The InPDouble HBT (DHBT) epi wafers were grown by Molecular Beam Epitaxy (MBE) and the structures consist of, from top to bottom, a highly-doped n-InGaAs emitter-cap layer, an n-InP emitter layer, a carbon-doped InGaAs base layer, an n-InP collector structure with an n-InGaAs/InAsGaAs grading transition layer between base and collector to eliminate electron blocking and an n-InP sub-collector layer. To provide optimum performance for specific applications, epi structures were designed to compromise the tradeoffs between cut-off frequency (Ft) and the collector-emitter breakdown (BVceo).Optical communication and digital circuits pursue higher operation frequency. In contrast, breakdown voltage is the dominating factor in RF power amplifiers (PA).Collector thickness is of great influence on BVceo[3]. Given that,two structures (EPI-1 and EPI-2) with different collector thickness were designedfor these two purposes. Between them, EPI-1 has similar epi structure with EPI-2 but with thinner collector thickness.