30 GHz polysilicon-emitter and single-crystal-emitter graded SiGe-base PNP transistors

The AC performance of SiGe-base PNP transistors has been limited by a valence band barrier at the base-collector junction caused by the retrograde Ge profile. A two-times improvement in f/sub T/ over previous work on SiGe-base PNPs is reported. The improvement was achieved by using a narrow base profile (W/sub B/=50 nm) and by grading the Ge across the neutral base and in the base-collector junction (the retrograde Ge profile). Similar results (f/sub T/=30 GHz) were achieved for both polysilicon emitter and single-crystal emitter PNP transistors with the same Ge retrograde profile. The position of the retrograde Ge profile in the base-collector junction was controlled by a self-aligned channeled boron implant through the SiGe base. Numerical simulations demonstrate that the peak f/sub T/ is very sensitive to the retrograde Ge position with respect to the base-collector doping profiles.<
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