Semiconductor Material

a semiconductor used in the manufacture of electronic instruments and devices. Crystalline semiconductor materials are predominantly used in semiconductor electronics. The crystal structure of most such materials has the tetrahedral coordination of atoms that is characteristic of the diamond structure.

Selenium has played an important role in the development of semiconductor technology. For a long time selenium rectifiers were the principal semiconductor devices in use.

As of the early 1970’s, the most widely used semiconductor materials were silicon and germanium. They are usually produced in the form of large single crystals doped with various impurities. Doped Si single crystals with a resistivity of 10^-3- 104 ohm-cm are obtained primarily by the Czochralski method, in which the crystal is pulled from a melt. Doped Ge single crystals with a resistivity of 0.1–45 ohm-cm are also produced by zone refining. As a rule, impurity atoms of group V of the periodic table (P, As, and Sb) impart n-type conductivity to silicon and germanium, and impurity atoms of group III (B, Al, Ga, and In) impart p-type conductivity. Si and Ge are commonly used to fabricate, for example, semiconductor diodes, transistors, and integrated microcircuits.

Chemical compounds of the type A^III Bv—that is, compounds of group III elements with group V elements—make up a large class of semiconductor materials. This class includes arsenides, phosphides, antimonides, and nitrides, for example, GaAs, InAs, GaP, InP, InSb, AIN, and BN, The compounds are obtained by various techniques for producing single crystals from the liquid and gas phases. The synthesis and growth of single crystals are usually carried out in closed vessels made of high-strength high-temperature chemically inert materials, since the pressure of the saturated vapor over a melt of such elements as P and As is comparatively great. Impurities of group II elements generally give these semiconductor materials p-type conductivity, and group IV elements impart n-type conductivity. The materials of this class are used primarily in semiconductor lasers, light-emitting diodes, Gunn diodes, and photomultipliers.

They are also used as film radiation detectors in the X-ray, visible, and infrared regions of the electromagnetic spectrum.

The most widely used type A^IIB^VI semiconductor materials are the compounds ZnO, ZnS, CdS, CdSe, ZnSe, HgSe, CdTe, ZnTe, and HgTe. Materials of this type are obtained principally by means of chemical reactions in the gas phase or by fusion of the components. Their resistivity and type of conductivity are determined not so much by the doping impurities as by the characteristic structural defects associated with the deviation of the materials from stoichiometric composition. The use of semiconductor materials of this type is connected primarily with their optical properties and photosensitivity. For this reason, the materials are used in such devices as photoresistors, photocells, electron-beam devices, infrared detectors, and light modulators.

Other semiconductor materials include some amorphous vitreous chalcogenide systems—such as alloys of P, As, Sb, and Bi with Ge, S, Se, and Te—and oxide systems—such as V₂O₅•P₂O₅•R_xO_y, where R are metals of groups I-IV, x is the number of atoms of the metal, and y is the number of oxygen atoms in the oxide. Such materials are used primarily as optical coatings in instrumentation.

The properties of semiconductor materials vary over a broad range with changing temperature. They also change under the action of, for example, electric and magnetic fields, mechanical stresses, and irradiation. This variation is made use of in designing various types of sensors.

Semiconductor materials are characterized by the following basic parameters: resistivity, type of conductivity, width of energy gap, concentration of charge carriers, carrier mobility, effective mass of carriers, and carrier lifetime. Some characteristics of semiconductors, such as energy-gap width and effective mass of the carriers, are relatively independent of the concentration of chemical impurities and the degree of perfection of the crystal lattice. Many parameters, however, are determined almost entirely by the concentration and nature of the chemical impurities and structural defects. (Some physical properties of important semiconductor materials are given in Table 1.)

In electronic devices, semiconductor materials are used both in the form of bulk single crystals and in the form of thin single-crystal and polycrystalline films applied to different substrates, such as insulating or semiconductor substrates. Such films range in thickness from fractions of a micron (μ) to several hundred μ. The semiconductor materials used in electronic equipment must have certain electrical and physical properties that are stable over time and resistant to ambient conditions during operation. Of great importance are the uniformity of the properties of semiconductor materials within a single crystal or a film and the degree of perfection of the crystal structure—for example, the dislocation density and the concentration of point defects.

Because of the high requirements for purity and structural perfection of semiconductor materials, the technology for the production of such materials is extremely complicated. High stability of such production conditions as constancy of temperature, flow rate of the gas mixture, and duration of the process is necessary. In addition, special conditions must be observed; for example, a certain level of cleanness must be maintained in the equipment and work area—no more than four dust particles larger than 0.5 μ, are allowed per liter of air. Depending on the size of the single crystals and the type of the semiconductor material, the length of the process of single-crystal growth ranges from a few tens of minutes to several days. The processing of semiconductor materials under industrial conditions involves cutting the materials with a diamond tool, grinding and polishing the surfaces of the materials with abrasives, heat treatment, and etching with alkalies and acids.

Control of the quality of semiconductor materials is extremely complicated and varied and is accomplished by means of special equipment. The main controllable parameters are chemical composition, type of conductivity, resistivity, carrier lifetime, carrier mobility, and level of doping. Optical, spectroscopic, mass-spectrometric, and activation techniques are normally used to analyze the composition of semiconductor materials. Electrical and physical characteristics are measured by probe methods or through the Hall effect. The perfection of single-crystal structure is investigated by X-ray diffraction techniques and optical microscopy. The thickness of layers is measured either by noncontact optical methods or by layer abrasion methods.