Nano-Electro-Mechanical Switches (NEMS) are among the most promising emerging devices due to their near-zero subthreshold-leakage currents. This paper reports device fabrication and modeling, as well as novel logic gate design using "laterally-actuated double-electrode NEMS" structures. The new device structure has several advantages over existing NEMS architectures such as being immune to impact bouncing and release vibrations (unlike a vertically-actuated NEMS) and offer higher flexibility to implement compact logic gates (unlike a single-electrode NEMS). A comprehensive analytical framework is developed to model different properties of these devices by solving the Euler-Bernoulli's beam equation. The proposed model is validated using measurement data for the fabricated devices. It is shown that by ignoring the non-uniformity of the electrostatic force distribution, the existing models "underestimate" the actual value of Vpull-in and Vpull-out. Furthermore, novel energy efficient NEMS-based circuit topologies are introduced to implement compact inverter, NAND, NOR and XOR gates. For instance, the proposed XOR gate can be implemented by using only two NEMS devices compared to that of a static CMOS-based XOR gate that requires at least 10 transistors. © Copyright 2010 ACM.

Time-dependent performance degradation due to transistor aging caused by mechanisms such as Negative Bias Temperature Instability (NBTI) and Hot Carrier Injection (HCI) is one of the most important reliability concerns for deep nano-scale regime VLSI circuits. Hence, aging-resilient design methodologies are necessary to address this issue in order to improve reliability, preferably with minimal impact on the area, power and performance. This work offers two major contributions to the aging-resilient circuit design methodology literature. First, it introduces a novel sensor circuit that can detect the aging of pipeline architectures by monitoring the arrival time of data signals at flip-flops. The area overhead of the proposed circuit is estimated to be less than 45% compared to that of previous approaches, which are over 95%. To ensure the accuracy of its operation, a comprehensive timing analysis is performed on the proposed circuit including the influence of process variations. As a second contribution, this work presents an innovative correction technique to reduce the probability of timing failures caused by aging. This method employs novel reconfigurable flip-flops, which operate as normal flip-flops as long as the circuit is fresh, but function as time-borrowing flip-flops once the circuit ages. This unique flip-flop design allows utilization of the advantages of the time-borrowing technique while avoiding potential race conditions that can be created by employing such a technique. It is shown via simulations that by employing the proposed design methodology, the probability of timing failures in the aged circuits can be reduced by as much as 10X for various benchmark circuits.

Nano-Electro-Mechanical Switches (NEMS) offer the prospect of improved energy-efficiency in digital circuits due to their near-zero subthreshold leakage and extremely low subthreshold swing values. Among the different approaches of implementing NEMS, laterally-actuated double-gate NEMS devices have attracted much attention as they provide unique and exciting circuit design opportunities. For instance, this paper demonstrates that compact XOR/XNOR gates can be implemented using only two such NEMS transistors. While this in itself is a major improvement, its implications for minimizing Boolean functions using Karnaugh maps (K-maps) are even more significant. In the standard K-map technique, which is used in digital circuit design, adjacent "1s" (minterms) are grouped only in horizontal and/or vertical directions; the diagonal (or zig-zag) grouping of adjacent "1s" is not an option due to the absence of compact XOR/XNOR gates. However, this work demonstrates, for the first time ever, that in lateral double-gate NEMS-based circuits, grouping of minterms is possible in horizontal and vertical as well as diagonal fashions. This is because the diagonal groupings of minterms require XOR/XNOR operations, which are available in such NEMS-based circuits at minimal costs. This novel design paradigm facilitates more compact implementations of Boolean functions and thus, considerably improves their energy-efficiency. For example, a lateral NEMS-based full-adder is implemented using less than half the number of transistors, which is required by a CMOS-based full-adder. Furthermore, circuit simulations are performed to evaluate the energy-efficiencies of the NEMS-based 32-bit carry-save adders compared to their standard CMOS-based counterparts.

