Visible Light-Emitting Diodes - The Formative Years

A brief history of visible light-emitting diodes (LED's) is given, from the first experimental observations of H.J.Round in 1907 to the mid-1970's when red and green emitters were in extensive production. Early investigations were empirical. This was changed with the invention of the transistor in 1947 by the demonstration of minority carrier injection at a forward-biased junction, followed by recombination. In 1952 the discovery of the semiconducting behaviour of III-V compounds introduced a new range of materials. Gallium nitride seemed attractive for light emission and was investigated at Philips and RCA laboratories but at the time proved to be too difficult for practical use. Gallium phosphide emerged as the most promising material and groups to investigate it were set up at SERL in England, Philips Central Research Laboratories in Germany and Bell Telephone Laboratories in the USA. Zinc and oxygen doping gave red emission. At Philips, the emphasis was on efficiencies. At SERL the emphasis was on reproducibility for manufacturable devices and when the conditions for zinc and oxygen doping were strictly controlled the world's first practical visible LED's were produced at the end of 1961. At Bell Telephone Laboratories progress was initially slow but with the advent of liquid-phase epitaxial growth production of red emitters on the scale required became possible. The accidental discovery of nitrogen doping of gallium phosphide at Bell led to the production of good green emitters. Until the end of the 1970's, gallium phosphide red and green emitters dominated the LED market. Subsequent developments to the present day are sketched in outline.