Comparative Area and Parasitics Analysis in FinFET and Heterojunction Vertical TFET Standard Cells

Vertical tunnel field-effect transistors (VTFETs) have been extensively explored to overcome the scaling limits and to improve on-current (I_ON) compared to standard lateral device structures for the future technologies. The benefits in terms of reduced footprint, high I_ON and feasibility of fabrication have been demonstrated in several works. Among various VTFETs, the asymmetric heterojunction vertical tunnel FETs (HVTFETs) have emerged as one of the promising alternatives to standard transistors for low-voltage applications. However, while such device-level benefits without parasitics have been widely investigated, logic-gate design with parasitics and layout implications are not clear. In this article, we investigate and compare the layouts and parasitic capacitances and resistances of HVTFETs with FinFETs. Due to the vertical device structure of HVTFETs, a smaller footprint is observed compared to FinFETs in cells with small fan-in. However, for high fan-in cells, HVTFETs exhibit area overheads due to infeasibility of contact sharing in parallel and series transistors. These area overheads also lead to approximately 48% higher parasitic capacitance and resistance compared to FinFETs when the number of parallel and series connections increases. Further, in order to analyze the impact of parasitics, we modeled the analytical parasitics in SPICE. The models for both HVTFETs and FinFETs with parasitics were used to simulate a 15-stage inverter-based ring oscillator (RO) in order to compare the delay and energy. Our simulation results clearly show that HVTFETs exhibit less delay at a V_DD < 0.45 V and higher energy efficiency for V_DDs in the range of 0.3V--0.7V, albeit at the cost of 8% performance degradation.