In this work, the selective Zn diffusion process is applied in the conventional p-i-n PD/HBT structure to improve the PD's performance. Without Zn diffusion, the top-illuminated PD need higher reverse bias to delay the screen effect. With Zn diffusion, the -3dB bandwidth can be extended to higher frequency at middle and high photocurrent under lower reverse bias. Even under higher reverse bias, it still has better performance at higher photocurrent. The -3dB bandwidth at 9 mA output current extends from 5 to 10.7 GHz at reverse bias of 1 V and from 4.7 to 11.5 GHz at reverse bias of 2 V.