SEMICONDUCTOR

any of a class of crystalline solids intermediate in electrical conductivity between a conductor and an insulator. Such a material can be treated chemically to transmit and control an electric current. Semiconductors are employed in the manufacture of various kinds of electronic devices, including diodes, transistors, and integrated circuits. The following article briefly treats semiconductors. Articles providing fuller treatment appear in the Macropaedia. For the electric properties of semiconductors, see Matter: Crystalline solids; for a description of semiconductor devices, see Electronics: Semiconductor devices. Semiconductor materials may be divided into two general groups: intrinsic and extrinsic. An intrinsic semiconductor exhibits a high degree of chemical purity (i.e., it has a ratio of impurities of only about one part in 1012). Its conductivity is poor and largely temperature-dependent. Some common intrinsic semiconductors are single crystals of silicon, germanium, and gallium arsenide. Such materials may be converted into the technologically more important extrinsic semiconductors by addition of small amounts of impurities, usually to a concentration of one part in 10 6. This process, known as doping, alters the electrical properties to produce much greater conductivity. For example, the atom of an intrinsic semiconductor such as elemental silicon has four electrons in its outermost shell. These electrons attach the silicon atom to its neighbouring atoms and are not free to move through the solid. Accordingly, pure silicon is a poor conductor. If phosphorus atoms with five outer electrons are substituted as an impurity for some of the silicon atoms, the fifth electron is not needed for binding to adjacent atoms and is free to move through the solid. Other types of impurity atoms, such as boron, have one less outer electron than does silicon. When they are substituted for some of the silicon atoms, each captures one electron from a neighbouring silicon atom, leaving an empty space. Such a hole behaves like a freely moving particle with a positive charge. The presence of these holes increases the ability of the silicon to conduct electric current. The substitution of as few as 10 atoms of boron per 1,000,000 atoms of silicon yields the resulting extrinsic silicon semiconductor. An extrinsic semiconductor is commonly classified as n- or p-type, depending on whether the impurity has an excess of negative charge (n-type) or a deficiency of negative charge (p-type). Under certain circumstances an intrinsic semiconductor may also be classified an n-type or p-type, since the mobility of electrons and holes can be different even though their carrier densities are nearly equal. In cases such as these, the conduction process is dominated by the carriers with the highest mobility.