Unveiling the Impact of IR-Drop on Performance Gain in NCFET-Based Processors

Abstract

Negative Capacitance Field-Effect Transistor (NCFET) pushes the sub-threshold swing beyond its fundamental limit of 60 mV/decade by incorporating a ferroelectric material within the gate stack of transistor. Such a material manifests itself as a negative capacitance (NC) that provides an internal voltage amplification for the transistor resulting in higher ON current levels. Hence, the performance of processors can be boosted while the operating voltage still remains the same. However, having a negative capacitance makes the total gate terminal capacitance larger. While, the impact of that on compensating the gained performance has already been studied in literature, this work is the first to explore the impact of negative capacitance on exacerbating the IR-drop problem in processors. In fact, voltage fluctuation in the Power Delivery Network (PDN) due to IR-drops is one of the prominent sources of performance loss in processors, which necessitates adding timing guardbands to sustain a reliable operation during runtime. In this work, we study NC-FinFET standard cells and processor for the 7 nm technology node. We demonstrate that NC, on the one hand, results in larger IR-drops due to the increase in current densities across the chip, which leads to a higher stress on the PDN. However, the internal voltage amplification provided by NC, on the other hand, compensates to some degree the voltage reduction caused by IR-drop. We investigate, from physics all the way to full-chip (GDSII) level, how the overall performance of a processor is affected under the impact that NC has on magnifying and compensating IR-drop.

Design Automation of Approximate Circuits With Runtime Reconfigurable Accuracy

“…In this section, we evaluate the efficiency of the proposed framework using industry-strength tools and benchmarks. We use two state-of-the-art technology standard cell libraries with differing characteristics: a conventional 7nm FinFET library [54] and its counterpart using Negative Capacitance Field-Effect Transistors (NCFET) [55], [56]. Six benchmarks are considered in our evaluation, i.e., adder, square root (sqrt), square, multiplier, multiply-accumulate (mac), and divider.…”

“…The FE layer manifests itself as a negative capacitance that magnifies the applied vertical electric field leading to an internal voltage amplification, instead of a voltage drop as is the case in any existing conventional CMOS technology. Such an internal voltage amplification significantly improves the electrostatic integrity of transistor, leading to larger ON current and thus a higher switching speed, while the operating voltage still remains the same [55]. In short, the presence of a negative capacitance inside the transistor's gate results in a much larger current that enables a higher switching speed and as a result the circuit can be clocked at a higher frequency while the voltage remains the same.…”

“…Because our implementation is technology independent, we can directly plug in NCFET-aware standard cell libraries inside RETSINA to automatically generate reconfigurable approximate circuits in the NCFET technology. In this work, we leverage NCFET-aware cell library developed in [55], [56], that was created based on the 7nm FinFET technology node using a physics-based NCFET compact model. This allows us to have fair and direct comparisons with the conventional FinFET-based circuits.…”

Impact of Negative Capacitance Field-Effect Transistor (NCFET) on Many-Core Systems

“…In practice, NCFET technology enables the transistor to reach the same ON current, without increasing the OFF current, but at a much lower voltage [2]. This is only possible due to steeper sub-threshold swing.…”

“…NCFET technology comes with an important side effect in which it increases the total capacitance of transistor. Such an increase can lead to reliability problems caused by IR-drop and voltage fluctuation during circuit's operation [2,18]. At the same time, because NCFET technology enables circuits to operate at lower voltages, it is expected that other reliability problems, related to lifetime, to become much less because all the underlying aging mechanisms, such as negative bias temperature instability (BTI) and hot-carrier injection (HCI), strongly depend on the operating voltage [20].…”