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Conceptual Physics Study Notes & Questions: Semiconductors (Chap. 25) 1) Pure semiconductors are good insulators, although some thermally induced conductivity occurs. Their valance electrons’ orbitals almost, but not quite, mix with neighboring atoms (which would allow currents to flow.) However if tiny amounts of alternative atoms are introduced into the semiconductor crystal, their valence orbitals do mix enough to allow current to flow. This is called doping (p532). 2) In n-type semiconductors, electrons carry current. In p-type semiconductors, holes—that is valence orbital vacancies—can perform the same function as carrying current. (p532). 3) Diodes are formed when n-type and p-type semiconductors are placed adjacent to one another. At the contact surface, electrons and holes, forming a depletion zone that has a small electric field across it, allow electrons to cross from n-to-p but not p-to-n. (NOTE: the current flow direction is actually in the opposite direction of electron flow.) 1 4) A diode rectifies AC current, only allowing current to flow in one direction (p534). 5) A transistor is a sandwich of p, n & p (or n, p & n) semiconductors. Normally, this construction is insulating—a voltage placed across the two end plates will not flow. A small metal electrode placed above the center material injects extra electrons in the center semiconductor which dramatically changes its conduction properties. Current now flows freely between the two end plates. By adjusting the center electrode voltage up and down, the large current between the two end plates can be adjust up and down. (p535) 6) In computer language, a bit is a “quantum of information.” In a physical system, like an integrated circuit (p538), a bit is a high or low voltage at a particular electrode position, at a particular moment in time. (p540). In a magnetic data storage medium, a bit of data is stored as a particular orientation of a magnetic field in a small patch of surface material. (p544) 2