A doubt regarding saturation mode of BJT

1. Feb 6, 2012

dexterdev

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

2. Feb 6, 2012

yungman

0.7V is only an approximation.The more exact equation is:

$$V_{BE}=V_T\; \ln [\frac {I_c}{I_s}]$$

The Vbc diode obey the same equation. Actually I am working with transistors of Ic at 50uA range and the measured Vbe is about 0.5V!!!

Last edited: Feb 6, 2012
3. Feb 6, 2012

Staff: Mentor

No one said there is heavy current flow from B-C. It's just not reverse biased. For an NPN, VBC = +0.5v is definitely a bias in the forward direction. But does not correspond to significant current flow through a silicon diode.

4. Feb 7, 2012