Semiconductors
In general, semiconductors are inorganic or organic materials that can control their conduction depending on chemical structure, temperature, illumination, and the presence of dopants. The name semiconductor comes from the fact that these materials have electrical conductivity between a metal, like copper, gold, etc., and an insulator, like glass. They have an energy gap of less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between the valence band and conduction band where electron states are forbidden. In contrast to conductors, semiconductors’ electrons must obtain energy (e.g., from ionizing radiation) to cross the band gap and reach the conduction band. The properties of semiconductors are determined by the energy gap between valence and conduction bands.
Theory of Semiconductors
The theory of semiconductors is based on the behavior of electrons and holes in a crystalline lattice structure. This theory is known as the electronic band structure.
The electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called band gaps or forbidden bands).
Semiconductors have a valence band, which is the highest energy band that is completely filled with electrons, and a conduction band, which is the next higher energy band that is empty or only partially filled with electrons. The energy gap between the valence and conduction bands is called the band gap.
At absolute zero temperature, all of the electrons in a semiconductor are in the valence band and there are no free electrons in the conduction band. However, at room temperature or higher, some electrons in the valence band can be excited by thermal energy or by an external energy source, such as light or an electric field, and jump into the conduction band, leaving behind a hole in the valence band.
The movement of these free electrons and holes in the crystal lattice structure of the semiconductor can be described by the laws of quantum mechanics. The behavior of these charge carriers is influenced by factors such as the crystal structure, the doping concentration and type, the temperature, and the presence of impurities or defects in the crystal lattice.
Intrinsic semiconductors have a perfectly balanced number of free electrons and holes, and their conductivity is determined by the intrinsic concentration of free electrons and holes, which increases exponentially with temperature. Extrinsic semiconductors, which are doped with impurities, have a much higher concentration of free electrons or holes, which dramatically increases their conductivity and makes them useful for electronic devices.
