When you provide enough energy to electrons in the valence band, they absorb it and move into the conduction band. The electron that moves into the conduction band becomes what we call a free electron, and the space it leaves behind in the valence band is referred to as a hole. So, as a result, you end up generating two types of charge carriers at the same time: free electrons and holes.

Recombination of carriers

On the other hand, the recombination of carriers is the process in which free electrons and holes are removed. This happens when a free electron in the conduction band falls into a hole in the valence band, leading to the elimination of both the free electron and the hole.

Electron-Hole Pairs

When we supply external heat energy to a semiconductor, some of its valence electrons get excited and move up to the conduction band, one by one. This leaves behind empty spaces in the valence band, which we call “holes.” The number of electrons that move to the conduction band depends on how much heat energy we provide to the semiconductor.

If only one electron moves to the conduction band, it creates one hole in the valence band. So, each time we see an electron moving, a hole is formed. This pair of an electron and a hole is called an “electron-hole pair.” The hole, which is the empty space in the valence band, behaves like a positive charge. It has a strong attraction towards nearby electrons in the covalent bonds.