Transient response of MOS capacitors under localized photoexcitation

Lateral surface transport of charges under the gate electrode of MOS capacitors from localized photogeneration of electrons and holes are studied by observing the transient photocurrent through the MOS capacitor. The photocurrent is highly non-linear under a step function illumination. It shows a large initial spike (phase I), whose length is independent of device area, followed by a slow decay (phase II), whose length increases for high light intensity and for large device area. Detailed time dependence data indicate that phage I comes from the flattening of the surface energy band in the illuminated area, which causes an initial surge of charges in the lateral direction driven by a constant voltage source, and phase II from the slow and areally uniform charging up of the entire interface under the gate. Theoretical calculations of the photocurrent waveforms were carried out using a simple transmission line model for phase I and one-lump model for phase II based on physical considerations. Excellent agreements between the theories and the observed photocurrent waveforms are obtained.