Transistor is a three terminal semiconductor device that amplify the current and used as a open close switch . Transistors plays a very important role in replacing vacuum tubes. Because transistors are ideal component for using in the digital circuits as switch.
A transistor can be used as a switch because its collector current is directly controlled by the base current . If the base current is greater than 0.7 volts than the transistor acts as a closed switch. To understand this let’s take a look of current characteristics of a transistor .
The characteristic curve above shows the three regions.
Saturation region Cut off region Active region
When we are using the transistor as switch we deals with only saturation region and cut off region.
Transistor in Cut off region :
In the cut off region the base voltage is less than 0.7 V so the base current is also negligible. In the transistor the collector current is directly proportional to the base current. So the collector current Ic is also negligible. But the collector to emitter voltage VCE is maximum. Which creates a large depletion region and no current flows through the transistor. So this region is called as cut off region.
The conditions for the transistor in the saturation region are Vb < 0.7 V, Collector Current ( Ic) = minimum, Collector to Emitter Voltage ( VCE ) = maximum.
Transistor in Saturation region :
In this mode the maximum base current is applied that results with high collector current and lower collector to emitter voltage. And this results a lower depletion region layer. By this the large amount of current can flow through the transistor.
Fig. Transistor in saturation region
The conditions for the transistor in the saturation region are Vb > 0.7 V, Collector Current ( Ic) = maximum, Collector to Emitter Voltage ( VCE ) = minimum.
When the base bias voltage is lower than 0.7 V. The transistor operated in the cut off region. But when the base bias voltage increases from 0.7 V the base saturated the collector and the current starts flow. In this state the transistor acts as a closed switch.