Conduction and Band Theory

Band Theory
Band theory is an extension of MO(molecular orbital) theory.

The more atoms that join together in a molecule the more MOs are created.
With the number of atoms in a sample of metal (moles * Avogadro's number) a large number of MOs are formed and thus the energies of the MOs are so closely spaced that they form a continuum or band.

According to band theory the lower energy MOs are occupied by the valence electrons. These make up the valence band.
The empty MOs of higher energy make up the conduction band.

Conduction
Conductors
In conductors such as metals there is no gap between the valence band and the conduction band. This means that electrons can enter the valence band when they receive even a tiny quantity of energy. Once into an orbital in the conduction band electrons can move through the sample freely, which is why electrons in metals are considered to be completely delocalised.
When heated the movement of atoms (vibration) interferes with the flow of electrons and thus decreases the metals conductivity.

Semiconductors
In semiconductors there is an energy gap between the valence and conduction bands. This gap is relatively small however, so thermally excited electrons are able to cross it, allowing a small current to flow. For this reason heating semiconductors has the opposite effect to heating conductors in that it increases, rather than decreases, electron flow.

Insulators
In insulators the gap between bands is too large for electrons to cross, so no current is able to flow.

Doped Semiconductors
Doping semiconductors involves adding small amounts of other elements to them to increase or decrease the number of valence electrons in the bands. P has more valence electrons than Si, thus when Si is doped with P the valence electrons of the P atoms enter orbitals in the conductor band. This bridges the gap between bands and allows for some electron flow, thus increasing conductivity. This is known as an n-type semiconductor because there are extra negative charges (electrons) present.

Ga has less valence electrons than Si, thus when Si is doped with Ga some orbitals in the valence band are empty, creating positive holes. Electrons from Si atoms can migrate to these positive holes, leaving positive holes from whence they came. Conductivity has again been increased because electrons are now able to flow. This is known as a p-type semiconductor (p for positive holes).

p-n junction
A p-n junction is formed when a p-type semiconductor is placed next to an n-type. These allow current to flow through in only one direction. If the positive terminal of a battery is placed next to the p-type portion and the negative next to the n-type, the electrons from the n-type will flow across to the positive terminal and the positive holes will move towards the negative.
If the positive terminal is placed next to the n-type portion the electrons will enter directly and no current will flow across the junction. The same applies to the positive holes moving to the negative terminal.