Regions of operation:
E < B < C
E < B > C
E > B < C
E > B > C
Bipolar transistors have five different regions of operation that are described by BJT junction biases.
The modes of operation can be explained in terms of the applied voltages (this explanation applies to NPN transistors; polarities are reversed for PNP transistors):
1. Forward active: base higher than as compared to emitter, collector higher than base (in this type the collector current is proportional to base current by βF).
2. Saturation: base higher than as compared to emitter, but collector is not higher than base.
3. Cut-Off: base lower than as compared to emitter, but collector is higher than base. It means the transistor is not letting conventional current to go via collector to emitter.
4. Reverse-action: base lower than as compared to emitter, collector lower than base: reverse conventional current undergoes transistor.