Question

What would happen if both junctions of a BJT are forward biased or reverse biased?

Answer 1

A BJT has four regimes of operation, depending on the four combinations of the applied biases (voltage polarities) to the emitter-base junction and the collector-base junction, as shown in the following table; ‘F’ and ‘R’ indicate forward bias and reverse bias, respectively.

BE junction	CB junction	Regime/Mode	Comment
F	F	Saturation	Small biasing voltage and large output current, that is, the transistor is in a conducting state and acts as a closed (or ON) switch.
F	R	Active or Normal or Forward	High current gain allows the transistor to amplify voltage variations applied to the base. Output current is proportional to base current.
R	R	Cutoff	The transistor is in an inactive state due to only reverse saturation currents in the reversebiased junctions. Corresponds to the open (or OFF) state of the transistor as a switch.
R	R	Inverted active (or Reverse)	Corresponds to the case where the roles of the collector and emitter are reversed with low emitter' efficiency due to lower doping of the collector than the base. Current gain is much lower than that in the active mode.