Properties: not as hard as metals, lower mp and bp but still solids, not ductile or malleable, poor electrical conductors - in many ways like a bad metal!
How do we explain this in terms of bonding?
Use band theory but with more basis orbitals : require two bands, one
filled (valence band) and one empty (conduction band).
If the band gap is large enough, the material is an insulator because no metal-like properties arise from a filled band.
If the band gap is on the order of thermal energies (small), then excitations of electrons from the valence band into the conduction band occurs.
where (Boltzmann population of the conduction band)
has the same properties as in a metal but the Boltzmann term dominates so the conductivity of a semiconductor increases with increasing temperature
Case 1: the impurity has a filled orbital in the semiconductor band gap
n-type carries are created thermally by excitation from the filled impurity orbital into the conduction band. The population of the valence band is undisturbed; since it is filled no charge can be transported through the valence band. The dopant is a species with more valence electrons than the semiconductor.
This is an n-type semiconductor.
Case 2: the impurity has an empty orbital in the band gap
At T > 0, the valence band is depopulated thermally creating p-type charge carriers (the electrons in the impurity orbital are not moblile, they are too far apart). The dopant must have fewer valence electrons than the semiconductor.
This is a p-type semiconductor
Valence Bond Theory description of semiconductors
n-type: extra electron on the dopant moves onto the host lattice and becomes mobile as a negative charge.
p-type: electron deficient dopant forms a new bond, requiring the use of an electron form the host, leaving a mobile positive charge.
This is a diode. Transistors are similar, but with three junctions (n-p-n or p-n-p)