Why we can't measure Vbe?

[Osawa_Odessa]

Can anyone help?:wink:

 

[rahdirs]

@Osawa_Odessa: I took some time to reply,but i found some excellent material from the book "Operation & Modeling of MOS Transistor" by Tsividis. Section 1.4 was on contact potentials.
I clearly explains why you measure Vsource if you keep Voltmeter across material K & L.

 

[Osawa_Odessa]

Thanks rahdirs!
And sorry for not metioning that I have this book.
Let's call the contact voltages as follows:
metal-p semiconductor junction: V1
p-n junction: Vbi (built in voltage)
n semiconductor -metal junction: V2
We have: V1 + Vbi + V2 =0. Therefore, we can't measure Vbi by using a normal voltmeter.
But I am still confused about Krichoff's law in post #20. Can you explain it now?
By the way, I have just seen you in allaboutcircuits.

 

[rahdirs]

Ok,taking help of attached image.

When there is no voltage source, writing KVL loop,
ψW,M1 + ψM1,M2 -ψW,M2 = 0 ----eq(1)

Now attach voltage source with positive terminal at M1 & negative terminal at M2.
Vf + ψW,M1 + (ψM1,M2 -Vf)-ψW,M2 = 0 as per equation (1).

Wrote Vf directly,otherwise had to write ψW,batt + Vf - ψW,batt = Vf.

 

[Osawa_Odessa]

Thanks, that make sense now.:smile:
I can sleep well tonight!