An intrinsic semiconductor is a semiconductor in which no other material is intentionally doped (similar to mixing). Example: Si, Ge. Notes: 1. It behaves as an insulator at absolute zero. 2. Electrons are excited by thermal energy. 3. They are different from pure semiconductors and may consist of some level of impurities. The conductivity of intrinsic semiconductor is more than that of a pure semiconductor as the impurities provide a few energy levels in the bandgap. Note: Pure semiconductors are semiconductors that have no impurities. Ideally, no semiconductor is pure in nature.
In its crystalline structure, Si and Ge tends to share one of its valence electrons and also take share of one electron from its neighbour atoms. As temperature is increased, some of the electrons break free and behave as conduction electrons. This creates a vacancy in the bond and creates a positive charge also known as a hole. It behaves as an apparent free charge similar to an electron and contributes to conduction. Notes: 1. Holes do not actually move in the semiconductor. The movement of electrons from the valence band to the conduction band and neighbouring atom can be viewed as movement of hole. 2. Mobility of holes is less than electrons.
Importance of holes
When an electron jumps from one atom to another atom with an available hole via the conduction band, it appears as if a hole has moved from the second atom to the first. Hence, holes also contribute to conduction in a semiconductor.
Total current in a semiconductor
Total current in the semiconductor is given by the sum of electron current and hole current in a semiconductor. I=Ie+Ih Note: Since the electrons and holes move in opposite direction and their charges are opposite, the net effect is the adding up of the two currents.
Generation or recombination of holes and electrons
Generation: When energy is supplied from external sources, electron jumps into the conduction band. This process is hence associated with the generation of an electron and a hole. Recombination: During the process of conduction of semiconductor, some electrons in the conduction band fall back to the valence band. This process is called recombination of an electron and a hole. Note: At equilibrium, rate of generation equals the rate of recombination of electron-hole pairs and net current is zero.
Law of Mass Action (Intrinsic Semiconductors)
According to the mass-action law: nenh=ni2 where ne: Electron concentration nh: Hole concentration ni: Intrinsic concentration For intrinsic semiconductors, ne=nh=ni