Photon Emission from Avalanche Breakdown in Silicon

Visible light is emitted from reverse-biased silicon p-n junctions at highly localized regions where avalanche breakdown is taking place. The emission occurs in both grown and diffused junctions. By using junctions diffused to a depth of only 2 microns below the crystal surface, it was established that the light sources are randomly spaced over the whole area of the junction as well as around the periphery where the junction intercepts the surface. The light sources are too small to be resolved under a high-power microscope. Their sites are reproducible with current cycling and their intensity and color are relatively insensitive to the field distribution, to the junction width, and to temperature. The number of light spots increases with the current rather than individual spots growing brighter. It is concluded that all the breakdown current is carried through the junction by these localized light-emitting spots. The spectral distribution of the light is continuous with a long tail extending to photon energies greater than 3. 3 ev. It is concluded that recombination between free electrons and free holes within the junction region is responsible for the light at the shorter wavelengths, the carrier energies in excess of the energy gap being supplied by the field. At longer wavelengths there appears to be a considerable contribution to the emission from intraband transitions. A tentative figure for the emission efficiency over the visible spectrum is one photon for every 10^8 electrons crossing the junction. The recombination cross section required is reasonable, being about 10^-22 cm^2.