Forward-bias tunneling at defect clusters in silicon emitter junctions

High values of the ideality factor n, in the range 1–6, have been found at low temperatures in highly doped n++p+ junctions with phosphorous and gallium or boron diffusion profiles. By studying the thermal activation of the forward current it has been possible to establish that tunneling related to charge carrier transport plays a decisive role for the generation of the high n factors. The tunneling component can be explained by the presence of local electric fields, much higher than the average field in the space-charge region, and the tunneling becomes more evident for p-n junctions with higher concentration of shallow dopants at the interception point between the p+ and the n++ concentration profiles. The use of a specially designed method based on deep-level transient spectroscopy has made it possible to conclude that the recombination centers, contributing to the recombination current, are positioned in the n++ phosphorous emitter and have the same properties for gallium and boron p base profiles. The concentration of isolated recombination centers decreases with increasing phosphorous concentrations due to clustering or precipitation, and increases the electric field constrictions.