Semiconductor

I

Introduction

Semiconductor, material able to conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminium are excellent conductors, but such insulators as diamond and glass are very poor conductors (see Insulation). At low temperatures, pure semiconductors behave like insulators. Under higher temperatures, or with the addition of impurities, or in the presence of light, the conductivity of semiconductors can be increased dramatically. The physical properties of semiconductors are studied in materials science and condensed-matter physics.

II

Conduction Electrons and Holes

The common semiconductors include chemical elements and compounds such as silicon, germanium, selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current. In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal together. (See Chemical Reaction). These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or “holes”, are left behind that also contribute to the flow of electricity. (These holes are said to be carriers of positive charge.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole pair is called the energy gap.

III

Doping

Free carriers of electrical charge can be introduced by adding impurities to, or “doping”, the semiconductor. The difference between the number of valence electrons of the doping material, or dopant (either donors or acceptors of electrons), and that of the host gives rise to negative (n-type) or positive (p-type) carriers of charge. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n-type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra electrons with negative charge. In p-type silicon, atoms with three valence electrons such as aluminium (Al) lead to a deficiency of electrons, or to holes, which act as carriers of positive charge. The extra electrons or holes can conduct electricity.

IV

p-n Junction

When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction. (A diode is a two-terminal device that has a high resistance to electric current in one direction but a low resistance in the other direction.) The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others. See Electronics; Rectification; Solar Energy.

Semiconductor devices have many applications in electrical engineering. Microelectronic engineering developments have yielded small semiconductor chips containing millions of transistors. These chips have made possible a high degree of miniaturization and complexity of electronic devices. More efficient use of such chips has been developed through what is called complementary metal-oxide semiconductor circuitry, or CMOS, which consists of pairs of p- and n-channel transistors controlled by a single circuit. Advanced layered semiconductor materials can be made using techniques such as molecular-beam epitaxy.

See also Computer; Integrated Circuit; Microprocessor.