When biasing voltages are applied, the emitter base junction becomes forward biased and collector base junction is reverse biased. Potential barrier and depletion region width reduces. As potential barrier is reduced, a large number of minority charge carriers are introduced in emitter and base regions. Few minority charge carriers reaching base recombines with majority carriers. The rest defuse to edge of depletion region where they are swept away to the collector region by the large electric field. Under this condition, the transistor is said to be in active region.
When VQE is increased, the injected minority charges into the base region increases and base – and collector currents also increases. The voltage Vcg decreases with increase in the collector current due to increasing drop in the external resistance Rc, as the collector base voltage Vcc is constant.
Hence potential barrier starts decreasing and at one point it becomes forward biased. Due to forward biasing there will be minority carrier injection into the base. Under this condition, the transistor is said to be in the saturation region.
