2.1 Electrons and Holes

We recall from the Periodic Table of Elements that silicon is classified as a semiconductor and it is widely used in the fabrication of modern technology electronic devices. Silicon has four valence electrons ^[*] and Figure 2.1 shows a partial silicon crystal structure in a two-dimensional plane where we observe that atoms combine to form an octet of valence electrons by sharing electrons; this combination is referred to as covalent bonding.

Figure 2.1: Partial silicon crystal structure

Thermal (heat) energy can dislodge (remove) an electron from the valence orbit of the silicon atom and when this occurs, the dislodged electron becomes a free electron and thus a vacancy (empty) space is created and it is referred to as a hole. The other electrons which stay in the valence orbit are called bound electrons. Figure 2.2 shows a free electron that has escaped from the valence orbit and the hole that has been created. Therefore, in a crystal of pure silicon that has been thermally agitated there is an equal number of free electrons and holes.

Figure 2.2: Free electron and...