Question

Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-n-p transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased.

Answer 1

In p−n−p transistor : →

The emitter base junction is always forward biased with voltage V_EE and the collector base junction is always reverse biased with voltage V_CC.

The holes in the emitter are pushed into the base by the positive terminal of battery of voltage V_EE. Since, base is thin and lightly doped so only few holes combine with electrons in the base. Thus, base current I_B is small. Since V_CC is quite large almost 99% of holes coming from emitter are collected by collector. For each hole reaching the collector, an electron is released from the negative terminal of the collector base battery to neutralize the hole. For each hole consumed in collector, a bond breaks in emitter and electron is released that enters positive terminal of emitter-base battery. Thus, we can say the current is carried by holes inside the transistor and by electrons in external circuit.