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.