Why diodes and other electronic devices does not obey ohms law?

Answer 1

Acc to ohm's law-" current flowing in a conductor is directly proportional to the voltage applied across it till its temperature , pressure and other physical parameters are kept constant".

This is the major factor ,current remains proportional to applied voltage till temperature is constant and while dealing with semiconductor devices like diodes and transistor , its temperature changes.

When current flows within a semiconductor, due to the motion of electrons and collisions in the it, heat is produced and temperature of the semiconductor increases.

As semiconductor have negative coefficient of resistance therefore its resistance starts decreasing with the increase in temperature. Hence with the decrease in resistance, its current starts increasing and device shows nonlinear behaviour.

that's why ohm's law is not valid for semiconductor.

while handling conductors, its temperature also changes as current flows from it but up to a certain range there is not an appreciable change in its resistance and it shows linear behaviour.

Answer 2

Ah, but they do, but in their own ways. Take for example a semiconductor (silicon) diode. It has a knee at about 700 mV. between zero and 700 mV it exhibits a high resistance, consequently little current flows due to Ohms law. Above 700 mV the slope resistance is much lower, so lotsa current flows due to Ohms law.

The 700 mV knee is due to the behaviour of electrons and holes in the junction region. All semiconductor devices exhibit this phenomenon in one way or another if they employ junctions of differently doped material.

In mathematical terms, a diode is a non-linear device, because the characteristic function of a diode is not a linear function (the graphs doesn't form a straight line). Ohm's law applies to every single point on that curve.