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In this study, two sets of double heterojunction bipolar transistors (DHBT's) with In0.52Al~Gao.48-~As collector and In0.53Al~Gao.q7-~As base were investigated, respectively. It is found that when the conduction band discontinuity of base collector junction is larger than 0.4 eV, an extra two dimensional electron gas (2DEG) will accumulate in the heterojunction notch and enhance the reach-through effect appearing in the output characteristics of DHBT's. This 2DEG effect is strongly dependent on the conduction band discontinuity. After comparing the characteristics of these two sets of DHBT's, we find that hot electron and high collector doping are two a p proaches which can be used to reduce the 2DEG effect..

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