N-type and P-type crystals: In a pure silicon crystal each silicon atom (Si) is surrounded by four valence electrons. In the N-doped semiconductor, illustrated in the left-hand figure, some silicon atoms are replaced by arsenic atoms (As), which have one extra valence electron. In the P-doped semiconductor, illustrated in the right-hand figure, some silicon atoms are replaced by boron atoms (B), which have only three valence electrons, resulting in a deficiency, or hole. In N-type semiconductors the extra valence electrons move as free negative charges and in P-type semconductors the holes move as free positive charges.


Semiconductor diode: In the figure on the left, electrons and holes are attracted away from the P-N junction. The transition region, with a low concentration of charges, is a layer of high resistance and no charge crosses the junction, i.e., no internal current exists. With the battery reversed, as in the figure on the right, the concentration of charges in the transition region is increased and an internal current is created.

semiconductor, solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction; insulation). At high temperatures its conductivity approaches that of a metal, and at low temperatures it acts as an insulator. In a semiconductor there is a limited movement of electrons, depending upon the crystal structure of the material used. The substances first used for semiconductors were the elements germanium, silicon, and gray tin. It was found that the incorporation of certain impurities in them enhances their conductive properties. The impurities either add free electrons or create holes (electron deficiencies) in the crystal structures of the host substances by attracting electrons. Thus there are two types of semiconductor: the N-type (negative), in which the current carriers (electrons) are negative, and the P-type (positive), in which the positively charged holes move and carry the current. The process of adding these impurities is called doping; the impurities themselves are called dopants. Dopants that contribute mobile electrons are called donor impurities; those that cause holes to form are acceptor impurities. Undoped semiconductor material is called intrinsic semiconductor material. Certain chemical compounds, including gallium arsenide, indium antimonide, and aluminum phosphide are semiconductors. Semiconductors are used to produce such electronic devices as diodes, transistors, and computer memory devices. The field of solid-state physics includes the study of semiconductors. See also integrated circuit.

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