Infrared LEDs
Techniques to make efficient p-n junctions with GaAs were rapidly developed during
the following years. GaAs was attractive because of its direct bandgap, enabling
recombination of electrons and holes without involvement of phonons. The bandgap is
1.4 eV corresponding to light in the infrared. In the summer of 1962, the observation of
light emission from p-n-junctions was reported [7]. A few months later, laser emission
in GaAs at liquid nitrogen temperature (77 K), was demonstrated independently and
almost simultaneously by three research groups at General Electric, IBM and the MIT
Fig. 1. Principle for light emission in a p-n junction. In a p-n junction biased with a forward voltage,
electrons are injected from the n- to the p-side, and holes are injected in the opposite direction.
Electrons recombine with holes and light is emitted (spontaneous emission). For efficient diodes it is
important that the semi-conductors have direct bandgaps. LEDs with indirect bandgaps require
phonon-assisted recombination, which limits the efficiency. The quantum efficiency of a LED is the
ratio of the number of emitted photons to the number of electrons passing through the contact in a
given time.
3
Lincoln Laboratory, in the U.S. [8-10]. It would be a few years, however, before laser
diodes became widely used. Thanks to the development of heterostructures (Nobel
Prize 2000 to Z.I. Alferov and H. Kroemer), and later quantum wells, allowing for a
better confinement of the carriers while reducing the losses, laser diodes could operate
continuously at room temperature, with applications in a large variety of areas.