There are two major types of semiconductors from which engineers can choose: a direct semiconductor and an indirect one. Both work in different ways with the conduction and valence bands, which are important in electrical energy, but both also attempt to find the minimal difference between the two bands. A direct semiconductor is used when the valence and conduction bands have the same momentum. This semiconductor is important in radiative recombination, in which an electron jumps from one band to the other.
In the science of energy, a direct semiconductor works with two bands. The lower band is known as the valance band. This represents the minimal amount of energy an electron has, because the electron is still bound to an atom; these electrons have very little momentum. The higher of the bands is the conduction band. This happens when the electron is freed from an atom and is able to freely move around in an atomic lattice, producing energy.
A direct semiconductor is used to mitigate energy from one band to another. To make the process of moving the electrons easier, the semiconductor finds the shortest way, or the path with the lowest amount of energy, for one electron to divide from the valence band and jump into the conduction band. The easiest path between the two is when the conduction band drops down and the valence band comes up, creating a small gap between the two energy bands.
The direct semiconductor can only be used if both bands perform this movement at the same time. This means both bands must have the same electron momentum. This is common mostly in low-power electronics and devices, because there is not much momentum required and the bands can commonly move in sync.
For the direct semiconductor to work, an electron needs to come up from the valence band. This causes it to rip a hole through the valence band, and the resulting energy becomes a photon. At the same time, the conduction band drops down to accept the energy, because it moves from the valence band and through the semiconductor.
An indirect semiconductor is similar to a direct one, but the difference is found in how the two bands move. In this schema, the valence and conduction bands move separately, so the valence will come up at one point, while the conduction will come down later. The indirect semiconductor takes the charge from the valence band, holds it and waits for the conduction band to drop to deposit the energy.