Negative capacitance in forward biased hydrogenated amorphous silicon p+-i-n+ diodes

We present experimental results and a physical explanation for the negative values of the small-signal capacitance of forward biased hydrogenated amorphous silicon (a-Si:H) p(+)-i-n(+) diodes. The device capacitance varies with the probe signal frequency. In the low frequency range the measured capacitance shows large negative values if the diode is sufficiently forward biased. Higher forward bias voltages shift this effect to higher frequencies. Time domain response of the device to a small voltage step and a Fourier transform of the resulting transient current provide a verification of the results. The time domain data also suggest the physical mechanism responsible for the overall dispersive behavior: separate and delayed trapping processes for holes and electrons at the interfaces with the two doped layers. (C) 1999 American Institute of Physics. [S0003-6951(99)00602-6].