Energy bands of semiconductors

Valence band 

• The energy band formed by a series of energy levels containing valence electrons is known as valence band.
• This band is always filled with electrons.
• This is the band of maximum energy.
• Electrons are not capable of gaining energy from external electric field.
• No flow of current due to electrons present in this band.
• The highest energy level which can be occupied by an electron in valence band at 0 k is called Fermi level.

Conduction band

• The higher energy level band is called the conduction band.
• It is also called empty band of minimum energy.
• In this band is partially filled by the electrons.
• In this band the electrons can gain energy from external electric field.
• The electron is the conduction bands are called the free electrons. They are able to move anywhere within the volume of the solid.
• Current flow due to such electrons.

Holes In Semiconductors

1. When an electron is removed from a covalent bond, it leaves a vacancy behind.
2. A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an electron.
3. Holes acts as virtual charge, although there is no physical charge on it.
4. Effective mass of hole is more than electron.
5. Mobility of hole is less than electron.