The collector characteristic curve shows the behavior of the transistor. To get these curves we bias a BJT transistor with a variable power supply and change the V_BB and V_CC. By variating these voltages, either transistor PN junctions may be forward or reverse biased. Based on the condition of the BJT transistor junction biasing, the transistor can operate in four regions of operation.

Transistor Regions of Operation

Suppose we have an NPN transistor connected with variable power supplies as the diagram shows.

Cutoff region

When the base power supply V_BB is set to zero, the current IB becomes zero. Both of the PN junctions of the BJT transistor become reverse bias and no collector current I_C flows. In this region, the transistor behaves as an open switch. And the V_CC voltage appears across the collector-emitter terminals V_CE. This region is the cutoff region.

Saturation region

When collector supply V_CC is set to zero, and base supply is set to produce a certain base current I_B. Both of the PN junctions become forward biased. The base current I_B flows through the emitter terminal because of low impedance and no current flows through collector I_C=0. This Transistor region of operation is the saturation region. In this region, both junctions are forward biased. Collector current I_C reaches a certain largest current independent of base current IB. In the saturation region, the voltage across collector-emitter terminals V_CE is zero.

The active or Linear region

By increasing the V_CC the voltage across collector-emitter terminals V_CE may increase. As the V_CE passes the 0.7v, the base-collector PN junction becomes reverse bias. And the base-emitter is forward bias. This mode of operation is the active or linear region of operation in the BJT transistor characteristic curve. By increasing the V_CE beyond 0.7v, the collector current remains constant for a given value of base current I_B. Increasing the V_CE can cause a very slight increase in I_C because of the widening of the base-collector depletion region. The following formula can calculate the active region collector current I_C.

Breakdown region

By increasing the V_CC, the V_CE can be increased which may increase the reverse bias of the base-collector junction. The high reverse bias may cause widening of the base-collector junction and finally break down the junction. As the BJT transistor enters the breakdown region, the collector current will increase rapidly as the curves diagram shows. A transistor is not used in the breakdown region.

Conclusion

Based on transistor characteristic curves has four regions of operation
Cutoff region work as an open switch
Saturation region work as a close switch
I_C remains constant for a certain amount of I_B active region
The Breakdown region of a transistor is not used

BJT Transistor regions of operation in the Characteristic Curve