transistor work in three different states, and they are, cut off , saturation and active. In active state transistor works as an amplifier while as a switch transistor work in cutoff (open switch) and saturation states (close switch). This is achieved by the DC biasing.
There are three parts to a transistor, the base, collector, and emitter. The collector is the electrical supply to be switched on & off. When switched on the emitter is the supply output. The base is the switch. When 5 volts (typically) are applied at the base, the transistor is in the switched on state and flow begins. When the voltage drops flow stops.
1. Cut-off Region
Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched "Fully-OFF".
2. Saturation Region
Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched "Fully-ON".