A doping concentration-dependent upper limit of the breakdown voltage–cutoff frequency product in Si bipolar transistors

Abstract Recent high-speed Si-based bipolar transistors apparently exceed the Johnson Limit in terms of breakdown voltage–cutoff frequency product, and this paper addresses the relevant issues. First, BVCES rather than BVCEO is shown to be the representative breakdown voltage in describing the breakdown-speed trade-off in collector design, since BVCEO is modulated by the current gain which is irrelevant of the collector design and also practical bipolar circuits are rarely operated with open-base condition for which BVCEO is defined. In the same context, it is suggested BVCES be employed in representing the upper limit of breakdown voltage–cutoff frequency product. Second, a collector doping concentration-dependent upper limit of BVCES·fT product is proposed incorporating the doping concentration-dependent critical electric field and accurate values for related device parameters. With this new approach, it is shown that the limit is far larger than the Johnson Limit and the limit is still yet to be reached.

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