a doubt regarding saturation mode of BJT

[DEV123]

Hi all,
I have a doubt regarding saturation mode of BJT. Saturation occurs when both emitter-base junction and base-collector junction gets forward biased. For forward biasing a silicon p-n junction only 0.7V is needed. So is for emitter-base junction. But not for base-collector junction. Because we know at saturation voltage across collector and emitter Vce is 0.2V. Using Kirchoff's voltage laws the voltage at base-collector p-n junction is less than 0.7V. Why is it lesser?

-Devanand T

 

[goldsmith]

Dear Devanand T
Hi
In fact , when you give , the 0.7 volt with max base current , to the BE junction , and give the positive voltage ( more than base voltage ) to the collector ( i'm talking about NPN transistor ) . thus the BE is forward and BC is reverse . but fact isn't it . the fact is inside the transistor . when you did that thing , inside the transistor , the after a little time , the BC , will going forward and BE will be forward . is it magic ?
see below please:


Best Wishes
Goldsmith

 

[DEV123]

Thankyou for your reply sir. I am totally confused now and lacking basics. Where can I find right explanations for my doubts.

Dear Devanand T
Hi
In fact , when you give , the 0.7 volt with max base current , to the BE junction , and give the positive voltage ( more than base voltage ) to the collector ( i'm talking about NPN transistor ) . thus the BE is forward and BC is reverse . but fact isn't it . the fact is inside the transistor . when you did that thing , inside the transistor , the after a little time , the BC , will going forward and BE will be forward . is it magic ?

Please can you explain the reason for magic?

 

[goldsmith]

Again Hi
Yes why not ? we can not analyze it simply because if you consider the base voltage 0.7 volt . thus the collector voltage is about 35 volt . and the difference between them should be BC voltage ( in reverse bias ) , but it is not occurs ! as you can see at my attachments , the voltage across the BC junction is in forward and about 670mv !!!!!!
So what do you think!!!!??????
Good luck
Goldsmith

 

[DartPlayer170]

Hi all,
I have a doubt regarding saturation mode of BJT. Saturation occurs when both emitter-base junction and base-collector junction gets forward biased. For forward biasing a silicon p-n junction only 0.7V is needed. So is for emitter-base junction. But not for base-collector junction. Because we know at saturation voltage across collector and emitter Vce is 0.2V. Using Kirchoff's voltage laws the voltage at base-collector p-n junction is less than 0.7V. Why is it lesser?

-Devanand T

First of all, the Vce(sat) is not constant at 0.2V - it varies with the base current.
Secondly, anything above 0V is forward biased, 0.7V is roughly the knee voltage for silicon pn junction to turn on. You are confusing forward bias with turn on voltage. For ex: if BE is forward biased at 0.7V and CE is 0.2V then CB is forward biased 0.5V - for CB to be reversed biased Vce > Vbe
Thirdly, the voltage across the forward biased pn junction will depend on the current through it - and for a saturated transistor little current is passing through CB junction - it is infact going straight through C to E. So it would be expected by the pn junction curve that Vcb < Vbe during saturation since Ibc < Ibe.

 

[DEV123]

thankyou guys for your explanations, helped a lot.