Transistor Regions of Operation

A transistor can be operated in three different regions as

• active region
• saturation region
• cut-off region

active region:

The transistor is said to be operated in active region when the emitter-base junction is forward biased and collector –base junction is reverse biased. The collector current is said to have two current components one is due to the forward biasing of EB junction and the other is due to reverse biasing of CB junction. The collector current component due to the reverse biasing of the collector junction is called reverse saturation current (I_CO or I_CBO) and it is very small in magnitude.

Saturation region:

Transistor is said to be operated in saturation region when both EB junction and CB junction are forward biased as shown. When transistor is operated in saturation region I_C increases rapidly for a very small change in V_C.

Cut-off region:

When both EB junction and CB junction are reverse biased, the transistor is said to be operated in cut-off region. In this region, the current in the transistor is very small and thus when a transistor in this region it is assumed to be in off state.