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Improved current gain and fT through doping profile selection in linearly graded heterojunction bipolar transistors

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4 Author(s)
Hafizi, M. ; TRW Inc., Redondo Beach, CA, USA ; Crowell, C.R. ; Pawlowicz, Leszek M. ; Kim, M.E.

Analytical and experimental results are used to show that extension of a thin p-doped layer of base doping into the graded-gap region, close to the base, of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor and removing n-type dopant from the rest of the linearly graded AlGaAs region improves current gain β and unity gain cutoff frequency fT. Current gain is significantly improved by reducing recombination near the metallurgical interface and using the effective electric field from the grading to accelerate electrons as they are injected into the p-base. The doping profile also inhibits the formation of a potential minimum in which electrons can be stored in close proximity to the base. This greatly improves fT, and does not hamper the current injection or increase the turn-on voltage. Space-charge recombination current is also reduced, due to the carrier density reduction associated with the effective electric field due to the graded gap

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Electron Devices, IEEE Transactions on  (Volume:37 ,  Issue: 8 )