The R-V curve of photodiode was fit by "lucky electron" model. It verifies that due to plenty of photo-generated carrier occurred in the depletion region, photocurrent multiplication induced by electron impact ionization is the primary reason of decreasing reverse-bias differential resistance. 采用luckyelectron模型对器件的R-V曲线进行了拟合,结果证实器件反偏微分电阻下降的主要原因是由于pn结耗尽区光生载流子的激增,碰撞电离导致的光电倍增效应所引起。