Electrical Engineering ⇒ Topic : Transistor

TRANSISTOR

William Shockley invented the first junction transistor in 1951. It is a semiconductor device and can amplify electronic signals such as radio and television signals. Transistor is an essential ingredient of every electronic circuit. To achieve amplification in the transistor, signal is passed from a region of low resistance to a region of high resistance. This concept of transfer of resistance has given the name transistor (Transfer-resistor).

There are two types of transistors: (i) Unipolar junction transistor and (ii) Bipolar junction transistor. The conduction of current takes place due to majority carriers, in unipolar junction transistor, whereas the conduction of current in bipolar transistor occurs due to both types of charge carriers. The bipolar junction transistor is referred to as BJT. There are two types of BJTs, i.e., (i) N-P-N and (ii) P-N-P shown in Figure 1 (a) and Figure 1 (b), respectively

                                                          figure (1) Construction of bipolar junction transistor.

The middle region of each transistor is termed as base (B) of the transistor. This region is very thin and lightly doped. The remaining two regions are called emitter (E) and collector (C) which are heavily doped. The doping level of emitter is slightly greater than that of the collector. The collector region area is slightly more than that of emitter. A transistor had two P-N junctions.One junction occurs between the emitter and the base which is known as emitter base junction.This is also simply called emitter junction (J_E). The other junction occurs between base and collector known as collector-base junction or simply collector junction (J_C. We can say that any transistor is like two P-N junction diodes connected back to back as shown in Figure (2).

                                           figure  (2) Analogy of two-diode transistor.

If no external voltage is applied to transistor, it is unbiased. There will be no current flowing from any of the transistor leads. There will be depletion regions at both the junctions shown in Figure (3)

                   Figure (3) Unbiased N-P-N junction.

The two P-N junctions of the transistor must be correctly biased with external voltages so that it can operate properly as an amplifier. The transistor can work in one of the three following regions depending on the external bias:

• Active region: Emitter junction is forward biased whereas collector junction is reverse biased.
• Cut-off region: Both emitter junction and collector junction are reverse biased.
• Saturation region: Both emitter junction and collector junction are forward biased.

Figure (4) shows the biasing for active region for both P-N-P and N-P-N transistors.

                                            Figure (4) Forward and reverse biasing of transistors.

In Figure (4) there are two externally applied voltages, i.e., V_EE and V_CC. These voltages bias the transistor. The operation of P-N-P is same as that of N-P-N except the roles of electrons and holes, the bias polarities and the current directions are all reversed. The emitter base junction is forward biased whereas the collector base junction is reverse biased. The action of transistor is discussed in the next article.

The externally applied voltages V_EE and V_CC, shown in Figure (4), bias the transistor in active region.