CHAPTER 5
The compound challenge

5.1 Why bother?

The dramatic commercial success of silicon in digital and power electronics has tended to leave its rivals trailing sadly behind. Was there really any need to ‘bother’ with an alternative material? The current world market for silicon devices is roughly $200 billion, while its nearest challenger GaAs can boast device sales of little more than $5 billion, a tiny enough fraction when viewed against silicon, though far from negligible in absolute measure—clearly, this size of market made it well worth someone’s while to bother! However, from our point of view, it is interesting to examine some of the reasons for, on the one hand, the dominance of silicon and, on the other, the necessity for other semiconductors to be involved at all.

There are, as we noted in Chapter 1, upwards of 600 known semiconductors, so why should just one of them assume such a dominant position? The answer is, of course, a commercial one. Silicon (obtained from sand) is not only one of the commonest elements on earth but is also, technologically speaking, the simplest semiconductor material with an appropriate band gap, making it significantly less expensive than all its rivals. As we have seen, germanium was the first semiconductor to be purified and made available in the form of high quality single crystals but its commercial promise was blighted by the problem of thermal runaway, inherent in its small band gap. Silicon was marginally more difficult to tame but came with a significantly larger gap and, as an unforeseen bonus, a stable oxide, which provided a low density of interface states. These advantages have allowed silicon to beat off all challengers in the area of what we might call (to beg most of the questions!) ‘conventional’ electronic devices, which may stimulate the reader into wondering whether it has any disadvantages. The answer is definitely ‘yes’; silicon is not quite an ideal semiconductor for commercial application. Briefly, it possesses only modest electron and hole mobilities and its band gap is indirect. The first of these defects renders it less than ideal for devices, which must operate at very high frequencies, while the second precludes its application to lasers and one or two other optoelectronic devices which demand a steep band edge and strong electron-photon coupling (i.e. a strong probability that electron-hole recombination will result in

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The Story of Semiconductors
Table of contents

Table of contents

  • Title Page iii
  • Preface v
  • Contents xi
  • Chapter 1 - Perspectives 1
  • Chapter 2 - The Cat's Whiskers 19
  • Chapter 3 - Minority Rule 47
  • Chapter 4 - Silicon, Silicon, and Yet More Silicon 93
  • Chapter 5 - The Compound Challenge 149
  • Chapter 6 - Low Dimensional Structures 213
  • Chapter 7 - Let There Be Light 277
  • Chapter 8 - Communicating with Light 331
  • Chapter 9 - Semiconductors in the Infrared 385
  • Chapter 10 - Polycrystalline and Amorphous Semiconductors 447
  • Index 503
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