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Unformatted text preview: in pocket calculators,
wrist watches and the alpha-numeric output displays of many instruments; they are also used in infrared-emitting diodes,
injection lasers, infrared detectors, photocathodes and photomultiplier tubes. An extremely elegant chemical solid-state
technology has evolved in which crystals of the required properties are deposited, etched and modified to form the
appropriate electrical circuits. The ternary system GaAsi -xP.r now dominates theZED market for a-numeric and graphic
displays following the first report of this activity in 1961. GaAsl,P, is grown epitaxially on a single-crystal substrate of
GaAs or GaP by chemical vapour deposition and crystal wafers as large as 20cmZ have been produced commercially. The
colour of the emitted radiation is determined by the energy band gap E,; for GaAs itself E, is 138kl mol-' corresponding
to an infrared emission (A 870 nm), but this increases to 184k.I mol-' for x -0.4 corresponding to red emission (A 650 nm).
For x > 0.4 E, continues to increase until it is 218kJmol-' for GaP (green, A. 550nm). Commercial yellow and green
LEDs contain the added isoelectronic impurity N to improve the conversion efficiency. A schematic cross-section of
a typical GaAsl-,P, epitaxial wafer doped with Te and N is shown in the diagram: Te (which has one more valence
electron per atom than As or P) is the most widely used dopant to give n-type impurities in this system at concentrations
of 1016-10i8atoms cm-3 (0.5-50ppm). The p-n junction is then formed by diffusing Zn (1 less electron than Ga) into
the crystal to a similar concentration. An even more recent application is the construction of semiconductor lasers. In normal optical lasers light is absorbed
by an electronic transition to a broad band which lies above the upper laser level and the electron then drops into this level
by a non-radiative transition. By contrast the radiation in a semiconductor laser originates in the region of a p-n junction
and is due to the transitions of injected electrons and holes between the low-lying levels of the conduction band and
the uppermost levels of the valence band. (Impurity levels may also be involved.) The efficiency of these semiconductor
injection lasers is very much higher than those of optically pumped lasers and the devices are much smaller; they are also
easily adaptable to modulation. As implied by the band gaps on p. 255, emission wavelengths are in the visible and near
infrared. A heterostructure laser based on the system GaAs-Al,Gai_,As was the first junction laser to run continuously
at 3 000K and above (1970).
In the two types of device just considered, namely light emitting diodes and injection lasers, electrical energy is
converted into optical energy. The reverse process of converting optical energy into electrical energy (photoconductivity
and photovoltaic effects) has also been successfully achieved by 111-V semiconductor systems. For example, the small
band-gap compound InSb is valuable as a photoconductive infrared detector, and several compounds are being actively
studied for use in solar cells to convert sunlight into useful sources of electrical power. Th...
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