An impurity atom in a semiconductor which can accept or take up one or more electrons from the crystal and become negatively charged. An atom which substitutes for a regular atom of the material but has one less valence electron may be expected to be an acceptor atom. For example, atoms of boron, aluminum, gallium, or indium are acceptors in germanium and silicon (illus.a), and atoms of antimony and bismuth are acceptors in tellurium crystals. Acceptor atoms tend to increase the number of holes (positive charge carriers) in the semiconductor (illus.b). The energy gained when an electron is taken up by an acceptor atom from the valence band of the crystal is the ionization energy of the atom. See also: Donor atom; Semiconductor

The potential difference at which an electrically stressed gas is tranformed from an insulator to a conductor. In an electrically stressed gas, as the voltage is increased, the free electrons present in the gas gain energy from the electric field. When the applied voltage is increased to such a level that an appreciable number of these electrons are energetically capable of ionizing the gas, the gas makes the transition from an insulator to a conductor; that is, it breaks down. The potential difference at which this transition occurs is known as the breakdown potential for the particular gaseous medium.

An electron tube in which a beam of electrons can be focused to a small cross section and varied in position and intensity on a display surface. In common usage, the term cathode-ray tube (CRT) is usually reserved for devices in which the display surface is cathodoluminescent under electron bombardment, and the output information is presented in the form of a pattern of light. The character of this pattern is related to, and controlled by, one or more electrical signals applied to the cathode-ray tube as input information. See also: Cathodoluminescence; Electron tube

Semiconductor devices wherein minority charge is stored in a spatially defined depletion region (potential well) at the surface of a semiconductor, and is moved about the surface by transferring this charge to similar adjacent wells. The formation of the potential well is controlled by the manipulation of voltage applied to surface electrodes. Since a potential well represents a nonequilibrium state, it will fill with minority charge from normal thermal generation. Thus a charge-coupled device (CCD) must be continuously clocked or refreshed to maintain its usefulness. In general, the potential wells are strung together as shift registers. Charge is injected or generated at various input ports and then transferred to an output detector. By appropriate design to minimize the dispersive effects that are associated with the charge-transfer process, well-defined charge packets can be moved over relatively long distances through thousands of transfers.

The branch of physics concerned with the motion of charged particles under the influence of electric and magnetic fields.

The electronic energy band of a crystalline solid which is partially occupied by electrons. The electrons in this energy band can increase their energies by going to higher energy levels within the band when an electric field is applied to accelerate them or when the temperature of the crystal is raised. These electrons are called conduction electrons, as distinct from the electrons in filled energy bands which, as a whole, do not contribute to electrical and thermal conduction. In metallic conductors the conduction electrons correspond to the valence electrons (or a portion of the valence electrons) of the constituent atoms. In semiconductors and insulators at sufficiently low temperatures, the conduction band is empty of electrons. Conduction electrons come from thermal excitation of electrons from a lower energy band or from impurity atoms in the crystal. See also: Band theory of solids; Electric insulator; Electrical conductivity of metals; Semiconductor; Valence band

An electrostatic potential that exists between samples of two dissimilar electrically conductive materials (metals or semiconductors with different electron work functions) that have been brought into thermal equilibrium with each other, usually through a physical contact. Although normally measured between two surfaces which are not in contact, this potential is called the contact potential difference. Its origin can be described in terms of the process necessary to bring the samples into equilibrium. Initially it is expected that mobile charge carriers (electrons or holes) will migrate from one sample to the other. If there is a net flow of electrons from material A to material B (see illustration), material B will become negatively charged and material A will become positively charged, assuming that they were originally neutral. This process is self limiting because a potential difference between the two samples will develop due to the charge separation and will grow to a value sufficient to stop further motion of the electrons from A to B.

A conducting disk with concentric inner and outer electrical contacts, which can be placed in a magnetic field parallel to its axis. The disk is named after O. M. Corbino, who in 1911 reported magnetoresistance measurements on several metals by using this configuration. In zero applied field the lines of current are simply radial (illus.a), but in the presence of an axial magnetic field they lengthen by spiraling (illus.b). This spiraling occurs because the geometry of the disk is such that the Lorentz force acting on the charge carriers is not counterbalanced by a Hall-effect electric field. The resistance of the disk increases as the field increases, largely as a result of the geometrical magnetoresistance effect associated with the lengthening of the current path.

A type of electrical conduction that generally occurs at or near atmospheric pressure in gases. A relatively strong electric field is needed. External manifestations are the emission of light and a hissing sound. The particular characteristics of the discharge are determined by the shape of the electrodes, the polarity, the size of the gap, and the gas or gas mixture.

A current-controlled switching device based on superconductivity for use primarily in computer circuits. The early version has been superseded by the tunneling cryotron, which consists basically of a Josephson junction. In its simplest form (see illustration) the device has two electrodes of a superconducting material (for example, lead) which are separated by an insulating film only about 10 atomic layers thick. For the electrodes to become superconducting, the device has to be cooled to a few degrees above absolute zero. The tunneling cryotron has two states, characterized by the presence or absence of an electrical resistance. They can be considered as the “on” and “off” states of the switch, respectively. Switching from on to off is accomplished by a magnetic field generated by sending a current through the control line on top of the junction. The device can switch in a few picoseconds and has a power consumption of only some microwatts. These properties make it an attractive switching device for computers, promising performance levels probably unattainable with other devices. See also: Josephson effect; Superconducting devices; Superconductivity