An n-p-n transistor is used as a switch in a circuit. The transistor is said to be in saturation when

In saturation, the transistor is fully ON and acts like a closed switch. This occurs when both the base-emitter and base-collector junctions are forward biased. In the active region, only the base-emitter junction is forward biased while the base-collector junction is reverse biased. In cut-off, both junctions are reverse biased.