A semiconductor device consisting of two P-N junctions formed by either a P-type or N-type semiconductor between a pair of opposite types is known as a transistor.

FIGURE 3.21

A transistor in which two blocks of N-type semiconductors are separated by a thin layer of P-type semiconductor is known as an NPN transistor.

A transistor in which two blocks of P-type semiconductors are separated by a thin layer of N-type semiconductor is known as a PNP transistor.

The three portions of a transistor are the emitter, base, and collector, shown as E, B, and C respectively in Figure 3.21.

The section of the transistor that supplies a large number of majority carriers is called the emitter. The emitter is always forward biased with respect to the base so that it can supply a large number of majority carriers to its junction with the base. The biasing of the emitter base junction of an NPN and PNP transistor is shown in Figure 3.22. Since the emitter is to supply or inject a large amount of majority carriers into the base, it is heavily doped but moderate in size.

The section on the other side of the transistor that collects the major portion of the majority carriers supplied by the emitter is called the collector. The collector base junction is always reverse biased. Its main function is to remove majority carriers (or charges) from its junction with the base. The biasing of collector base junctions of an NPN transistor and a PNP transistor is shown in Figure 3.21 above. The collector is moderately doped but larger in size so that it can collect most of the majority carriers supplied by the emitter.

The middle section, which forms two P-N junctions between the emitter and collector, is called the base. The base forms two circuits, one input circuit with emitter and the other an output circuit with collector. The base emitter junction is forward biased providing low resistance for the emitter circuit. The base collector circuit is reversed biased, offering a high-resistance path to the collector circuit. The base is lightly doped and very thin so that it can pass on most of the majority carriers supplied by the emitter to the collector.

Operation of an NPN Transistor

An NPN transistor circuit is shown in Figure 3.22. The emitter base junction is forward biased while the collector base junction is reversed biased. The forward biased voltage v_eb is quite small, where as the reversed biased voltage v_cb is considerably high.

FIGURE 3.22

As the emitter base junction is forward biased, a large number of electrons (majority carriers) in the emitter (N-type region) are pushed toward the base. This constitutes the emitter current i_.e.. When these electrons enter the P-type material (base), they tend to combine with holes. Since the base is lightly doped and very thin, only a few electrons (less than 5%) combine with holes to constitute base current i_b. The remaining electrons (more than 95%) diffuse across the thin base region and reach the collector space charge layer. These electrons then come under the influence of the positively based N-region and are attracted or collected by the collector. This constitutes collector current i_c.

Thus, it is seen that almost the entire emitter current flows into the collector circuit. However, to be more precise, the emitter current is the sum of the collector current and base current i.e.,

i_e=i_c+i_b.

Operation of a PNP Transistor

A PNP transistor circuit is shown in Figure 3.23 below. The emitter base junction is forward biased while the collector base junction is reverse biased. The forward-biased voltage v_eb is quite small, where as the reverse-biased voltage v_cb is considerably high.

As the emitter base junction is forward biased, a large number of holes (majority carriers) in the emitter (P-type semiconductor) are pushed toward the base. This constitutes the emitter current i.e., when these electrons enter the N-type material (base), they tend to combine with electrons. Since the base is lightly

FIGURE 3.23

doped and very thin, only a few holes (less than 5%) combine with electrons to constitute base current i_b. The remaining holes (more than 95%) diffuse across the thin base region and reach the collector space charge layer. These holes then come under the influence of the negatively based P-region and are attracted or collected by the collector. This constitutes collector current i_c.

Thus, it is seen that almost the entire emitter current flows into the collector circuit. However, to be more precise, the emitter current is the sum of the collector current and base current i.e.,

i_e= i_c+i_b.

FIGURE: 3.24   Integrated chips