Double-diffused graded SiGe-base bipolar transistors

Self-aligned SiGe/Si bipolar transistors have been fabricated using a single-polysilicon, double-diffused process with the base in a graded SiGe layer to improve base transit time. To remain compatible with homojunction bipolar technology, undoped SiGe base and Si emitter layers were deposited by selective epitaxy at temperatures of 700-750/spl deg/C in a commercial epitaxial reactor. Maximum cutoff frequencies of 40 and 50 GHz were observed for devices with collector-emitter breakdown voltages (BV/sub CEO/) of 4.2 and 3.0 V, respectively. Preliminary results indicate that the addition of Ge to the base of these transistors did not degrade the long-term device reliability.<
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