Forward bias electron injection into the p side of a p-n homojunction was shown to result in an improved response of the ZnO photodiodes. Injection of about 25C of charge yielded a nearly 2.5-fold increase of photocurrent at 350nm. This improvement was correlated with the increase of the diffusion length of minority electrons in p-type ZnO:Sb as determined by electron beam induced current measurements. It is suggested that the increase of the diffusion length is related to the carrier trapping on nonionized acceptor levels.