Design of BiCMOS SRAMs for high-speed SiGe applications

Abstract

This study documents the speeds of various SRAM buffer memories that are possible in a contemporary fast SiGe heterojunction bipolar transistor (HBT) BiCMOS process. An SRAM in a 0.13 μm HBT BiCMOS technology using current mode logic (CML)-style circuits serves as a basis for the discussion. This basic SRAM design features a CML decoder, CML word line driver, bipolar sense amplifier for achieving high speed and CMOS 6T memory cells for high density. The BiCMOS technology is especially useful for realising ultra-high-speed SRAMs for low level cache memory in high-clock rate computer systems, but when reorganised can also be utilised in analogue-to-digital converter (ADC) systems to store digitalised data. Speed and power tradeoffs can be made using different bias strategies, CML logic levels and different generations of SiGe HBTs. A demonstrated 128 kb SRAM macro consumes 2.7 W at 4 GHz using a -3.4 and -1.5 V supply voltage for the bipolar and CMOS circuits, respectively, and has dimensions of 3.5 mm × 3.6 mm by using IBM 8HP SiGe technology, which provides an HBT with a fT of 210 GHz. This macro can be integrated into large scale, ultra-wide bus SRAMs using heterogeneous silicon and 3D technology. Simulation indicates that with the next generation of SiGe HBTs, this SRAM macro can operate at 5 GHz, while consuming the same amount of power or alternatively consume 0.73 W, which is 73% less power consumption compared to 8HP, while operating with the same frequency of 4 GHz. Reorganising the memory for a 4 way-interleaved ADC, it can accept data written at 9.5 GS/s for 8HP designs, and 11.9 GS/s for 8XP designs.