A silicon-based PIN photodetector structure with the characteristics of high responsibility and high cut-off frequency fabricated with the PERL (the passivated emitter and the rear locally-diffused) technologies is introduced in this paper. After making some tiny adjustments of the structure, Silvaco software is used to simulate three similar structures of PIN photodetector by measuring the spectral response, dark current, cut-off frequency and dc characteristics. From the results, it is concluded that PIN photodetector with the middle shallow diffusion area is superior to the other two ones in dark current, at least 35% lower. The responsibility of these three kinds of detector reaches the degree of 0.3 A/W, and the highest spectral response is around 600 nm. Their cut-off frequencies are all over 108Hz.