Termal shunt stabilization of multiple part heterojunction bipolar transistors

Structure and fabrication details are disclosed for AlGaAs/GaAs microwave HBTs having improved thermal stability during high power operation. The use of a thermal shunt joining emitter contacts of a multi-emitter HBT is shown to improve this thermal stability and eliminate "current-crush" effects. A significant reduction in thermal resistance of the disclosed devices is also achieved by spreading the generated heat over a large substrate area using thermal lens techniques in the thermal shunt. These improvements achieve thermally stable operation of AlGaAs/GaAs HBTs up to their electronic limitations. A power density of 10 mW/ mu m2 of emitter area is achieved with 0.6 W CW output power and 60% power-added efficiency at 10 GHz. The thermal stabilization technique is applicable to other bipolar transistors including silicon, germanium, and indium phosphide devices. The disclosed fabrication sequence employs an improved two-step polyimide electrical isolation planarization sequence in preparation for fabrication of the thermal shunting element.