Cut off, Active & Saturation Region of Transistor

In this theory, cut off region, active region and saturation region of the transistor is explained.

Active Region, Cut off Region and Saturation Region of Transistor

Cut off region of Transistor

• When load line intersect I_B = 0, it is known as cut off region of the transistor. 
• As the base current is zero, only small collector leakage current flows. The base emitter junction does not remain in the forward biased because the base current is zero. 
• The collector to emitter voltage is equal to V_CC.

Saturation Region of Transistor

• When the load line intersect I_B = I_{B ( saturation )} , it is known as saturation region of the transistor. 
• As the base current is maximum, the collector current also becomes maximum.  
• The collector to emitter junction does not remain in the reverse biased condition therefore the transistor action is also lost. 
• If the base current increases beyond saturation level, the collector current does not increase because of collector to base junction is not remain reverse biased.

         I_C = V_CC / R_C

        V_CE = V_{CE ( knee )}

Active Region of Transistor

• It is a region between active and cut off of the transistor output characteristic. 
• The base to emitter junction remains in forward biased condition whereas the collector to emitter junction remains in the reverse biased condition.

Current During Different Region of Transistor

Active region

Collector current I_C = βI_B

• The transistor works as an amplifier in the active region

Cut off region

• As there is no base current, the collector current also becomes zero.

Saturation region

• The transistor behaves as a closed switch between collector and emitter. The collector – emitter path is short circuited.
• Every transistor consists of two diodes emitter – base diode and collector to emitter diode in the common emitter configurations.

Region	Base – emitter diode	Emitter – collector diode
Cut off	OFF	OFF
Saturation	ON	ON
Active	ON	OFF

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