Abstract: This paper first gives the calculating method that includes the analysis formula, that is,
R_outer=R_S·L·(L_E²+2L_EW-W²·L_n(L_E/W))/(N·(W-L_E))³
and Gummul numbers from SEDAN.
Results show that the resistance relates to the area of the photo-excited region, the side length of the emitter region, the position of the emitter region, and the emitter current. We also find that the optimizing position of the emitter region in the square photo-excited region is in the center; and that the optimizing ratio of the side length of the photo-excited region W to the side length of the emitter region is 7.578425:1. These results will be used to design low noise, high frequency phototransistors.

Key words: modelling, R_B, low noise, phototransistor