Tunnel FETs for Ultra-Low Voltage Digital VLSI Circuits: Part II-Evaluation at Circuit Level and Design Perspectives

In Part II of this paper, the potential of tunnel FETs (TFETs) for ultra-low voltage (ULV)/ultra-low power (ULP) operation at 32-nm node is investigated through Verilog-A simulations of appropriate reference circuits. Critical issues arising at ultra-low voltages are analyzed, including static robustness of TFET logic gates, performance degradation, and sensitivity to process variations. Guidelines to design ultra-low energy standard cell libraries are derived. The minimum energy point is analyzed in a wide range of conditions, and guidelines for microarchitectural optimization for ultra-low energy are introduced. Voltage scalability of static RAM memories is also analyzed as main limitation to aggressive voltage scaling of very large scale integration (VLSI) systems, and improved precharge schemes are introduced to reduce leakage. The impact of variations of the main device parameters on VLSI digital circuits is investigated to identify the most critical variations that need to be controlled at process level. This investigation permits to understand the potential of TFETs and their advantages over traditional devices within a unitary framework that is based on fair design and comparison from device to circuit level, as well as to develop clear design perspectives in the context of ULV/ULP VLSI digital circuits.