Numerical solutions for surface electric field distributions in avalanching p-i-n power diodes

The results of a two-dimensional finite-difference analysis of the electric field distribution near the surface of reverse-biased p-i-n power diodes are presented. The boundary conditions encountered in fully passivated and encapsulated devices have been modelled as closely as possible, thereby enabling the effects of device polarity, electrode overhang and dielectric coating to be investigated. Our calculations show that the peak field near the surface is less than the peak bulk field for all positive bevel angles, whereas with negative bevel angles it is significantly less than the peak bulk field only for shallow bevel angles of 10° or less. Metal overhang and shallow doping profiles have been shown to reduce the field near the surface. It is concluded that a 50° positive bevel angle with metal overhang and shallow doping profile represents the optimum design. Experimental devices made to this design using two deep diffusions gave higher breakdown voltages with harder and more stable characteristics than devices with steeper doping profile, or with steeper bevel angle or with metal underhang.