A degradation mechanism in InGaP/GaAs heterojunction bipolar transistor (HBT) was investigated. The current gain of the HBT grown at a V/III ratio of 0.7 for the base layer decreased when annealed at 670 degrees C. The samples grown at V/III ratios > 1.0 showed constant photoluminescence intensity independent of annealing temperature, indicating that high thermal stability is obtained, while the V/III ratios < 1.0 showed significant reductions by thermal annealing. Infrared (IR) absorption measurement revealed that the intensity of C-H and C-2-H complexes in carbon-doped GaAs decreased when annealed at > 550 and 650 degrees C, respectively. We suggest that the decomposition of the C-2-H complex creates defect centers, which lead to the current gain reduction. (c) 2008 Elsevier B.V. All rights reserved.