| Summary: | The dc and ac performance of advanced SiGe:C heterojunction bipolar transistors (HBTs) featuring transit frequency (<inline-formula> <tex-math notation="LaTeX">$f_{\text {T}}$ </tex-math></inline-formula>) and maximum oscillation frequency (<inline-formula> <tex-math notation="LaTeX">$f_{\text {max}}$ </tex-math></inline-formula>) of 300 and 500 GHz was characterized from 298 K down to 4.3 K. At 4.3 K, the transit frequency <inline-formula> <tex-math notation="LaTeX">$f_{\text {T}}$ </tex-math></inline-formula> increases by 65% from measured 317 GHz (at 298 K) to 525 GHz. The increase of <inline-formula> <tex-math notation="LaTeX">$f_{\text {T}}$ </tex-math></inline-formula> starts to saturate below 73 K. The physical reasons for the temperature variation of the experimental characteristics and simple model extensions for improving existing compact models (CMs) are discussed so that they can be used for estimating circuit design results at cryogenic temperatures (CTs). The model extensions are verified using experimental data. To the best of our knowledge, this is the first demonstration of modeling advanced SiGe:C HBTs for such a wide temperature range from deep cryogenic to room temperature (RT) (4.3–298 K).
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