SMALL-SIGNAL CAPACITANCE AND CONDUCTANCE OF BIASED A-SI STRUCTURES

Small-signal capacitance and conductance of experimental samples of a- Si n-i-n structures were measured in a wide frequency range under vari ous bias conditions. The measured capacitance at low frequencies great ly exceeds the expected value derived from the Delta Q/Delta V ratio, where Delta Q is a change of the trapped charge corresponding to a cha nge Delta Q of the applied voltage. This capacitance increases with th e steady-state bias and decreases with the frequency of the measuring signal. The measured low-frequency small-signal conductance equals the differential conductance obtained hum the steady-state current-voltag e characteristics, but it increases with the rising frequency of tile measuring signal. A small-signal analytical model of an a-Si n-i-n str ucture is developed which agrees well with the experimental results. W ith this model, the high capacitive effect of the n-i-n device al low frequencies is explained on the basis of a phase shift which rises fro m the delayed capture-emission mechanism of carriers in the idealized states, Using this model, it is shown that an increasing frequency of small-signal excitation moves the energy region of gap states engaged in the delaying action toward the conduction band, resulting in a decr easing capacitance and an increasing conductance of the a-Si n-i-n str ucture. (C) 1996 American Institute of Physics.