BJT problems on graph understanding?

Question

For the output characteristic graph of a Bipolar Junction Transistor, common base configuration, why there is collector current flow when the Vcb(Collector-Base Voltage) is zero?

Also for the difference between the output characteristic graph of common base and common emitter, the saturation region.

Common base's saturation region happens when Vcb < 0 while common emitter's saturation region happens when 0< Vce(Collector-emitter voltage) < 0.7.
Any reason and explaination?

Answer 1

Because, in the common-base configuration, the BE junction is still forward biased, and Ic = Is*e^(Vbe/Vt -1). The input in a CB setup is taken at the emitter, at a voltage level that is negative with respect to the base, which keeps the be junction forward biased.

Also, from my graphs, saturation in the common - base configuration occurs when Vcb < ~-0.4 to -0.5 volts, not 0 volts. This makes sense, because if the Vbe = 0.7 volts, that would correspond to the Vce being at about 0.2 to 0.3 volts. Saturation in the CE configuration happens when Vce< 0.2 to 0.3 volts, not 0.7 volts.