Bump formation was observed for the first time on the surface of biased Si avalanche photodiodes (APDs) fabricated using millisecond-pulse laser ablation, and the mechanism of the phenomenon was studied experimentally and theoretically. Surface maximum temperatures and damaged areas were tested under different laser pulse durations. The surface maximum temperature was similar to 1500 K, which is lower than the melting point of Si when the laser pulse duration and energy density were 1.0 ms and 50 J/cm(2), respectively, but there was a damaged area on the surface. A longitudinal temperature distribution simulation showed that the laser ablation first occurred at the PN junction of the APD, and the mechanism of the phenomenon was Joule heating. This result differs significantly from results obtained for laser ablation of other photodetectors. (C) 2018 Elsevier Ltd. All rights reserved.