In order to study the observed frequency variation of small-signal admittance of forward- biased amorphous silicon (a-Si:H) p-i-n diodes, we performed time-resolved measurements of currents induced by application of a small voltage step superimposed on a constant DC bias. The small amplitude ensures a linear behavior of the system under study. The transient current response includes all the details necessary to explain the stationary response to small sine-wave excitation. Details are obtained through a Fourier transform of the transient current. The real and imaginary parts of the resulting complex current are related to the capacitance and conductance spectra. This approach can explain phenomena taking place in the stationary regime such as negative capacitance values measured at low frequencies.

In the frequency domain, measured capacitances do show negative values, as well as a clear dependence on the applied forward bias voltage. Namely, higher voltages extend the region in which the phenomenon occurs to higher frequencies of the probe signal. In the time domain, all measured transient currents exhibit common features such as an initial decay after the displacement current and, for higher DC biases, a gradual increase. A physical explanation of the current transient is proposed which accounts for the dependence on applied DC bias. A mathematical model is used to show how a delayed increase in the current leads to a modulation of the capacitance and conductance in the frequency domain, the two being related to each other. This finally allows the comprehension of the observed frequency domain behavior.