CHAPTER 7
Let there be light

7.1 Basic principles

One of the more exciting and appealing aspects of semiconductor technology concerns the possibility of converting electrical power directly into light power. We have already met the GaAs light emitting diode (LED) in Chapter 5. A forward biased p-n junction emitting ‘light’ in the infrared part of the spectrum (λ = 880 nm), it made its debut in 1955, though this was simply a natural development of an already well-established tradition of light emitting devices. The first report of electroluminescence appears to have occurred as early as 1907 when an Englishman, H. J. Round (who had been at one time an assistant to Marconi) reported the results of experiments on SiC, observations which were later given a more scientific basis by the Russian, Oleg Losev of the Nizhegorodskaya Radio Laboratory who recognized, during the 1920s, the distinction between forward and reverse bias. However, it was not until 1951 that Kurt Lehovec and colleagues (Fort Monmouth Signal Corps Engineering Laboratory) discussed the effect in terms of minority carrier injection across a p-n junction (using insight gained from their work on the still new transistor). This was followed soon afterwards by the observation of forward bias electroluminescence in Ge and Si by Haynes and Briggs at Bell Laboratories and, by 1955, in a number of III-V materials such as GaAs, GaSb, and InP.

It was very quickly realized that, if similar devices could be made using semiconductor materials with wider band gaps, they would emit radiation within the visible waveband (λ = 700–400 nm, corresponding to hv = 1.77–3.10 eV). Early examples were SiC, ZnS, and GaP, though, because ZnS could not be doped p-type, the emission, in this case, took the form of AC electroluminescence, widely developed during the 1950s. The bright yellow emission was pleasant to the eye but technical difficulties and lack of proper understanding of its mechanism militated against its wider acceptance. SiC p-n diodes were capable of generating blue light, though with very low efficiency, while it was not until the 1960s that GaP became sufficiently well developed as a material to allow its red and green luminescence to be commercialized. Both SiC and GaP are indirect gap materials which make it difficult to generate light with high efficiency—the first direct gap visible emitter was GaAsP which

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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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