Base-collector current in BJT Saturation

  • #1
125
9
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.
 

Answers and Replies

  • #2
LvW
879
237
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.
 
  • #3
125
9
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.
 
  • #4
DaveE
Science Advisor
Gold Member
1,794
1,454
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, βf & βr 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).
 
Last edited:
  • #5
125
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Ah ok thanks Dave!
 

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