The negative capacitance as often observed at low frequencies in semiconducting devices is explained by bipolar injection in diode configuration. Numerical calculations are performed within the drift-diffusion approximation in the presence of bimolecular recombination of arbitrary strength. Scaling relations for the characteristic frequency with bias, sample dimensions, and carrier mobilities are presented in the limits of weak and strong recombination. Finally, impedance measurements conducted on a light-emitting diode and photovoltaic cell based on low-mobility organic semiconductors are modeled as a function of bias and temperature, respectively.