Effect of exponentially graded base doping on the performance of GaAs/AlGaAs heterojunction bipolar transistors

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Abstract
The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5*10/sup 19/ cm/sup -3/ at the emitter edge to 5*10/sup 18/ cm/sup -3/ at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1*10/sup 19/ cm/sup -3/, the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40%.