# V. simple question about diodes i-v graph.

1. Oct 18, 2009

### earlofwessex

so i've seen this graph all over the internet, but nowhere seems to explain why the current after +0.6v rises exponentially and not just straight up. is it mainly due to the internal resistance of the diode or is there significant inductance/capacitance in there as well. whats the dominating effect(s)?

I'm guessing at that voltages significantly higher than Vf its just the internal resistance (r=1/gradient), but the curve leading up to Vf, whats causing it?

http://www.yegopto.co.uk/media_yegOpto/image/400%20x%20400%20images/volt_current%20graph.jpg [Broken]

also, whats a typical internal resistance for a power diode?

thanks

Last edited by a moderator: May 4, 2017
2. Oct 18, 2009

### f95toli

No, it is a microscopic effect. It has to do with the shape of the bands (the density of states) in the semiconductor; you can't really think of it in terms of "macroscopic" variables like capacitance etc.
It is not too difficult to derive the correct expression using a band model and you should be able to find that derivation in just about any book on semiconductor physics.

The fact that is rises exponentially is essentially just a consequence of the electrons obeying Fermi-Dirac statistics.

3. Oct 18, 2009

### uart

No the exponential relationship is not due to series resistance. It's true that bulk resistivity in the neutral regions can have a significant effect in some diodes at high current levels (making the I/V characteristic less steep than predicted by the exponential relationship) but no the exponential relation is not due to resistance. It's a lot more complicated than that. Start with this for example : http://books.google.com.au/books?id...resnum=2&ved=0CA8Q6AEwAQ#v=onepage&q=&f=false

4. Oct 18, 2009

### dlgoff

The 0.6 volts is called the built-in potential and depends on the type of semiconductor material in the diode.
http://en.wikipedia.org/wiki/Diode#Semiconductor_diodes"
And as f95toli implies, you would need to study the junction theory to see why the curve is exponential.

Last edited by a moderator: Apr 24, 2017
5. Oct 18, 2009

### earlofwessex

thanks guys.

not so simple question after all then, it's going to take me a long while to understand all that.

6. Oct 18, 2009