THERMAL STABILIZATION OF ALGAAS/GAAS POWER HBTS USING ETA-ALXGA1-XAS EMITTER BALLAST RESISTORS WITH HIGH THERMAL COEFFICIENT OF RESISTANCE

The use of n-AlxGa1-xAs an emitter ballast layer material for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is proposed and demonstrated. A high thermal coefficient of resistance can be obtained owing to the temperature dependence of the relative populations of the conduction band Gamma, L and X valleys if the Al mole fraction x of the n-AlxGa1-xAs ballast layer is adjusted to give the appropriate intervalley energy separation. Al0.3Ga0.7As and Al0.35Ga0.65As emitter ballast layers incorporated in HBT structures have an average thermal coefficient of resistance of about 8 x 10(-3)degrees C-1, which is significantly higher than that of GaAs ballast resistances. The temperature stabilizing effect of n-AlxGa1-xAs ballast layer is assessed by measuring the d.c, current-voltage characteristics of single-emitter finger and multi-emitter finger HBTs. Large power HBTs with integrated n-AlxGa1-xAs ballast resistors have been fabricated and an output power of up to 10 W at 0.9 GHz has been obtained.