ABSTRACT
In this paper we report on the performance of InGaP/GaAs HBTs with different compositionally graded InxGa1-xAs base regions. In previous work we have shown that a large built-in base field of ~ 8.6 kV/cm significantly improved the HBT current gain and unity current gain cutoff frequency. To further characterize the base transport in these HBTs, the magnitude of the E-field in the base and its contribution to electron transport have been studied. HBTs with the base graded linearly from GaAs at the emitter-base junction to InxGa1-xAs at the base-collector junction were fabricated and tested. As the base field was increased, the dc current gain increased by nearly 75% and the unity gain cutoff frequency increased by over 20% compared to a device with a standard non-graded base design. The saturation of the current gain and the base transit time at higher fields indicate that velocity-field characteristics play a significant role in base transport.
