Measured and modelled characteristics are reported for a new geometry for contacting AlGaAs/GaAs heterojunction bipolar transistors (HBTs), which provides lower thermal resistance and lower emitter inductance than does the conventional approach. Common-emitter connected HBTs are mounted emitter side down on a metal heatsink. which also serves as electrical ground. Connections to the base and collector are made with substrate vias that extend through the GaAs substrate from the backside of the wafer. This configuration was measured to have values of thermal resistance as small as 0.23 K/mW for a 132μm 2 emitter area HBT, in agreement with simulations. An output power per unit emitter area of 2.9mW/μ 2 : (total power of 370mW) at 9 GHz was observed.