Saturation region of a transistor

[titangsu]

I have not any clear idea about the saturation region of a transistor. Can anyone help me in detail? What parameter actually becomes saturated in this region? Why are both the junctions becomes forward biased? Is it possible to widen this region for a particular transistor?

 

[phyzguy]

I assume from the question that you mean a bipolar transistor. Bear in mind that "saturation" means something different for MOS and bipolar transistors. The word "region" doesn't refer to a region of the transistor, it refers to a region of the voltage-current parameter space. When the voltage on the collector drops below the voltage on the base, then the base/collector junction becomes forward biased and by definition this is the saturation region. When this happens, carriers are no longer traveling as intended from the emitter to the collector, and the current gain drops drastically.

It corresponds to the left hand region on the attached I-V plot, which I copied from Wikipedia, http://en.wikipedia.org/wiki/Bipolar_junction_transistor#Regions_of_operation