Extraction of the average collector velocity in high-speed NpN InP/GaAsSb/InP DHBTs

In "type-II" NpN InP/GaAsSb/InP double heterostructure bipolar transistors (DHBTs), the p⁺ GaAsSb base conduction band edge lies ΔE_C above the InP collector conduction band: a small ballistic energy ΔE_C is imparted to collected electrons as they are launched into the collector. The high initial velocity should in principle reduce the collector signal delay time. In the present work, we extract the average collector electron velocity in high-speed InP/GaAsSb/InP DHBTs, and find a peak average velocity approaching 4 × 10⁷ cm/s across a 2000 Å InP collector. This finding provides the first evidence of the performance advantage afforded by abrupt type-II B/C junctions for collector transport when compared to the conventional graded \´launchers\´ required when a GaInAs base layer is used.