# Solid State Physics - p-n junctions

1. Nov 17, 2011

### GrantB

1. The problem statement, all variables and given/known data

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?

2. Relevant equations

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

3. 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.

2. Nov 17, 2011

### cbetanco

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

3. Nov 17, 2011

### GrantB

I'm running into a problem.

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

Last edited: Nov 17, 2011
4. Nov 17, 2011

### cbetanco

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

5. Nov 17, 2011

### GrantB

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!