When energy is supplied to a semiconductor crystal, a single event of covalent bond breaking generates two charge carriers: an electron in the conduction band and a hole in the valence band (as shown in the figure).
The electron and hole are produced simultaneously as a pair, a process known as electron-hole pair generation.
This process can be represented by the following equation:
Covalent bond + Thermal energy → (Electron + Hole) pair
At any temperature T, the number of electrons generated would be equal to the number of holes produced. If n denotes the concentration of electrons in the conduction band and p is the concentration of holes in the valence band, then
n = p
After generation, the charge carriers move independently. Electrons move in the conduction band, while holes move in the valence band. Their motion is random within their respective bands, as long as no external electric field is applied.
It is likely that an electron in the conduction band may lose energy due to collisions with other particles in the lattice and fall into the valence band (refer to the Figure).
When a free electron falls into valence band, it merges with a hole. This process is called recombination.
Electron + Hole → Covalent bond + Energy
In the process the electron-hole pair disappears and energy is released. The released energy is mainly in the form of thermal energy.
At a steady temperature a dynamic equilibrium exists which balances the two processes of electron-hole pair generation and electron-hole recombination.
Just as thermal energy generates electron-hole pairs, light radiation can also produce electron-hole pairs in a semiconductor.
Both electrons and holes are carriers in semiconductors and participate in the conduction of electricity.
