An efficient process design parameter T, the width ratio of the selective diffusion mask window and the electrode, is presented. The results show that there always exists an optimum T value for different substrate concentrations and doping concentrations to obtain the maximum short-circuit current. As the selective doping concentration is 1×1018~1×1019 cm-3, the optimal T value is 1; when the selective doping concentration is 1 × 1020 cm-3 , the optimal T value is less than 0.5. Increase of the electrode width makes the photocurrent decrease, but it has little effect on the optimal T value. Increasing the selective doping concentration causes decrease of the optimal T value.