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?
1\$\begingroup\$ I have not seen any such characteristic in any BJT datasheet. \$\endgroup\$– Ignacio Vazquez-AbramsSep 22, 2015 at 8:14
1\$\begingroup\$ Your question is rather unclear actually. Are you talking about saturation current in the reverse-active region, i.e. in which collector and emitter are purposefully switched? Datasheets hardly ever give any parameters for that region. Or are you taking about reverse saturation current in collector-base junction in the active region? \$\endgroup\$– FizzSep 22, 2015 at 8:30
\$\begingroup\$ Its actually reverse saturation current in collector-base junction in the active region..Edited the question now \$\endgroup\$– Ishara AbeykoonSep 22, 2015 at 8:35
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
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
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.