(This is one of the Expected Question for AISSCE Physics 2011)
Explain the action of transistor as a switch.
To answer this question (with proper understanding) you should have an idea of the three regions in the VI characteristics of a transistor. viz; Cutoff region, Active region and the saturation region.
When the input bias of a CE transistor (Vbe) is very low, the transistor does not conduct. So the output voltage (Vce) will be almost equal to Vcc, the output bias voltage.
By increasing the input voltage, the transistor can be brought to saturation region when the transistor will be conducting well. Now the pd across the transistor (Vce) will be very low.
This is the working principle of transistor as a switch.