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InP/GaAsSb/InP double heterojunction bipolar transistors (DHBTs) are some of the fastest bipolar transistors ever fabricated, with current gain cutoff and maximum oscillation frequencies simultaneously exceeding 300 GHz while maintaining breakdown voltages BVCEO >6 V. InP/GaAsSb/InP DHBTs are particularly appealing because excellent device figures of merit are achievable with relatively simple structures involving abrupt junctions and uniform doping levels and compositions. This is a tremendous manufacturability advantage and the reason why some organizations have moved aggressively toward GaAsSb DHBT production despite a relative scarcity of information on the physical properties of the GaAsSb alloy in comparison to GaInAs. The present paper reviews some of the key concepts associated with the use of GaAsSb base layers, and discusses the physical operation of InP/GaAsSb/InP DHBTs. In particular, we describe the implications of the staggered band lineup at the E/B and B/C heterojunctions for charge storage in the devices, and show that InP/GaAsSb/InP DHBTs offer inherent advantages from that point of view. We also show that GaAsSb-based DHBTs can be expected to display better scalability than GaInAs-based devices because of their inherently superior base ohmic contacts.