If a crystal of germanium or silicon is doped in such a way that n-type regions are formed at its two sides with a very thin p-type region in between, it forms an n-p-n transistor. Alternatively the crystal can be formed such that it can have p-type regions at the two sides with a thin n-type region in the Centre. Such a transistor is known as p-n-p transistor. Both these types of transistors along with their symbols are shown in fig. 19.15.
The central region of the transistor is known as base and the regions on its either sides are called emitter and collector. In fig. 19. 15 these regions are shown by letter B, E and C respectively.
Transistor action:
It can be seen in fig. 19.15 (A,B) that a transistor consists of two junctions one is the emitter base junction (EB) and the other is collector base (CB). Generally, the emitter base junction is forward biased, while the collector base junction is reversing biased. The biasing scheme for an n-p-n transistor has been shown in fig. 19. 16. The potential VCB that reverse biases the collector base junction is many times larger than the potential VEB that forward biases the emitter base junction.
As the emitter base junction is forward biased, so its resistance gets very low and a stream of electrons is injected from the emitter into the base. Thus current begins to flow through this junction. Resistance RE is used to control this current. As the base region is very thin, so practically almost all the electrons injected into the base are attracted towards the collector due to its large positive potential and very few electrons enter into the base circuit. In this way a current IE, injected from the emitter into the base, which is actually divided into the collector current Ic and base current IB, therefore, IE = Ic + IB
Note that in fig. 19.16 direction of current is that of electronic current. The importance of the transistor is based upon the fact that value of the collector current Ic is many times greater than the base current. So by applying a small voltage Vi in the base circuit, if IB is changed slightly, then the collector current Ic changes significantly. This gives raise a big change in potential Vo across the resistance RC. Thus a small change Vi in the base circuit, after increasing many times, appears as a voltage Vo in the collector circuit. In this way the transistor acts as an amplifier.
