# Homework Help: Series resistance of Schottky diode

1. Oct 23, 2011

### ffesty

1. The problem statement, all variables and given/known data
I have plotted the I-V characteristics of a Schottky diode under strong forward bias (0-0.7V, conduction begins at about 0.3V). I have to find the series resistance of the diode, rc.

2. Relevant equations
My notes give the equation $I_{F}=I_{S}\exp\left(\frac{e\left(V_{F}-Ir_{c}\right)}{\eta kT}\right)$, then they rearrange it to $I_{F}r_{c}=V_{F}-\frac{\eta kT}{e}\ln\left(\frac{I_{F}}{I_{s}}\right)$ and say 'hence, estimate the series resistance of the diode'.

3. The attempt at a solution
My problem is I can't work out how to get from the curve I have plotted to a value of rc. I could just take the gradient of the curve where it is most linear and treat that as the series resistance but I don't think that's right, as the question clearly wants me to use their rearranged formula. I have all the data for n, k T, etc, so I could just plug the numbers in for each data point and get an average rc, but then there would have been no point in drawing a graph?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 24, 2011

### rude man

My notes give the equation $I_{F}=I_{S}\exp\left(\frac{e\left(V_{F}-Ir_{c}\right)}{\eta kT}\right)$, then they rearrange it to $I_{F}r_{c}=V_{F}-\frac{\eta kT}{e}\ln\left(\frac{I_{F}}{I_{s}}\right)$ and say 'hence, estimate the series resistance of the diode'.

Why should rc necessarily be constant? If they define "resistance of the diode as rc" then rc is by no means constant with forward current, courtesy the second term in your last equation above. So really there is no "the" resistance, just a continuum of resistances varying with forward current.

Actually, I'm not sure calling rc the diode resistance is a good idea to begin with. rc is neither Vf/If nor dVf/dIf, the latter being the slope of your V-I curve for the diode. Usually, one or the other matters most.