A Device for Noncontact Determination of the Photosensitivity Distribution on Areas of n+–p(n)–p+-Type Silicon Structures

A device is described that allows one to measure the photosensitivity contrast over the area of n+–p(n)–p+ silicon structures without contacts. The structure is placed between the capacitor plates and is locally illuminated on one side by two intensity-modulated lasers. The laser wavelengths are 1064 and 808 nm. The radiations of the first and second lasers are absorbed in the volume of the base region and only near its illuminated surface, respectively. The local photosensitivity is determined by the ratio of the amplitudes of the modulations, at which the total variable photo-emf vanishes. This compensation allows one to avoid an error that is associated with shunting of the illuminated area of the structure by the rest of it due to currents in the n+ and p+ layers. The photosensitivity contrasts measured by the proposed compensation method and the standard method based on short-circuit current measurements were compared on n+–p–p+ single-crystal silicon structures. The difference was no more than 6%, which is consistent with the calculations.