Significant time constants defined by high-current charge dynamics in advanced silicon-based bipolar transistors

Analytical expressions for the time constants in advanced silicon-based bipolar transistors defined by the charge dynamics in the base-collector junction space-charge region (/spl tau//sub C/) and by the charge modulation in the quasi-neutral base (/spl tau//sub BM/) are derived based on an accounting for the high-current-induced perturbation of the space-charge region. The derivations show that voltage drops in the intrinsic and extrinsic collector regions and in the extrinsic emitter region are important in defining /spl tau//sub C/ and /spl tau//sub BM/, and that /spl tau//sub BM/ is approximately proportional to collector-current density. Application of the results to an aggressive SiGe-base HBT technology shows that /spl tau//sub C/ and /spl tau//sub BM/ are comparable to the base transit time, and hence that they are significant in defining high-current speed of the HBT.<
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