The Energy Band Model
The Valence Band:
The valence band is the lower band of allowed states.
In the drawings it is depicted by a line labeled by Ev
which represents the highest energy state in the valence band. Since electrons
have a tendency to fill the lowest available energy states,the valence band
is always nearly completely filled with electrons, especially as the temperature
falls toward 0K. As the temperature rises or light is introduced,
electrons can absorb the energy and leave the valence band to
rise up to the conduction band. When an electron gains enough energy, greater
than the band gap energy, and gets to the upper band, it is free to move,
becoming a carrier and therefore increasing the conductivity of the semiconductor.
When electrons leave the valence band they leave behind a hole
which can move about the crystal, also adding to the conductivity.
To see what this looks like, take a look at the
demo of a bond breaking
and entering the conduction band leaving a hole behind in the valence band.
To run the demo, you will need the
plug-in for your browser. It should run properly on the Vectras, but
it will not run on the Sun workstations. After you're done viewing, use
the "back" button on your browser to return to this page.