A detailed theoretical investigation is made into the effects of steady state illumination on the MOS-capacitor characteristics. Solutions are given for the C–V characteristics in the low and high frequency limits, and a general solution for the total admittance is developed in terms of a simple lumped circuit model, which is approximately valid for all gate voltages and for test frequencies below the majority carrier relaxation frequency. Suggested techniques for determining the lumped circuit elements are noted and a scheme for evaluating the light perturbed carrier concentration directly from capacitance data is outlined. The band to band communication, which is enhanced by illumination, is studied assuming a single level recombination center, and the effect of the center's parameters and the light intensity on the recombination resistance is deduced. Finally, the expected dependence of the perturbed carrier concentrations on the light intensity, light wavelength, and incident light orientation is derived.