CURRENT GAIN OF GRADED ALGAAS/GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS WITH AND WITHOUT A BASE QUASI-ELECTRIC FIELD

Graded AlGaAs/GaAs heterojunction bipolar transistors with and without a base quasi-electric field are fabricated to investigate the various components of base current. The experimental results demonstrate that the base current for devices with extrinsic base surface passivation is dominated by base-emitter space-charge recombination current, rather than base bulk recombination current. Both a simple theoretical calculation and SEDAN (SEmiconductor Device ANalysis) simulations are used to support this finding. SEDAN simulations also indicate strong effects of hole barrier lowering which reduces device current gain when the current gain approaches values of 1000 and when the maximum aluminum composition in the AlGaAs emitter is less-than-or-equal-to 30%. The experimental finding that space-charge recombination current dominates the base current in passivated graded HBT's agrees well with published theoretical work. This work and other published experimental and theoretical works will also be compared and discussed.