Accurate delay models for digital BiCMOS

A detailed transient analysis of the MOSFET-BJT combination prevalent in digital BiCMOS gates is presented. The analysis accounts for high-level injection leading to BJT beta roll-off, base pushout leading to BJT f/sub T/ roll-off, short-channel behavior of the MOS drain current, and parasitic capacitances at the base and output. Based on the transient analysis, a piecewise delay expression is derived that shows excellent agreement with measured gate delay and with SPICE simulated delay. The comparisons are made for a wide range of circuit parameters in the gate, namely, MOSFET/BJT size, load capacitance, and supply voltage for both 1- and 0.6- mu m BiCMOS technologies. The model is used to optimally size gates, and to determine circuit and device design guidelines to minimize the delay degradation at reduced supply.<
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