Compound Semiconductor Heterostructures

Researchers have long recognized that so-called III-V compounds such as GaAs, AlSb and InP make excellent semiconductors. Possessing equal numbers of atoms of elements in the third and fifth column of the Periodic Table, they have the same tetrahedral crystal structure as carbon and silicon - and therefore similar physical properties. Working at Siemens-Schuckert research laboratory in Erlangen, Germany, Heinrich J. Welker pioneered the understanding of these compounds in the 1950s. He and his colleagues established that they often have wider band gaps and greater carrier mobilities than semiconductors made of germanium and silicon. Although III-IV compounds are not likely to displace silicon in traditional semiconductor uses, their versatile band structures have been widely exploited for high-frequency oscillators and opto-electronic devices.

After the invention of semiconductor lasers in 1962, it took almost another decade to achieve room-temperature devices. In 1963 Herbert Kroemer, then at Varian, and Zhores Alferov in Leningrad suggested that higher-performance lasers could be realized using a three-layer "double heterostructure" in which a narrow-band-gap semiconductor is sandwiched between two wide-band-gap layers. For example, a thin layer of p-type GaAs might be sandwiched between n- and p-type layers of AlGaAs. Higher populations of electrons and holes can be confined within the inner layer, which also serves as a waveguide for radiation from electron-hole recombination.

At the 1967 Solid-State Device Research Conference in Santa Barbara, Jerry Woodall reported that his IBM group had developed liquid-phase epitaxy to grow such AlGaAs layers on GaAs substrates. Using such an approach, Izuo Hayashi and Mort Panish of Bell Labs built a successful double-heterostructure laser that operated at room temperature, announcing their breakthrough at the 1970 Device Research Conference in Seattle. At about the same time, Alferov's group at the Ioffe Physico-Technical Institute reported similar results.

Improved techniques since then have allowed researchers to grow extremely thin layers on GaAs and InP substrates. In the late 1970s and early 1980s, Al Cho pioneered molecular-beam epitaxy at Bell Labs, while Russell Dupuis and Harold Mansevit developed metal-organic chemical vapor deposition at Rockwell. Extremely delicate, detailed heterostructures (often with one or more dimensions measured in nanometers) have been fabricated using these techniques. To describe the movement of electrons and holes in such tightly confined spaces requires use of quantum mechanics, hence these structures are often called quantum wells.

Practical applications of these heterostructures began to emerge in the 1980s. With the thin-layer control that had become available, multiple-cavity quantum-well lasers and vertical-cavity surface-emitting lasers were now possible. In the latter devices, cavity mirrors are formed by alternating layers of compound semiconductors with substantially different refractive indices. These heterostructure-growth technologies also permitted a rich field of device research and the production of a new family of high-frequency transistors. In addition to the heterojunction bipolar transistor, a variety of high-electron-mobility transistors found use in microwave applications.

The highest expression of the heterostructure art developed so far is the quantum cascade laser developed in the mid-1990s by a Bell Labs team led by Federico Capasso. This structure contains hundreds of ultrathin layers of gallium indium arsenide and aluminum indium arsenide, each at most a few nanometers thick, laid down by molecular-beam epitaxy. The result is a sequence of many similar, quantum wells, each with a precisely determined energy level. A single electron descending this "quantum staircase" emits dozens of identical photons at infrared wavelengths.

Today semiconductor lasers flash away at the heart of opto-electronic devices ranging from CD and DVD players to laser printers and bar-code scanners. Along with a great variety of semiconductor photodetectors, they are the critical active elements in fiber-optic systems that span the globe, providing broadband telecommunications on vast highways of light.

Exercise 5

Give antonyms of the following words.

1. equal (same)	9. possible	17. to find
2. to understand	10. to appear	18. high
3. often	11. alternating	19. at most
4. wide	12. latter	20. to descend
5. likely	13. different	21. to emit
6. to displace	14. new	22. far
7. thin	15. to permit	23. great
8. to unite	16. to add	24. active

Exercise 6

Render the meaning of each paragraph in one or two sentences.

E.g. Having excellent semiconductor properties, caused by their versatile band structures, III-V compounds are used for high-frequency oscillators and opto-electronics devices.

LESSON 16

Exercise 1

Form new words by means of word-building affixes.

Evolve, oxide, passive, dissipate, maintain, primary, sequential, result, certain, create, pattern, cross, wire, purpose.

Exercise 2

Translate the following paying attention to tenses passive.

1.The machine with a computing speed of 35.6 trillion mathematical operations per second has been installed at the Earth Simulator Research and Development Center in Yokohama, where it will allow climate changes to be simulated. 2. Some of these broadband systems use regions of the spectrum that have, in the past three decades, been opened for public and private use. 3.Human body’s most common molecules, including water, have an irregular distribution of charge, so that they are influenced by an electric field or a magnetic one. 4.When a processor or other chip component asks for the data, the stored charge is removed from the memory cell, sensed, and sent to the output pins of the array. 5. An economically attractive approach is to base multiplexing systems on channels that are spaced apart, and for some years companies have been deploying such systems while a standard was being developed. 6.The circuit is thus also balanced as far as the signal is concerned. 7. When a grounded-emitter transistor is driven from a very high impedance source, i.e. a constant current generator, the collector current is determined principally by the base current and the device’s current gain. 8. To the best of my knowledge, the influence of input stage bias current on amplifier distortion was dealt with and worked out to any extent in only a few works. 9.Since the biasing system has been described above, only the remaining subsystems are dealt with here. 10.Once power output and impedance range are decided, the heat sink thermal resistance to ambient is the main variable to manipulate. 11. Resistor R has been increased to minimise power dissipation, as there seems to be no significant effect on linearity, however with the resistor omitted altogether, linearity will be affected. 12. If the compositional variation of the heterostructure is compressed right at the emitter-to-base junction of a bipolar transistor, so that carriers are injected from a wider-gap emitter into a narrower-gap base, the quasi-electric fields become quasi-electric potential barriers. 13.Though Kroemer wasn’t pleased by Varian’s decision, the Gunn effect (a phenomenon in which microwave oscillations are produced when a certain voltage is applied to opposite faces of a semiconductor) which had just been discovered, interested him. 14. Things at Fairchild had not gone well, because the company was dedicated to silicon technology and Kroemer’s interests had long been elsewhere. 15. Though his name had been mentioned over the years, Kroemer knew that the Nobel Prise was almost invariably awarded for fundamental discoveries, not for applied research. 16.Now the line’s capacitance is being lowered by changing the material that insulates it from the surrounding silicon chip as well as from the neighbouring wire.

Exercise 3

Match Ukrainian translations to the following English phrases.

thermal oxidation	1. бiполярні КМОН транзистори
standby operation	2. введення iонiв
BiCMOS transistors	3. iзоляцiйна чи провiдна плiвка
photoresist	4. режим очiкування
insulating or conductive film	5. термiчне окислення
lithography and etching	6. свiтлочутливий шар
ion implantation	7. лiтографiя та витравлювання

Exercise 4

Pay attention to translation of the following phrases.

breaching the micron level -	перейшовши мiкронний рiвень;
by exposing a light-sensitive layer (photoresist) with an image of this pattern - шляхом освiтлення фоторезисту, на який нанесена потрiбна схема.