AbstractIn 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 −3 dB 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 −3 dB 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. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2200–2202, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24531