While it is easy to understand an excited electron in a single atom, it is important to note the context of electron excitation in a solid such as a semiconductor. Common semiconductor materials such as silicon (Si on the Periodic Table of Elements) have four valence electrons in their valence shell (the outermost orbitals), and this means that in a slab of silicon material all the atoms bond to each other, sharing four electrons each (observe the center atom in the figure of Covalent Bonds). Silicon is therefore known as a Group IV semiconductor, because of its group on the Periodic Table and its number of valence electrons.
When we think of electron excitation in semiconductor materials such as Si, we must take into account these bonds. Here are some points regarding the bond model of a group IV semiconductor:
- When the temperature or provided energy is sufficiently high enough to excite some electrons, these electrons will be able to break a bond between two crystal atoms.
- Once “freed”, the electron can move to through the crystal and contribute to the current flow (to be discussed in Conduction in Semiconductors). As for the broken bond, it can be viewed as a hole to be filled in by a neighboring electron. In this way, not only do electrons flow, but also holes.
It is important to note that this bond model is not valid for all semiconducting material, however it is useful for illustrating the effects of impurities or dopants (see “Impurities and Dopants”) in a semiconductor.
Above (Ref. 1, M.A. Green, UNSW): In Figure (a) above, the bonds between silicon atoms are unbroken. In (b), however, sufficient energy was given to break a bond, resulting in an electron moving freely in the semiconductor and the hole, or broken bond, moving the opposite direction.
1. Green, Martin A. Solar Cells: Operating Principles, Technology, and System Applications. Englewood Cliffs: Prentice-Hall, Inc., 1982. Full book ordering information at www.pv.unsw.edu.au.