Influence of negative corona-charged surface protective material on surface depletion region width

The change of the surface depletion region width (SDRW) of a pn junction for high voltage silicon rectifiers caused by a negative corona-charged surface protective material has been studied. Negative corona charge makes the surface charge density of a SiO₂-insulator interface negative and thus, cause the SDRW to expand. The photocurrent method is used to measure the SDRW of reverse-biased pn function with positive bevel angle mesa structure. The change to SDRW can be caused by the decay of the surface charge density. The expanse of the SDRW tends to saturate as the voltage of the corona is increased. The decay process is different with different surface protection materials. Numerical calculation were performed and the surface charge density of negative corona-charged insulators was found to be approximately -10¹¹ charges/cm²