Base-collector current in BJT Saturation

[Helena Wells]

TL;DR Summary

This thread is about the base-collector current in BJT saturation

I can't find the formula for the current flowing from the base to the collector in saturation mode of BJT.Any help?There are many references to the emitter current in the forward active mode but none for the base-collector current in saturation mode.Help appreciated.

 

[LvW]

You are looking for a formula?
The other way round: A pretty large base current is an indication for forward operation of the base-collector junction. It is common practice to say (for example) that a base current which is app. 1/10 of the collector current is a good and safe indication for a forward biased B_C junction. But this is a "rulre of thumb" only - a factor of 0.05 is also good enough.

 

[Helena Wells]

yeah just write there is a formula for the relationship between collector , emitter current and base current in forward active mode i was wondering if there is for saturation.

 

[DaveE]

yeah just write there is a formula for the relationship between collector , emitter current and base current in forward active mode i was wondering if there is for saturation.

It is just the forward biased diode equation. The BC junction is forward biased in saturation and there can't be any "transistor" action because the voltage polarities aren't right. The device is completely flooded with carriers flowing through both the BE and BC junctions.

You can find some of the equations here: http://web.mit.edu/6.012/www/SP07-L18.pdf

edit: Removed some of the words, which aren't correct. The device is still a transistor, β_f & β_r still matter. But in normal application circuits it isn't working like a transistor, the collector current is normally determined by eternal things (resistors and voltages).

 

[Helena Wells]

Ah ok thanks Dave!