Reverse saturation current of BJT?

Question

This might be a basic question...but I'm having a hard time understanding reverse saturation current in collector-base junction in the active region(Is), Could anyone explain about it? Its temperature dependent right? If so is there a fixed value at room temperature? How does it change over different transistors?

Answer 1

The reverse saturation current in the collector-base junction is origined by the diffusion of minority carriers from the neutral regions to the depletion region.

It is very dependent from specific parameters of the junction itself, such as the donor and acceptor concentrations, the diffusion coefficients of holes and electrons, the cross-sectional area.

In general it must be kept small, since you don't want a transistor conducting when the base-emitter junction is not directly polarized.

It is exponentially dependent form temperature (as a general rule it doubels every 10°C), and independent from Vcb

Answer 2

When in a physical transistor emitter current is reduced to zero, then the collector current is known as ICBO (approximately equal to ICO). Reverse collector saturation current ICBO also varies with temperature, avalanche multiplication and variability from sample to sample. Consider the circuit shown in fig. 4. VBB is the reverse voltage applied to reduce the emitter current to zero. IE = 0, IB = -ICBO If we require, VBE = - 0.1 V Then - VBB + ICBO RB < - 0.1 V

Fig. 4

If RB = 100 K, ICBO = 100 m A, Then VBB must be 10.1 Volts. Hence transistor must be capable to withstand this reverse voltage before breakdown voltage exceeds.