A COLLECTOR DESIGN STUDY FOR GAAS/GE/GAAS DOUBLE HETEROJUNCTION BIPOLAR-TRANSISTORS

We analyze the effect of the conduction band offset in the collector heterojunction in NpN GaAs/Ge/GaAs double heterojunction bipolar transistors (DHBT). Despite excellent diode characteristics in both the emitter-base and base-collector diodes, the GaAs/Ge/GaAs DHBTs exhibit relatively low common-emitter dc current gains and a lack of collector current saturation with increasing collector-emitter bias. Simulations indicate that this is at least in part attributable to a reduction in collection efficiency caused by the large collector-base conduction band offset (0.26 eV) between the Ge base and the GaAs collector. A modified collector design, which incorporates higher doping in the region of the GaAs conduction band spike, is proposed to decrease the width of the barrier to electrons. Simulations predict that the collection efficiency will be greatly improved with the incorporation of a modified collector.