Visible LEDs
Following early experiments at the end of the 1950s [11], progress in making efficient LEDs using GaP (indirect bandgap equal to 2.2 eV) was made in parallel by three research groups from Philips Central Laboratory in Germany (H.G. Grimmeiss), the Services Electronics Laboratories (SERL) in the UK (J.W. Allen) and Bell telephone laboratories in the USA (M. Gershenzon) [12-14]. They had different objectives, ranging from communication, lighting and television to indicator lamps for electronics and telephones. Using different dopants (e.g. Zn-O or N) at various concentrations, different wavelengths were generated ranging from red to green. By the late 1960s a number of manufacturers in different countries were making red and green LEDs based on GaP.