The continuity equation of minority carriers in n-type germanium is solved for the stationary case, in which the volume recombination is assumed to be of the recombination-center type and the surface recombination is introduced in the boundary condition. An expression for the photoconductivity is obtained from this solution.By substituting the known absorption data and the volume lifetime of minority carrier in the sample into the expression of photoconductivity and by choosing suitable values of surface recombination velocity, photoconductive spectra are calculated and are compared with the experimental curves. The agreement is satisfactory for wave-lengths greater than 1.2 μ. Below 1.2 μ, the experimental curves fall well below the calculated curves. It is thought that other types of volume recombination may exist.The photoconductive spectra usually have a maximum outside the absorption edge. By measuring the relative height of the maximum, the surface recombination velocity can be obtained. This method is suitable for medium and large values of surface recombination velocity.The photoconductivity drops sharply on both sides of the maximum in case of large values of recombination velocity. In such cases, the energy gap of the semiconductor can be obtained in a simple way. For germanium the energy gap is 0.83 eV for direct transition and 0.635 eV for indirect transition.From this investigation, the energy gap of CdS is reestimated to be 2.53 eV and that of Cu2O greater than 2.10 eV.
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