CHM 501 Lecture

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)