TABLE OF CONTENTS In Chap. 4 we studied electrons and holes located around defects. Since these electrons and holes are immobile they are known as bound electrons and holes, respectively. In contrast, electrons in the conduction band and holes in the valence band of a semiconductor can carry electrical current. Hence they are referred to as free carriers. In this chapter we will study the effect of an external electric field on free carriers in a semiconductor. The response of these carriers to an electric field depends on the field strength. We will first consider the case of weak electric fields, where the behavior of carriers can be described by Ohm's law. Under high electric fields, carriers in a semiconductor can acquire so much energy that their average kinetic energy becomes higher than that of the lattice. Such energetic electrons are known as hot electrons. It is very difficult to calculate their properties analytically, therefore our discussions of hot electrons will be qualitative.
Let F represent a weak external static electric field applied to a semiconductor. We can assume without loss of generality, that this semiconductor contains only free electrons (i. e., it is an n- type semiconductor). For simplicity, we will assume that the concentration of free electrons is low enough that we can neglect their interactions with each other (such as collision and screening effects). We will also neglect local field effects due to ionic charges, i. e. the field experienced by...
