Series resistance of Schottky diode

[ffesty]

Homework Statement

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, r_c.

Homework 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'.

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?

 

[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.