Микроэлектроника / Speedier Semiconductor Chips
.docxSpeedier Semiconductor Chips
The ongoing microelectronics revolution was ushered in over 50 years ago by the introduction of silicon-based semiconductor chips. The circuits speeds in some advanced computer equipment are now approaching the theoretical limits of silicon, and for many years scientists have been experimenting with faster-working alternative materials. Harris Microwave Semiconductor, of Milpitas, Calif, recently introduced two digital integrated circuits made from one exotic alternative to silicon: gallium arsenide.
Electronic chips made from gallium arsenide have been available in the past, but usually only on a prototype basis. The new Harris chips, both of which are designed for use in sophisticated telecommunication equipment and military electronic systems are the first commercially available off-the shelf gallium-arsenide 1С chips. The manufacturer says they work five times faster than the speediest of today's silicon-based counterparts.
Silicon and Germanium
The two elements we can now concentrate on, as by far the most important semiconductors, are silicon and germanium. Silicon is one of the most plentiful elements in the world, but occurs in chemical compound such as sand (silica), from which it is difficult to extract pure silicon. The element can be isolated by the reduction of silica in an arc furnace. It then contains small quantities of calcium, iron, aluminium, boron and phosphorous as principle impurities. Alternatively, silicon can be prepared by the pyrolytic reduction of silicon tetrachloride and in this way the material can be obtained free from analytically detectable quantities of boron and phosphorus.
Germanium is comparatively rare but it is rather easier to refine. It should perhaps be mentioned that the list of semiconductors given is not confined to elements; increasing attention is being paid to semiconductor compounds such as indium antiminide and other compounds of group III with group V elements.
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Semiconductors as Materials
A semiconductor is a material having a resistivity in the range between conductors and insulators and having a negative temperature coefficient. The conductivity increases not only with temperature but is also affected very considerably by the presence of impurities in the crystal lattice.
Types of semiconductor material commonly used are elements falling into group IV of the Periodic Table, such as silicon or germanium. The donor and acceptor impurities are group V and group III elements, respectively, differing in valency by only one electron.
Certain compounds such as gallium arsenide (Symbol: GaAs) which has a total of eight valence electrons, also make excellent semiconductors.
GaAs is a direct-gap III—V semiconductor that has a relatively large band gap and high carrier mobility. The relatively high carrier mobility allows the semiconductor to be used for high-speed applications and because of the large energy gap it has a high resistivity that allows easier isolation between different areas of the crystal. The conduction band is a two-state conduction band; some electrons therefore are 'hot' electrons, i.e. they have small effective mass and higher velocity, this resulting in the Gunn effect.
GaAs is difficult to work since diffusion of impurities into the material is extremely difficult. Epitaxy, or ion implantation must therefore be used to produce areas of different conductivity type. The main uses for gallium arsenide have been as microwave devices, such as Gunn diodes or IMPATT diodes, but lately it has been used as a MESFET (a GaAs junction field-effect transistor) for high speed logic circuits.