Types of Semiconductors

An intrinsic semiconductor is a highly pure material with no impurities, such as pure silicon. At absolute zero, it behaves as a perfect insulator because all valence electrons are bound and the conduction band is empty. When temperature increases, thermal energy excites electrons from the valence band to the conduction band, creating electron-hole pairs that enable electrical conduction.

Doping introduces impurities into an intrinsic semiconductor to change its electrical properties. Pentavalent dopants donate excess electrons, creating N-type semiconductors with electrons as majority carriers. Trivalent dopants create holes as majority carriers, forming P-type semiconductors. This process fundamentally alters the material's conductivity and Fermi level position.

The Fermi level describes the probability of electron occupancy at different energy levels at thermal equilibrium. In intrinsic semiconductors, it is positioned at the midpoint of the band gap. In N-type semiconductors, the Fermi level shifts near the conduction band, while in P-type semiconductors, it shifts towards the valence band, reflecting the dominant charge carrier type.

Electron-hole pairs (EHPs) form when thermal energy excites electrons from the valence band to the conduction band, leaving holes behind. Electrons move freely in the conduction band while holes act as positive charge carriers in the valence band. EHP creation enables electrical conduction in semiconductors and their generation and recombination rates must balance at thermal equilibrium.

Intrinsic carrier concentration (ni) is temperature-dependent and increases as temperature rises. At higher temperatures, more thermal energy excites electrons to the conduction band, increasing the number of free electrons and holes. This relationship is expressed mathematically using band gap energy, temperature, and material constants, directly impacting the semiconductor's conductivity.

N-type semiconductors are created by doping with pentavalent impurities, which donate excess electrons as majority carriers. P-type semiconductors result from trivalent dopants that create holes as majority carriers. The Fermi level in N-type materials shifts toward the conduction band, while in P-type materials it shifts toward the valence band, determining their electrical behavior.

At thermal equilibrium, electron-hole pairs are generated at rate gi and recombine at rate ri. These rates must be equal to maintain a stable carrier concentration and prevent continuous changes in the semiconductor's electrical properties. The recombination rate depends on the product of electron and hole concentrations, ensuring the system remains in balance.