Q. Explain working of Bipolar junction transistors?

The family of bipolar junction transistors has two members: the npn BJT and the pnp BJT. Both types contain semiconductor junctions which operate with bipolar internal currents consisting of holes and electrons. These are illustrated in Figure along with their circuit symbols.

The emitter in the circuit symbol is identi?ed by the lead having the arrowhead. The arrow points in the direction of conventional emitter current ?ow when the base-emitter junction is forward biased. A transistor can operate in three modes: cutoff, saturation, and active. In the active mode, for an npn BJT, the base-emitter junction (BEJ) is forward-biased by a voltage vBE, while the collector-base junction (CBJ) is reverse-biased by a voltage v_CB. Thus for an npn BJT, as shown in Figure (a), i_B and i_C are positive quantities such that i_B + i_C = i_E. For a pnp BJT on the other hand, in the active region, the base-emitter and collector-base voltages are negative, and  currents i_B, i_C, and i_E are all negative quantities such that i_E = i_B + i_C, that is to say, the bias voltages as well as current directions are reversed compared to those of an npn BJT.