Optical beam induced currents in MOS transistors

The effect of the optical beam induced current (OBIC effect) is discussed considering a MOS structure. Compared with the simpler case of a pn junction the MOS transistor shows a complex behaviour leading to various results on the output current. It can be shown that this depends strongly on the device parameters, the spot position and the outer circuit voltage supply. A device analysis techniques resulting from these aspects is dicussed which allows to observe the digital logic states of the circuit under test. It thus serves as a contactless test technique inside the IC.