The formation of an photosensitive device due to the local breakdown in an MOS structure with an impurity
containing oxide layer has been monitored. A stepwise breakdown of the oxide layer resulted in the formation
of a diode like characteristics with further on stable current-voltage characteristics. Under illumination with
white and blue light we found a high photosensitivity of the resulting structure, probably due to the formation
of a local p-n junction due to out-diffusion from the oxide of n-type dopants into the underlying silicon
substrate. Using a blue light LED illumination during the monitoring of the device formation enables the
identification of the moment, when a high ratio between photo- and dark current is obtained.