The effect of the sheet resistance of the diffused region of a p-n junction photodiode on the diode response is discussed. A differential equation is obtained for diffused region potential in terms of its sheet resistance and the manner in which the diode is illuminated. For zero external bias and low light levels or for large back-bias, the differential equation is linear. The linear equation has been solved for steady illumination and for sinusoidally varying illumination, both falling uniformly on the diode. Current voltage equations and equivalent circuits are obtained for these cases. In the ac case, the equivalent series resistance due to the diffused region sheet resistance and equivalent junction capacitance are found to be frequency dependent. The frequency dependence is interpreted as a decrease in effective diode area at high frequencies. The frequency at which this effect begins to be important is the reciprocal of the product of the diffused region sheet resistance, the junction capacitance per unit area, and the square of the diode width. The effect is slightly dependent on diode and contact geometry; both linear and circular geometries are discussed.