Abstract: An apparatus for making ac surface photovoltage (SPV) measurements of a specimen of semiconductor material under variable dc bias voltage conditions includes a light source whose output beam is intensity modulated, a reference electrode, a guard electrode, a back electrode, a first voltage for biasing the reference electrode with a variable dc voltage and a second voltage separate from the first voltage for biasing the guard electrode such that an accumulation layer is established in the area on the specimen controlled by the guard electrode to prevent flow of carriers between the area controlled by the reference electrode and the rest of the specimen.

Abstract: A method and apparatus are described for the nondestructive readout of a latent electrostatic image formed on a sheet or layer of insulating material. A sheet or layer of semiconductor material is disposed in relatively close proximity to the insulating material. A latent electrostatic image formed on the insulating material (by any known means) causes a surface depletion layer to be produced by induction at the surface of the semiconductor material. The location and distribution of the accumulated charges on the semiconductor material are read out as analog electrical signals corresponding to the AC surface photovoltage induced on the semiconductor material as the semiconductor material is scanned with a low intensity modulated light beam of appropriate wavelength, the magnitude of the analog signals depending on the local charge density. The analog electrical signals so obtained are then digitized, processed and stored and/or displayed.