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- An electronic semiconductor is typically a valence crystal whose conductivity depends markedly on temperature and on the presence of minute amounts of foreign impurities. The ideal crystal at the absolute zero is an insulator. When the valence bonds are completely occupied and there are no extra electrons in the crystal, there is no possibility for current to flow. Charges can be transferred only when imperfections are present in the electronic structure, and these can be of two types: excess electrons which do not fit into the valence bonds and can move through the crystal, and holes, places from which electrons are missing in the bonds, which also behave as mobile carriers. While the excess electrons have the normal negative electronic charge -e, holes have a positive charge, +e. It is a case of two negatives making a positive ; a missing negative charge is a positive defect in the electron structure. The bulk of a semiconductor is electrically neutral; there are as many positive charges as negative. In an intrinsic semiconductor, in which current carriers are created by thermal excitation, there are approximately equal numbers of excess electrons and holes. Conductivity in an extrinsic semiconductor results from impurity ions in the lattice. In n-type material, the negative charge of the excess electrons is balanced by a net positive space charge of impurity ions. In p-type, the positive charge of the holes is balanced by negatively charged impurities. Foreign atoms which can become positively charged on introduction to the lattice are called donors; atoms which become negatively ionized are called acceptors. Thus donors make a semiconductor n-type, acceptors p-type. When both donors and acceptors are present, the conductivity type depends on which is in excess. Mobile carriers then balance the net space charge of the impurity ions.
- John Bardeen: Semiconductor Research Leading the Point Contact Transistor. Nobel Prize Lecture (11 December 1956).
- One of the remarkable and dramatic developments in recent years has been the application of solid state science to technical developments in electrical devices such as transistors. The study of semiconductors led to the discovery of their useful properties and to a large number of practical applications. ... The semiconductor substances in most common use today are silicon and germanium. These elements crystallize in the diamond lattice, a kind of cubic structure in which the atoms have tetrahedral bonding with their four nearest neighbors. They are insulators at very low temperatures—near absolute zero—although they do conduct electricity somewhat at room temperature. ... somehow put an extra electron into a crystal of silicon or germanium which is at a low temperature ... The electron will be able to wander around in the crystal jumping from one atomic site to the next. Actually, we have looked only at the behavior of electrons in a rectangular lattice, and the equations would be somewhat different for the real lattice of silicon or germanium. All of the essential points are, however, illustrated by the results for the rectangular lattice.
- Richard Feynman: (1965). 14–1. Electrons and holes in semiconductors in Chapter 14. Semiconductors, The Feynman Lectures on Physics, Volume III, Quantum Mechanics
- A consequence of the discovery of electricity was the observation that metals are good conductors while nonmetals are poor conductors. The latter were called insulators. Metallic onductivity is typically between 106 and 104 (Ω cm)–1, while typical insulators have conductivities of less than 10–10 (Ω cm)–1. Some solids with conductivities between 104 and 10–10 (Ω cm)–1 are classified as semiconductors. ... semiconductors have an energy gap while semimetals and metals have no such gap. However, very impure semiconductors show a more or less metallic behavior and with many substances, the art of purification is not so far advanced that a distinction can easily be made. The transition between semiconductors and insulators is even more gradual and depends on the ratio of the energy gap to the temperature of investigation. Very pure semiconductors may become insulators when the temperature approaches the absolute zero.