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?
Reverse saturation current of BJT
bjtcurrentsaturationtransistors
Related Solutions
I think you are asking how beta can be less in saturation than in active mode when it appears from a calculation of Ic that Ic is highest in saturation. If that is your question, the answer is that in saturation, if you increase the base current this fails to further increase the collector current. So in betasat = Ic/Ib, Ib increases but Ic does not, hence betasat reduces.
It can be confusing because in a MOSFET the saturation region is something else and they call the "linear" region what would be the "saturation" region in a BJT. Why oh why?
Here's my simplified picture of things for a BJT: -
Note that all the curves for different base currents do not overlap as is commonly shown. If they did overlap there would be no BJT based 4-quadrant multipliers (Gilbert cell). They rely on the saturation region being able to modulate the current for a given CE voltage. Anyway, that's a bit off the mark for your question.
The saturation region does include the scenario when CB is forward biased but I don't think this is particularly helpful - the saturation region (or close to it) must still encompass normal transistor amplification and, as far as I know, this cannot happen when collector and base are forward biased.
Why doesn't further increase in base current cause changes in collector current?
It does up till the point when the collector-base junction is forward biased. The curves look bunched in your diagram (and this is an error basically) but they are still different and for a given low voltage across C-E, the current is proportional to that voltage AND the base current.
Hope this helps.
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Best Answer
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