An Electron is defined as a negative charge or negative polarity atomic particle. The electrons are free or attached to the nucleus of an atom. They exist in the various energy level or energy band of an atomic particle or atom. The movement of electrons generates an electric current in the semiconductor material. The charge on an electron is termed as the unit electric charge.
When energy is supplied to a semiconductor, a valence electron is lifted to a higher energy level, the departing electron leaves a vacancy in the valence band. This vacancy is known as Hole. Thus, in other words, we can define hole as a vacancy left in the valence band because of the lifting of an electron from the valence band to a conduction band is called as a Hole.
Whenever, some external energy in the form of heat energy is supplied to a semiconductor the valence electrons are lifted up to the conduction band one after the other leaving behind a vacancy in the valence band called the Hole. The number of electrons to be lifted from the valence band to the conduction band depends upon the quantity of external energy supplied to the semiconductor.
If only one electron is lifted to the conduction band, then one hole is created in the valence band, thus, each time an electron-hole pair is formed. The vacancy created by the electron in the valence band known as hole acts as a positive charge. It has a strong tendency to attract the electrons from the nearby covalent bonds.
Recombination of Electron and Hole
When some external energy is supplied to a semiconductor, the electron of the valence band is lifted to the conduction band and become free leaving behind the holes in the valence band. The orbit of the conduction band in which free electrons are moving is larger as compared to the orbit of the valence band in which the holes are formed.
The one atom of the conduction band orbit may intersect with the hole orbit of another atom. As a result of this intersection, the conduction band electron falls into a hole. This merging of the free electron and a hole is called Recombination of Electron and Hole. When the recombination takes place, the hole does not move anywhere, it just disappears.
This recombination process takes place continuously in a semiconductor and fills every hole. However, the incoming heat energy keeps producing new holes by lifting valence electrons up to the conduction band forming electron-hole pair. The creation of electron-hole pairs and their recombination goes on continuously.
The average timing between the creation and the disappearing of an electron-hole pair is termed as Lifetime. The lifetime varies from nanoseconds to several microseconds depending upon the various factors such as shape, size, crystal structure of the semiconductor material.