Study of capacitance in hydrogenated amorphous silicon phototransistors for imaging arrays

Abstract In this paper we report on the study of capacitance in hydrogenated amorphous silicon phototransistors in order to determine their applicability in large area imaging systems. Measured capacitance values exceed the geometrical one at low frequencies of the probe signal, both in the dark and under illumination. In particular, capacitance values in excess of 60 μF/cm 2 are measured under 220 μW/cm 2 illumination at 600 nm. The experimental data have been reproduced by a numerical device simulator, which takes into account the distribution of defects in amorphous materials. We have found that capacitance is mainly determined by the trapping and release processes occurring in the base and at the interfaces between the intrinsic and the n-layers of the device. At these interfaces, the Fermi level lies in correspondence with the band tails, whose high number of defects causes a large variation of trapped charge in response to the a.c. applied voltage.