Transistor as Switch: A transistor can be used for two types of applications viz. amplification and switching. For amplification, the transistor is biased in its active region. For switching applications, transistor is biased to operate in the saturation (full on) or cut-off (full off) region.
(i) Transistor in cut-off region (Open switch):
In the cur-off region, both the junctions of transistor are reverse biased and very small reverse current flows through the transistor. The voltage drop across the transistor (VCE) is high, nearly equal to supply voltage VCC. Thus, in cut-off region the transistor is equivalent to an open switch as shown in fig.(a).
(ii) Transistor in Saturation region (Closed switch):
When Vin is positive, a large base current flows and transistor saturates. In the saturation region, both the junctions of transistor are forward biased. The collector current is very large, the voltage drop across the transistor (VCE) is very small, of the order of 0.2V to 1 V, depending on the type of transistor. Thus in saturation region, the transistor is equivalent to a closed switch.