Measurement of base and collector transit times in thin-base InGaAs/InP HBT

The influence of base thickness reduction on performances of heterojunction bipolar transistors (HBTs) is examined. HBT structures are grown, with a base thickness in the range 25-65 nm and doping concentration from 3/spl times/10/sup 19/ to 6/spl times/10/sup 19/ at/cm/sup 3/. Base transit time is accurately extracted from total base-collector transit time, and described using a simple drift-diffusion approach. This model, however basic, shows very good agreement with measurements when usual parameter values are used. A 0.13-ps transit time reduction is measured when thinning the base from 65 to 25 nm. The thinnest base structure presents a 0.08 ps transit time, allowing a 250 GHz f/sub t/ operation at 270 kA/cm/sup 2/ emitter current density.