Semiconductors Energy Level Diagrams
Description:
Energy Level
In an isolated atom, the energy processed by the electrons in the same orbit is almost equal.
When the atoms are placed in a crystal form the energy level of each electron changes due to the effect of other closely placed atoms.
There are two types of electrons present in an atom
Valence electron − An atom requires eight electrons in the outer most orbit to be stable. So the atom shares electrons with another atom to attain stability, this is known as valence fulfilment. In valence fulfilment the electron that is shared between the two atoms is known as valence electron.
Free electron − If an electron receives energy externally such as due to heat the electron moves out of the valence band and becomes free. The free electron has a greater energy than the valence electron.
When electric current is passed through the material the free electrons move towards the positive direction of current resulting in conduction of current through the material.
In crystal formation the energy level of electron can be categorised into two distinct energy levels or energy bands.
Valence Band − Valence band contains valence electrons. The valence band can be completely filled with electrons or sometimes partially filled with electrons but it is never empty. As these are valence electrons they are not affected by the electric filled.
Conduction Band − Conduction band contains free electrons. It can be empty or partially filled with electrons. As these are free electrons they conduct electricity through the material.
Forbidden Band / Energy Gap (Eg) − The forbidden band is completely empty as there are no electrons in it. To move an electron from the valence band to the conduction band an energy equal to the energy gap is required.