3.6 Limitations of Ohm’s Law

Although 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. The deviations broadly are one or more of the following types:

• \(V\) ceases to be proportional to \(I\) (Fig. 3.5).

• 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 reversing the direction of \(V\) keeping its magnitude fixed, does not produce a current of the same magnitude as \(I\) in the opposite direction (Fig. 3.6). This happens, for example, in a diode which we will study in semiconductor chapter.

• The relation between \(V\) and \(I\) is not unique, i.e., there is more than one value of \(V\) for the same current \(I\) (Fig. 3.7). A material exhibiting such behaviour is GaAs.