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 Ш 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 Ш-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.

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Speedier Semiconductor Chips

The ongoing microelectronics revolution was ushered in some 30 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 IC chips. The manufacturer says they work five times faster than the speediest of today's silicon-based counterparts.

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2.17. Ответьте на следующие вопросы:

1. What new possibilities did the advent of the transistor open? 2. What are semiconductors? What are the main properties of the semiconductors? 3. What is the operation of a semicon­ductor based on? 4. What are donors? What are acceptors? 5. What makes silicon an indispensable material in microelectron­ics? 6. What are attractive characteristics of GaAs? In what way can it compete with germanium? with silicon?

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1.... have been developed; 2.... make it possible (to do) ...; 3.... has allowed the further reduction...; 4.... the need was clearly evi­dent for...; 5.... are discussed...

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1. From the information available in the literature, CMOS chip under consideration demands substantial design efforts. 2. Time delays associated with interconnections made of different materials have been considered. 3. The proper choice of the mate­rial within the constraints (ограничение) placed by the fabrica­tion technology existing can result in minimization of the RC de­lay time. 4. The area occupied by a MOS transistor can be made smaller by shortening its channel width and length leading to a faster device. 5. Smaller dimensions, larger chip size, and circuit innovations in question all contribute to the progress of integra­tion and the generation of a larger number of components on a single chip. 6. The long distance line voltage drop will increase with scaling mentioned above. 7. Polysilicon to be used meets all of the requirements addressed above.

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The first transistor developed was the junction transistor. Nearly all transistors today are classed as junction transistors.

Through the years there were developed new types of junction transistors that performed better and were easier to construct. When first introduced the junction transistor was not called that; it was the "cat's whisker" used in the first radio receivers in the 1920s. Shockley and his crew resurrected (возродить) it, a mere imposing name sounded much more scientific. The junction tran­sistor of 1948 was further modernized in 1951, with the develop­ment of the "grown" transistor. The technology for manufacturing transistors steadily improved until, in 1959, the first integrated cir­cuit was produced — the first circuit-on-a-chip.

The integrated circuit constituted another major step in the growth of computer technology. Until 1959 the fundamental logi­cal components of digital computers were the individual electrical switches, first in the form of relays, then vacuum tubes, then tran­sistors.

In the vacuum tubes and relay stages, additional discrete com­ponents such as resistors, inductors and capacitors were required in order to make the whole system work. These components were about the same size as packaged transistors. Integrated circuit technology permitted the elimination of some of these compo­nents and "integration" of most of the others on the same chip of semiconductor that contains the transistor. Thus the basic logic el­ement — the switch, or "flip-flop", which required two separate transistors and some resistors and capacitors in the early 1950s, could be packaged into a single small unit in 1960. That unit was half the size of a pea.

The chip was a crucial (важный) development in the acceler­ating pace of computer technology. With integrated circuit tech­nology, it became possible to jam (зд. размещать) more and more elements into a single chip. Entire assemblies or parts could be manufactured in the same time that it previously took to make a single part. Clearly, the cost of providing a particular computing function decreased proportionally. As the number of components on an integrated circuit grew from a few to hundreds, then thou­sands, the term for the chip changed to microcircuit.

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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 phosphorus 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 antimonide and other compounds of group Ш with group V elements.

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