# Solid State Physics - p-n junctions

## Homework Statement

An ohmmeter is sometimes used to determine the "direction" of a diode by connecting the ohmmeter to the diode one way and then reversing the ohmmeter leads. If the ohmmeter applies an emf of .5V to the diode in order to determine resistance, what would be the ratio of reverse resistance to forward resistance at 300K?

## Homework Equations

I=I0(ee$\varphi$/kBT-1) ?

## The Attempt at a Solution

I know that for reverse bias, the potential $\varphi$ is negative, and for forward bias, it is positive. But that's basically as far as I've gotten.

I am mainly confused as to what the relationship between the potential and emf is, and how that leads you to getting the resistance.

I think I've forgotten too much of my previous physics class...

Thanks for the help.

## Answers and Replies

Use Ohm's Law to write $R=\frac{V}{I}=\frac{V}{I_0 e^{qV/kT}-1}$. I used V instead of $\phi$ and q instead of e

I'm running into a problem.

When I take e-qV/kT-1 for the reverse bias it gives me a negative number.

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You also have to take V in the numerator to be negative, then you will get a positive number

You also have to take V in the numerator to be negative, then you will get a positive number

Ahh, thank you.

I am getting an answer of 2.5x108 when the answer is 2.4x108. Although, I think its a rounding (error) somewhere since what I'm doing seems correct.

Thanks!