current and the results are plotted as below: