Effect of base thickness reduction on high speed characteristics and base resistance of InGaAs/InP heterojunction bipolar transistor

Influence of base thickness reduction on performances of IC-oriented OPTO+ DHBT technology is presented. HBTs structures are grown, with base thickness in the 25 - 65 nm range and doping concentration from 3 /spl times/ 10/sup 19/ at/cm/sup 3/ to 6 /spl times/ 10/sup 19/ at/cm/sup 3/. Associated base transit time reduction and cutoff frequencies increase are measured. The thinnest base structure presents a 0.08 ps transit time, allowing a 250 GHz ft operation at 270 kA/cm/sup 2/ emitter current density. A measurement method allowing to distinguish between intrinsic and extrinsic base resistance is presented. Base resistance of our devices are then extracted, and are shown to increase on thinnest structures with doping level allowing a current gain above 30. This base resistance increase is linked to a combined increase of lateral access resistance through the layer and increase of base electrode contact resistance.