# anyone here a guru in semiconductor physics?

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#### jinyong

##### Newbie level 6
Is anyone here a guru in semiconductor physics? I have a question on diode physics if anyone could help please respond.

#### jinyong

##### Newbie level 6
For a PN junction diode, the total potential across the semiconductor equals the built-in potential minus the applied voltage.

Phi = Phi(built-in)-Va(voltage applied)

My question is what happens when the Va exceeds the Phi(built-in)? Will the Phi = negative? But how come they always assume drop of ~0.7V for diodes.

Too see what I'm talking about refer to equation 4.2.1 of

Thanks.

#### nxing

When va exceeds phi(build-in), the diode will be forward biased. and you gonna see lot amount of current. The junction can only take 0.7v, after that, it's hardly you can increase voltage applied to the junctin(otherwise, you can damage the diode).
Check any book about how the diode is operated, you will find the answer.

#### jinyong

##### Newbie level 6
Why is the formula Phi = Phi(built-in)-Va(voltage applied) ?

If Va > Phi(built-in) then wouldn't Phi be negative?

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