Extraction of the average collector velocity in high-speed "Type-II" 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 injection energy ΔE_C is thus imparted to electrons as they are launched into the collector. The resulting high initial velocity should in principle reduce the collector signal delay time in comparison to the case where thermal electrons are accelerated by the collector electric field alone. We extract the bias dependence of the average collector electron velocity in high-speed InP-GaAs_0.62Sb_0.38-InP DHBTs, and find a maximum average velocity reaching 4×10⁷ cm/s across a 2000 Å InP collector. This finding provides evidence of the performance advantage afforded by abrupt type-II base/collector (B/C) junctions for collector transport when compared to other B/C junctions.