Question

Draw the symbolic representation of a (a) p-n-p (b) p-n-p transistor, Why is the base region of transistor thin and lightly doped? With proper circuit diagram, show the biasing of a p-n-p transistor in common base configuration. Explain the movement of charge carriers through different parts of the transistor in such a configuration and show that I_E = I_C + I_B

Answer 1

Symbol of PNP transistor

Symbol of PNP transistor

The thin and lightly doped base region offers lesser number of majority charge carrier in base region which reduces the possibility of combination of electron hole pair in base region and hence, reducing the base current. Also, this leads to increase collector current. Hence, current gain increase in transistor.

The forward bias of emitter-base circuit move holes towards collector. The base region is very thin and lightly doped, only 5% of holes combines with the electrons in base region and remains nearly 95% holes enters into the collector region under the influence of V_EB. The excess of holes (95%) in collector region get combined with the electron drawn from the negative terminal of V_CB and get annihilated. I_E = I_C + I_B.