Field effect is a principle that refers to the modulation of conductance of a material through the use of an external electric field.

Overview

In an insulator, most atoms hold on to their electrons tightly. Contrary to an insulator, in a conductor, the valence electrons of the atoms are loosely bound and are free to roam through the lattice of the material. Those free electrons are always in random and ccontinuous motion due to the thermal energy of the conductor.

As they travel in random motion, free electrons collide with each other. In between collisions, electrons acquire relatively high velocity. However, due to their random motion, those charge carriers continue to suffer collisions so frequently that the net flow in any particular direction is roughly zero. That is, those electrons do not go anywhere.

The average distance traveled by those free electrons between any two successive collisions, or the mean free path (, [cm]), can be calculated as the product of the time between collisions (, [s]) and the thermal velocity (, [cm/s]). For example, silicon at ambient temperature (somewhere around = 10^7 cm/s) with = 0.1ps would mean the average distance traveled is 10nm.

When an external electric field is applied to the conductor, that is when some potential difference is applied across the semiconductor, each of the free electrons in the conductor now experience a force in the direction opposite to the direction of the electric field.

That is, when under the effect of an electric field, the free electrons now flow in the a direction that is opposite to the direction of the electric field, in addition to their thermal velocities.