Why Does Current Rise Exponentially in a Diode After Reaching +0.6V?

[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. what's 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, what's causing it?


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


also, what's a typical internal resistance for a power diode?

thanks

 

[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.

 

[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

 

[dlgoff]

The 0.6 volts is called the built-in potential and depends on the type of semiconductor material in the diode.

The current–voltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary “cut-in” voltage is defined as 0.6 to 0.7 volts. The value is different for other diode types — Schottky diodes can be rated as low as 0.2 V and red or blue light-emitting diodes (LEDs) can have values of 1.4 V and 4.0 V respectively.

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.

 

[earlofwessex]

thanks guys.

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

 

[Bob S]

Look at this URL
http://www.allaboutcircuits.com/vol_3/chpt_3/1.html
and scroll down to the diode equation. kT at room temperature is about 26 millivolts.
For some reason, I cannot open Wiki URL in post #4.
Bob S