Semiconductors, in the science and technology of materials, are materials, belonging to the category of semimetals, which can have a resistivity higher than that of conductors and lower than that of insulators; resistivity depends directly on temperature.
The conductivity of semiconductors grows with temperature (i.e. the temperature coefficient of resistivity is negative) in the temperature range of normal interest, it grows with the illumination of the material and can be modified even by several orders of magnitude by appropriately varying the concentration of chemical impurities present in the material. Some semiconductor materials – at first germanium, soon after silicon and for some important applications, gallium arsenide – are the protagonists of advanced technological developments. They constitute the microscopic circuits engraved on chips: microprocessors, semiconductor memories, electrooptic amplifiers, microwave integrated circuits, semiconductor lasers, etc.. Another potentially important field is photovoltaic conversion for the production of electricity.
They are the basis of all major solid-state electronic and microelectronic devices such as transistors, diodes, and light-emitting diodes (LEDs). Semiconductor physics is the branch of solid state physics that studies the electrical properties of semiconductors.
The properties of these materials become interesting if they are properly doped with impurities. Their characteristics such as resistance, mobility, concentration of charge carriers are important to determine the field of use. The response of a semiconductor to a forcer depends on its intrinsic characteristics and on some external variables such as temperature.
One of the first experimental observations on the characteristics of semiconductor materials can be attributed to M. Faraday, who in 1833 found that silver sulfide has a negative temperature coefficient of resistivity, unlike the usual conductors, in which resistivity increases with temperature. In 1839 A. E. Becquerel made known his findings on the photovoltaic effect; in 1873 W. Smith showed that the resistance of selenium could be reduced by illuminating the material. Subsequently F. Braun noted that the resistance of metal-galena, or metal-pyrite contacts, depends on the magnitude and sign of the applied voltage; A. Schuster made similar observations for contacts between a copper oxidized conductor and a non-oxidized one. The first selenium photocell was built by W. G. Adams and R. E. Day in 1876, and the first selenium rectifier may be attributed to C. E. Fritts who built it in 1883.
In 1879 E. H. Hall discovered in metals the effect that bears his name, the discovery had great importance for the study of the mechanism of conduction in solids and, when it was applied to semiconductor materials, in the thirties appeared significant differences compared to what had been observed until then with metals: in particular some semiconductor materials behaved as if the current was carried by positive charges instead of negative (free electrons) as in metals. In any case the number of charge carriers (responsible for the flow of electric current inside the materials) per unit volume was much lower in semiconductors than in metals and increased rapidly with temperature, while in metals this number is practically constant and generally equal to one electron per atom.
Two semiconductor elements, silicon and germanium, were studied extensively at Bell Telephone Laboratories and Purdue University; the efforts were concentrated on germanium for its lower melting point and after a decade of research for the purification of this material and countless experiences, J. Bardeen and W. H. Brattain succeeded in finding the best solution. H. Brattain succeeded to realize the first transistor in 1947. Their work was soon followed by the hypothesis that charge carriers of opposite sign to those characterizing the material could be injected into germanium and transferred from one electrode to another. This concept was emphasized in a paper by W. Shockley, G. L. Pearson, and J. R. Haynes, published in 1949.
Most of the possibilities of semiconductors were outlined explicitly or implicitly in the synthesis of device theory published by Shockley at the same time. Developments in semiconductor materials have been directed toward improving their performance, particularly switching speed, an essential parameter of two-state (two voltage levels) based logic circuits. It was also sought to expand the possibilities of application, in particular in relation to the emission and absorption of electromagnetic radiation, with a view to this goal was synthesized in the fifties the gallium arsenide that is the constituent of LEDs and semiconductor lasers (for example, compact discs). Finally, manufacturing techniques were refined (essentially planar technology), which allowed the industrial construction of chips with millions of transistors and, at laboratory level, chips with a million microlasers.
Subsequently, they began to study “quantum” semiconductor devices using very thin films of atoms (mono- or diatomic layers as opposed to the traditional thinner layers consisting of 3-400 atomic planes). At these levels quantum effects begin to dominate over “classical” behaviors of solid-state physics. In particular, it is possible to affect band characteristics with potential advantages in miniaturization and speed. Researches – in addition to germanium, silicon and gallium arsenide – are also addressed to other basic materials and in particular to diamond that, in the presence of impurities, is semiconductor.
Simple semiconductors and composite semiconductors
Among the materials that have semiconductor properties are silicon, germanium, indium antimonide, gallium arsenide and also rare earth elements and their compounds. To the simple semiconductors, that is made of a single species of atoms, belong elements of the IV group of the periodic system, while the compound semiconductors are normally formed by combinations of elements of the III and V group, or II and VI group.
The crystalline lattice of silicon is the best known and most widely used semiconductor element. Each silicon atom divides its four valence electrons with the four adjacent atoms, thus forming regions of relatively high electronic density (two valence electrons for each region) that connect the various atoms of the crystal and form the so-called covalent bonds. The crystal lattice can be represented by a two-dimensional model in which the circles represent the silicon atoms deprived of the four valence electrons and therefore contain a positive electric charge equal to four times that of the electron.
At the temperature of absolute zero the valence electrons, engaged in covalent bonds, cannot receive energy due to thermal agitation; therefore, if they do not receive energy from outside, they cannot go out from their assigned regions, corresponding to the lowest values of potential energy. At absolute zero, silicon (like any other semiconductor) therefore behaves as an insulator. The situation changes at much higher temperatures, for example at room temperature, because the equilibrium condition of the regions that form the covalent bonds can be destroyed by thermal agitation: in fact some of the electrons that form the covalent bonds can acquire enough energy to escape from the original region and move freely in the other regions of the lattice. It is possible to “break” some of the covalent bonds.
The destruction of a covalent bond gives rise to two distinct processes: the electron that has acquired sufficient energy to break the covalent bond moves freely through the crystal lattice; the broken bond can be restored by a valence electron engaged in another bond with one of the adjacent atoms; this electron neutralizes an atom temporarily deprived of an electron, but at the same time leaves behind a new electron vacation, i.e. a gap (or hole). While the first process is similar to the one which is responsible for conduction phenomena in metals, the second one represents a different kind of electronic movement which has the effect of moving a gap from one atom to another. Therefore it can be thought that the destruction of a covalent bond involves the simultaneous release of two charge carriers of opposite sign: an electron (negative carrier) and a gap (positive carrier).