Aggressively scaled type-I and type-II InP-based single and double-heterojunction bipolar transistor technologies are compared for ultrahigh-speed digital applications. Device and circuit figures-of-merit, as well as thermal characteristics are studied on the basis of improved hydrodynamic (HD) device models. Device simulations were calibrated against experimental device data for 80 Gb/s operation, and circuit simulations checked for validity against real circuit prototypes. The excellent agreement was used as the basis for performance optimization and scaling, and ensures our study if firmly grounded in reality. The investigation provides a needed perspective on the relative impact of device figures-of-merit, band structure, breakdown voltage, and material properties on the ultimate performance of practical digital circuits: We show that the type-II DHBTs appear to be the most promising technological approach to achieve multiplexer bit rates towards 300 Gb/s with realistic scaling parameters. (c) 2007 Elsevier Ltd. All rights reserved.