Small-signal parameters of an abrupt p-n junction diode under forward bias are computed numerically by solving Poisson's equation and continuity equations together. The parameter values are compared with those obtained with analytical expressions. It is found that the depletion capacitance is underestimated when the depletion approximation is used. The diffusion conductance and capacitance values agree with theory only in a limited range of forward bias voltages, and this range becomes smaller as the minority carrier lifetime decreases. The reasons behind the discrepancy are discussed.