Silicon bipolar transistor: a viable candidate for high speed applications at liquid nitrogen temperature
ABSTRACT Despite its inherent speed advantage over CMOS technologies under loaded conditions, the silicon bipolar transistor historically has been dismissed as a viable candidate for digital applications in the 77 K environment. The principal reason for this is the well documented degradation in the device current gain at low temperatures. It is demonstrated in this paper that this conclusion is no longer valid with respect to state-of-the-art devices. The transistors used in this investigation have sufficient current gain at 77 K for most digital applications without intentional profile modification. Emitter coupled logic (ECL) circuits switch at < 100 ps speeds at 77 K, and reduced logic-swing operation offers the benefits of an attractive power-delay product. This paper examines the physics, design and performance issues associated with the low temperature operation of silicon bipolar transistors, and discusses the potential advantages of such devices for high speed applications in future low temperature computer systems.
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ABSTRACT: For pt.I see ibid., vol.40, no.3, p.525-41 (1993). The circuit performance issues associated with optimizing epitaxial Si- and SiGe-base bipolar technology for the liquid-nitrogen temperature environment are examined in detail. It is conclusively demonstrated that the notion that silicon-based bipolar circuits perform poorly at low temperatures is untrue. Transistor frequency response is examined both theoretically and experimentally, with particular attention given to the differences between SiGe and Si devices as a function of temperature. ECL and NTL ring oscillator circuits were fabricated for each of the four profiles described in pt.I. The minimum ECL gate delay for a SiGe base is essentially unchanged from its room-temperature value. ASTAP models were used to explore circuit operation under typical wire loading. The results indicate that epitaxial-base bipolar technology offers significant leverage for future cryogenic applicationsIEEE Transactions on Electron Devices 04/1993; · 2.06 Impact Factor
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ABSTRACT: The DC design considerations associated with optimizing epitaxial Si- and SiGe-base bipolar transistors for the 77-K environment are examined in detail. Transistors and circuits were fabricated using four different vertical profiles, three with a graded-bandgap SiGe base, and one with a Si base for comparison. All four epitaxial-base profiles yield transistors with DC properties suitable for high-speed logic applications in the 77-K environment. The differences between the low-temperature DC characteristics of Si and SiGe transistors are highlighted both theoretically and experimentally. A performance tradeoff associated with the use of an intrinsic spacer layer to reduce parasitic leakage at low temperatures and the consequent base resistance degradation due to enhanced carrier freeze-out is identified. Evidence that a collector-base heterojunction barrier effect severely degrades the current drive and transconductance of SiGe-base transistors operating at low temperatures is providedIEEE Transactions on Electron Devices 04/1993; · 2.06 Impact Factor