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 I. A material exhibiting such behaviour is GaAs.