Amorphous silicon-based phototransistors are studied as an alternative solution to replace pixel-level amplifiers simplifying large-area imaging systems. We report electrical characterization by means of current-voltage and capacitance measurements. The measured capacitance increases with decreasing frequency of the probe signal and values largely exceeding the geometrical one at low frequencies have been achieved both in the dark and under illumination. In particular, values in excess of 200 μF/cm2 are measured under 220 μW/cm2 illumination at 600 nm at 100 mHz. The capacitance dependence on frequency is interpreted in terms of trapping and release kinetics processes in the base and of the gain of the device.