Ohm’s law has been found valid over a large class
of materials, there do exist materials and devices used in electric circuits
where the proportionality of V and I does not hold. Such conductor are called

**non Ohmic conductors**. The deviations broadly are one or more of the following type V ceases to be proportional to I
The relation between V and I depends on the sign
of V. In other words, if I is the current for a certain V, then reveres the direction
of V keeping its magnitude fixed, does not produce a current of the same
magnitude as I in the opposite direction.

The relation between V and I is not unique, i.e.,
there is more than one value of

*V*for the same current**. A material exhibiting such behaviour is GaAs.***I*
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