We have used energy bands and gaps to explain the spectral properties of an incandescent lamp and LEDs. Recall that the conduction and valence energy bands and the resulting energy gap result from interactions of numerous atoms that are very close together - a situation that occurs in solids. As we have seen, the energy gap of the solid materials (called semiconductors) that make up the LED determine the energies (and thus, colors) of light emitted by the LEDs.
A solid can be a pure material in which all atoms are the same element. As a result, each nucleus of the atom contained in this solid has the same electrical charge. Thus, each atom in this solid has identical properties. The interactions among these atoms create the energy bands and gaps that we have studied. Modern technology can create materials that are very close to being all identical atoms. These pure materials have light emitting properties much like we have studied in previous activities.
For today's technology pure materials are not the most valuable. Instead, a wide range of devices --- from LEDs to computer chips --- use almost pure materials into which a small quantity of a different element --- an impurity has been introduced. Then materials with different impurities are joined.
Suppose we start with a pure material and add atoms of a different element. These different elements will have a different number of electrons than the atoms of the original material. We place the impurities into two groups:
Both the donors and acceptors have zero electrical charge. They have more or less charge in the nucleus to balance the more or fewer electrons.
Example
Most semiconductors are made from silicon that has an impurity introduced. Silicon has four valence electrons; each atom of silicon can contribute four electrons to creating bonds with neighboring atoms.
Suppose we introduce phosphorus as an impurity. Phosphorus has five valence electrons - four bond with neighboring silicon atoms and one electron is left over. Phosphorus is a donor atom. The energy levels of the solid shift slightly in the negative direction.
Arsenic has three valence electrons. The effect is opposite of the phosphorus, as arsenic is an acceptor atom. The energy levels shift in the positive direction.
Only a small amount of impurity atoms are required to shift the energy levels. One impurity atom in every million silicon atoms is enough to make a difference.
Common Names
n-type material: extra electrons
p-type: shortage of electrons
Real semiconductor chips (including LEDs) are made of n-type
and p-type materials. Simply, we take one of each type and stick
them together.