A mechanism of current gain degradation caused by highly Si-doped n-GaAs sub-collector layer in InGaP/GaAs heterojunction bipolar transistor was investigated. Current gain decreased with increasing carrier concentration in the sub-collector layer grown at a V/III ratio of 20, suggesting that Si-related defects act as a source of reduction in current gain. We suggest that Ga interstitial, which is generated together with Ga vacancy, is the origin to reduce current gain. It was found that current gain increased with decrease in the V/III ratio of the sub-collector layer from 20 to 1.3. Carbon atomic concentration was 2×10 16 cm -3 for the V/III ratio of 20, while it increased up to 2×10 18 cm -3 for the V/III ratio of 1.3. We suggest that formation of Ga interstitial and carbon complexes in the sub-collector layer prevent Ga interstitial diffusing into the base layer, which leads to current gain degradation.