Accurate fitting of measured current-voltage [I(V)I(V)] data is crucial to the correct analysis and understanding of metal-insulator–metal (MIM) diodes, especially for optical rectennas. With the commonly used polynomial fitting of the I(V)I(V) data, the order of the fit can drastically affect the diode performance metrics such as resistance, responsivity, and asymmetry. Additionally, the resulting fitting coefficients provide no useful parameters. An exponential-based equation can fit the I(V)I(V) data well, can avoid artifacts from the choice of order of the polynomial, and allows for the accurate calculation of diode performance metrics directly from the fitting coefficients. Connecting the performance metrics to fitting coefficients shows a correspondence between zero-bias responsivity and asymmetry at any given voltage.