Dynamic gates have been excellent choice in the design of high-performance modules in modern microprocessors. The only limitation of dynamic gates is their relatively low noise margin compared to that of standard CMOS gates. Traditionally, this issue has been resolved by employing a pMOS keeper circuit that compensates for leakage current of the pull-down nMOS network. In the earlier technology nodes, the keeper circuit could improve reliability of the dynamic gates with minor performance penalty. However, aggressive scaling trends of CMOS technology along with increasing levels of process variations have reduced effectiveness of the traditional keeper approach. This is because to maintain an acceptable noise margin level in deep sub-100 nm technologies, large pMOS keepers must be employed, which generates substantial contention between the keeper and the pull-down network, and hence results in severe loss of performance and high power consumption. This problem is more severe in wide fan-in dynamic gates due to the large number of leaky nMOS devices connected to the dynamic node. In this paper, a novel variation-tolerant keeper architecture is proposed, which is capable of significantly reducing contention and improving performance and power consumption. Using circuit simulations, the overall improved characteristics of the proposed keeper are demonstrated in comparison to those of the traditional as well as several state-of-the-art keepers. The proposed keeper exhibits the lowest delay deviation under different levels of process variations. Also, it is shown that for an eight-input or gate, in presence of 15%Vthfluctuations, the proposed architecture can lead to 20%, 15%, and more than 40% reduction in power consumption, mean delay, and standard deviation of delay, respectively, when compared to the traditional keeper circuit.

For the first time, a new source of random threshold voltage (Vth) fluctuation in emerging metal-gate transistors is identified, analytically modeled and investigated for its device and circuit-level implications. The new source of variability, christened Work-Function Variation (WFV), is caused by the dependency of metal work-function on the orientation of its grains. A statistical framework is developed, which enables estimation of the key parameters of work-function distribution by identifying the physical dimensions of the devices and properties of materials used in the fabrication. This paper offers three major contributions for process, device and circuit designers. First, the proposed model can be employed to identify suitable materials and fabrication processes that can reduce the impact of Vth fluctuation due to WFV. For instance, four types of metal nitride gate materials (TiN and TaN for NMOS and WN and MoN for PMOS devices) are studied and it is shown that TiN and WN result in lower Vth fluctuation. Second, device engineers can benefit from the result of this work by evaluating the WFV level of various types of classical or non-classical metal-gate CMOS transistors. As an example, it is shown that FinFET transistors are less affected by WFV compared to FD-SOI and Bulk-Si devices due to their larger gate area. Third, circuit designers can utilize this model to investigate the impact of such a variation on the key performance and reliability parameters of the circuits. For instance, an SRAM cell is analyzed in the presence of Vthfluctuations due to WFV and it is shown that such variations can result in considerable performance and reliability degradation.

Integration of nano-electro-mechanical switches (NEMS) with CMOS technology has been proposed to exploit both near zero-leakage characteristics of NEMS devices along with high ON current of CMOS transistors. The feasibility of integration of NEMS switches into a CMOS process is illustrated by a practical process flow. Moreover, co-design of hybrid NEMS-CMOS as low power dynamic OR gates, SRAM cells, and sleep transistors is explored. Simulation results indicate that such hybrid dynamic OR gates can achieve 60--80% lower switching power and almost zero leakage power consumption with minor delay penalty. However, the hybrid gate outperforms its CMOS counterpart both in terms of delay and switching power consumption with increase in fan-in beyond 12. Additionally, it is shown that the proposed hybrid SRAM cell can achieve almost 8X lower standby leakage power consumption with only minor noise margin and latency cost. Finally, application of NEMS devices as sleep transistors results in upto three orders of magnitude lower OFF current with negligible performance degradation as compared to CMOS sleep switches.

Substantial increase in leakage current and threshold voltage fluctuations are making design of robust wide fan-in dynamic gates a challenging task. Traditionally, a PMOS keeper transistor has been employed to compensate for leakage current of pull down (NMOS) network. However, to maintain acceptable noise margin level in sub-100 nm technologies, large PMOS is necessary, which results in substantial contention (during pull down) and severe loss of performance. In this paper, a novel keeper architecture is proposed which is capable of significantly reducing the contention and improving the performance and power consumption. Using circuit simulations, superior characteristics of the proposed keeper is demonstrated in comparison to those of the traditional as well as state-of-the-art keepers. It is shown that for an 8-input OR gate, in presence of 15% Vth fluctuations, the proposed architecture can lead to 20%, 15%, and more than 40% reduction in power consumption, mean delay, and standard deviation of delay, respectively, when compared to traditional keeper circuit.