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I would like to ask question about the resistance of a diode under different regime. Surely, in reverse bias, it has a breakdown voltage, and in forward bias,it rises exponentially according to the exponential relationship according to the theory ( ideal Shockley diode equation ) . But what happens at the flat band situation? I do not understand why it should obey Ohm's law well when the fermi levels are split by eV=eVo, where Vo is the barrier height (when applied bias V is zero). (see Fig.1.)

Any Justifications with the physics of the charge carriers or using formulas would be much appreciated.

enter image description here

Thanks

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  • $\begingroup$ A diode does not obey ohms law. What is your question here exactly? $\endgroup$
    – boyfarrell
    Commented Jan 6, 2015 at 9:50
  • $\begingroup$ @boyfarrell No, it does.. After it reaches the flat band region $\endgroup$ Commented Jan 6, 2015 at 18:32
  • $\begingroup$ I think you are thinking about this in the wrong way. If you hold a diode at any particular point on the IV curve it has constant resistance but don't confuse this with ohms law! Ohms law states that for any applied bias the resistance is constant. This is clearly false for a diode. $\endgroup$
    – boyfarrell
    Commented Jan 6, 2015 at 18:45
  • $\begingroup$ But +1 for your question from me because it illustrates very well this confusing point. $\endgroup$
    – boyfarrell
    Commented Jan 6, 2015 at 19:01

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I'll upgrade my comment to an answer.

You are thinking about this in the wrong way.

If you operate a diode at a fixed point on the IV curve (that is by sourcing or sinking current as needed), then the device's resistance is constant. For example, you might want to do this with a PN junction solar cell; they are operated at the maximum power point.

However, this does not mean that the diode obeys Ohms Law, that is a property of the whole IV curve, not just a single point.

Ohms Law states that for any applied bias the resistance is constant. This is clearly false for a diode because if the bias changes, even by a little bit, the resistance changes exponentially (in forward bias).

I think the confusion is arising from the fact that you are mentally equating the band structure of a forward bias diode (at the flat band point) to that of a simple resistor. And based on this fact, extrapolating to the statement that the diode must be Ohmic.

tl;dr Any device held at a particular point on it's IV curve has constant resistance, this doesn't mean that all electronic components are resistors.

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