Making Semiconductors more Conductive by introducing Partially Filled States
Dec 09, 2024
More partially filled states ⇒ better conduction as the electrons have more places to be.
We can either add electrons (into more bands) or remove them (from existing, filled bands).
Doping
(Assuming that the main atom in our lattice is silicon).
The idea is to replace some of the silicon atoms in our lattice with atoms with atoms with a different number of electrons in their valence shell.
The dopants must be similar to silicon, since they must fit within the lattice structure.
Adding Electrons into the Conduction Band (Donor Impurities)
If we manage to add some electrons into the conduction band, then the band has way more free spaces than electrons,
So the electrons can be transported easily between spaces ⇒ conduction.
If the dopant has more valence electrons than silicon then the extra electrons will be left with no space.
These electrons sit near the conduction band, ready to jump into the conduction band where they can move around freely.
This “sitting below the conduction band” brings the Fermi Level upward.
In this case, the primary charge carriers will be electrons, since they can move freely in the conduction bands. Hence, silicon doped with donors are called n-type semiconductors.
Eg: phosphorus (more electrons, weak enough to let go).
Remove Electrons from the Valence Band (Acceptor Impurities)
If we remove a few electrons from the valence band, then these are equivalent:
A full valence band minus a few electrons.
An empty valence band with a positive charge.
If the dopant has fewer valence electrons than silicon, then an empty space is freed up in the impure lattice.
This presence of empty spaces brings the Fermi Level lower.
This vacant space (hole) move around (in the sense that electrons move into it), hence this is a p-type semiconductor since it’s relatively positively charged (positive charge carrier).
Eg: boron (less electrons, strong enough to bind another).