A simplified energy band diagram used to describe semiconductors. Shown are the valence and conduction band as indicated by the valence band edge Ev and the conduction band edge Ec. The vacuum level, EVACUUM and the electron affinity, c are also indicated on the figure.
The diagram identifies the almost-empty conduction band simply by a line which indicates the bottom of the conduction band and is labeled Ec. Similarly the top of the valence band is indicated with a line labeled Ev. Note: The actual bandstructures of semiconductors is more complex than the reader is lead to believe by the discussion above. So, semiconductors distinguish themselves from metals and insulators by the fact that they contain an "almost-empty" conduction band and an "almost-full" valence band. This also means that we will have to deal with the transport of carriers in both bands.
To facilitate the discussion of the transport in the "almost-full" valence band we will introduce the concept of holes in a semiconductor. It is important for the reader to understand that one could deal with only electrons (since these are the only real particles available in a semiconductor) if one is willing to keep track of all the electrons in the "almost-full" valence band.
The concepts of holes is introduced based on the notion that it is a whole lot easier to keep track of the missing particles in an "almost-full" band, rather than keeping track of the actual electrons in that band. We will now first explain the concept of a hole and then point out how the hole concept simplifies the analysis.
Holes are missing electrons. They behave as particles with the same properties as the electrons would have occupying the same states except that they carry a positive charge. This definition is illustrated further with the figure below which presents the simplified energy band diagram in the presence of an electric field.