Simple “flattened” model of Si crystal

At 0K:

•All bonds are satisfied –>all valence electrons engaged in bonding.

•No “free”electrons

At finite temperature

 

•Finite thermal energy

•Some bonds are broken

•“free” electrons –Mobile negative charge, -1.6 x 10-19C

•“free” holes –Mobile positive charge, +1.6 x 10-19C

Caution: picture is misleading! Electrons and holes in semiconductors are “fuzzier”: they span many atomic sites