What is transistor saturation?

Why is the current so high in the saturation region of Common-Base output characteristics of a NPN transistor?

• The saturation region, in which both the collector and emitter junctions are forward-biased, lies in the portions of the curves wherein Vcb is just very slightly above 0 V and Ie > 0. When both are forward biased then how can so much current be flowing through the circuit.

• Answer:

  This is a touchy-feely kind of question that you usually only see in fourth-rate textbooks.    There does NOT have to be a "lot" of current to saturate a transistor, for instance if the collector resistor is 1 megohm fed from 1 volt and the transistor beta is 1,000, the collector and emitter currents are 1 microamp and the base current is 1 picoamp in saturation.  None of those are "high" currents.   In general when a transistor is saturated there is only a minimum, like 0.3 volts collector to emitter so the emitter and collector currents are as high as they can go given the collector resistance.