The thermal annealing effect on InGaP/GaAs heterojunction bipolar transistor (HBT) grown at V/III ratios of 0.7 and 25 in the base layer was investigated. Hydrogen concentration for the V/III ratio of 0.7 showed about two times-higher than that for the V/III ratio of 25. Current gain for the V/III ratio of 0.7 decreased with hydrogen concentration reduced. However, current gain for the V/III ratio of 25 did not change even though hydrogen concentration reduced at 7 x 10(17)cm(-3). Infrared absorption (IR) measurement revealed that the V/III ratio of 0.7 contained C-H and C-2-H complex, while the V/III ratio of 25 only contained C-H complex. The C-H complex intensity was decreased when annealed at 600 degrees C for 5 min. However, the C-2-H complex intensity still remained. From gain drift dependence on hydrogen concentration, it was found that the C-2-H complex did not contribute to gain drift. We suggest that decomposition of the C-2-H complex causes defect centers, which leads to current gain degradation. (c) 2006 Elsevier B.V. All rights reserved.