One of the properties that define semiconductors is its ability to become a conductor under certain conditions. In conductors, electrons flow freely; however, in semiconductors, electrons can flow freely only once excited to a certain energy level. Before we understand these special “conditions”, we must understand how electrons are actually excited.
Above: An electron (the red and yellow particles) around the nucleus of an atom are excited to a higher energy state, or orbital, when it absorbs a photon, and is relaxed to a lower energy state when it emits a photon.
Borrowing an analogy from Martin Green, we can picture an atom as a parking structure, with each level being an energy state (or orbital). When all the parking spots on the lower level are filled, there is no room for the electrons to move. However, when one car moves to a higher level (when an electron is struck by a photon of sufficient energy), there is more room on both the higher and lower levels for cars (electrons) to move around. The vacancies left behind on the lower level by electron excitation are called holes. These travel through a semiconductor just as electrons travel, and are thus also treated as charge carriers, only with a positive charge.
Above (Ref. 1, M.A. Green, UNSW): In Figure (a), the lower level represents the valence shell. When the electron (car) is excited, it moves to the next level in the “garage”, as in (b). The vacancy, or hole, then travels in lower level as the electrons move to fill the space, and the electron can move freely in the upper level.
Since free electrons and holes are generated simultaneously, a pure (intrinsic) semiconductor must have an equal number of free electrons and holes at all times. The concentration of free electrons (or holes) at any given time will be constant. This is known as the intrinsic concentration, ni discussed in Intrinsic Carrier Concentration.
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.