Ohm's law (R=V/I) is used very often in electronics. However, do not always use it since it is NOT always true. For Ohm's law, we assume that the conduction electrons in the metal are free to move, and electrons collide only with atoms of the metal but not with one another. Without electric field in the conductor, electrons move thre in random directions with an approximate speed of 1.6*106 m/s. When we apply an electric field to the conductor, the electrons modify their random motions slightly and they drift very slowly– in a direction opposite to the field. The average drift speed is about 5*10–7 m/s. This number is very small compared to 1.6*106 m/s, and that is when Ohm's law works. The materials that follows Ohm's law (typicaly called "ohmic conductors") will therefore have a `rho` value (resistivity) that is independent of the electric field being applied. For semiconductors, these rules don't work, and so they don't obey Ohm's law.
[Be aware that `rho=m/(e^2ngamma) ]`