The post-annealing effect on the dark current of the InGaAs waveguide photodiodes, which are developed for 40-Gbps optical receiver applications, is experimentally investigated. The interesting experimental phenomena were observed that the dark current is significantly decreased and the breakdown voltage is slightly increased after annealing at 250 and 300 degrees C whereas the dark current and the breakdown voltage are almost constant after annealing at 200 degrees C. Based on the experimental results, the post-annealing is more effective for the dark current improvement than the conventional curing process. (c) 2006 Elsevier Ltd. All rights reserved.