Semiconductors

A semiconductor is a material whose electrical conductivity value falls between a conductor such as an iron rod and an insulator such as glass. Semiconductors are employed in the manufacture of various kinds of electronic devices such as diodes, transistors and integrated circuits. These devices have found wide applications because of their reliability, compactness, power, efficiency and low cost. They are widely used as power devices, optical sensors and light emitters including solid – state lasers. They have a wide range of current and voltage handling capacity and can be easily integrated into complex but readily manufacturable microelectronic circuits.

DOPING

In semiconductor production, doping is the process of intentionally introducing impurities into an intrinsic semiconductor for the purpose of modulating it’s electrical, optical and structural properties. The doped material is referred to as extrinsic semiconductor. Semiconductor devices can display a range of useful properties such as passing current more easily in one direction than the other, showing variable resistance and sensitivity to light or heat. Because the electrical properties of semiconductors can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching and energy conversion. The conductivity of silicon is increased by adding a small amount of pentavalent atoms such as arsenic, antimony and phosphorus or trivalent atoms such as boron, gallium and indium in a process known as doping which leads to the formation of extrinsic semiconductors. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is known as degenerate semiconductor.

INTRINSIC SEMICONDUCTORS

An intrinsic semiconductor also called an undoped or i- type semiconductor is a pure semiconductor with no significant impurities present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. In intrinsic semiconductors the number of excited electrons and the number of holes are equal. This may even be the case after doping the semiconductor though only if doped with donors and acceptors equally. The electrical conductivity of intrinsic semiconductors can be due to the crystallographic defects or electron excitation. In this semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band.

EXTRINSIC SEMICONDUCTORS

An extrinsic semiconductor is one in which impurities have been added or doped. During manufacture a trace element of a chemical called a doping agent has been added chemically to the crystal for the purpose of giving it different chemical properties than the pure semiconductor crystal which is intrinsic semiconductor. In an extrinsic semiconductor these foreign dopant atoms in the crystal lattice provide the charge carriers which carry electric current through the crystal. The doping agents used are of two types which result in two types of extrinsic semiconductors. An electron donor dopant is an atom which when added to the crystal releases a mobile conduction electron into the crystal lattice. An extrinsic semiconductor which is doped with electron donor atoms is called n-type semiconductor because the majority of the charge carriers in the crystal are negative. An electron acceptor dopant is an atom which accepts an electron from the lattice, creating a vacancy where an electron is called a hole which can move through the crystal like a positively charged particle. An extrinsic semiconductor which has been doped with electron acceptor atoms is called p-type semiconductor because the majority of charge carriers in the crystal are positive holes.

P- TYPE SEMICONDUCTORS

P-type semiconductors are created by doping an intrinsic semiconductor with an electron acceptor element during manufacture. The term p-type refers to positive charge carriers known as holes. When compared to n-type semiconductors the p-type semiconductors have a larger hole concentration than electron concentration. In p-type semiconductors holes are the majority carriers and electrons are the minority carriers. Dopants used for p-type semiconductors include boron, gallium and indium.

N-TYPE SEMICONDUCTORS

N-type semiconductors are created by doping an intrinsic semiconductor with an electron donor element during manufacture. The term n-type comes from the negative charge carriers known as electrons. In n-type semiconductors electrons are the majority charge carriers while holes are the minority charge carriers. Common dopants used for n-type semiconductors are phosphorus, arsenic and antimony.

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