Semiconductors
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.
= ne
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
Doping
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.
Semiconductor junctions
This is a diode. Transistors are similar, but with three junctions (n-p-n or p-n-p)