Application of micro-airbridge isolation in high speed HBT fabrication

A new selfaligned AlGaAs/GaAs heterojunction bipolar transistor (HBT) fabrication process is presented, featuring a deep wet etching that undercuts the base and the emitter interconnects, and forms micro-airbridges for device isolation. The advantages of this isolation process include critical lithographies on flat surfaces, capacitance reduction, and compatibility with thick collector layers. 1.5 × 10 μm2 HBT devices, grown by MOCVD, exhibit an Fmax of 120 GHz.