Arijit Raychowdhury

A 45 nm Resilient Microprocessor Core for Dynamic Variation Tolerance

A 45 nm microprocessor core integrates resilient error-detection and recovery circuits to mitigate the clock frequency (FCLK) guardbands for dynamic parameter variations to improve throughput and energy efficiency. The core supports two distinct error-detection designs, allowing a direct comparison of the relative trade-offs. The first design embeds error-detection sequential (EDS) circuits in critical paths to detect late timing transitions. In addition to reducing the Fclk guardbands for dynamic variations, the embedded EDS design can exploit path-activation rates to operate the microprocessor faster than infrequently-activated critical paths. The second error-detection design offers a less-intrusive approach for dynamic timing-error detection by placing a tunable replica circuit (TRC) per pipeline stage to monitor worst-case delays. Although the TRCs require a delay guardband to ensure the TRC delay is always slower than critical-path delays, the TRC design captures most of the benefits from the embedded EDS design with less implementation overhead. Furthermore, while core min-delay constraints limit the potential benefits of the embedded EDS design, a salient advantage of the TRC design is the ability to detect a wider range of dynamic delay variation, as demonstrated through low supply voltage (VCC) measurements. Both error-detection designs interface with error-recovery techniques, enabling the detection and correction of timing errors from fast-changing variations such as high-frequency VCC droops. The microprocessor core also supports two separate error-recovery techniques to guarantee correct execution even if dynamic variations persist. The first technique requires clock control to replay errant instructions at 1/2FCLK. In comparison, the second technique is a new multiple-issue instruction replay design that corrects errant instructions with a lower performance penalty and without requiring clock control. Silicon measurements demonstrate that resilient circuits enable a 41% throughput gain at equal energy or a 22% energy reduction at equal throughput, as compared to a conventional design when executing a benchmark program with a 10% VCC droop. In addition, the microprocessor includes a new adaptive clock control circuit that interfaces with the resilient circuits and a phase-locked loop (PLL) to track recovery cycles and adapt to persistent errors by dynamically changing Fclk f°Γ maximum efficiency.

Modeling of metallic carbon-nanotube interconnects for circuit simulations and a comparison with Cu interconnects for scaled technologies

Semiconducting carbon nanotubes (CNTs) have gained immense popularity as possible successors to silicon as the channel material for ultrahigh-performance field-effect transistors (FETs). On the other hand, their metallic counterparts have often been regarded as ideal interconnects for future technology generations. Owing to their high current densities and increased reliability, metallic single-walled CNTs (SWCNTs) have been subjects of fundamental research, both in theory, as well as experiments. Metallic CNTs have been modeled for radio-frequency (RF) applications using a transmission-line model. In this paper, we present an efficient circuit-compatible RLC model for metallic SWCNTs, and analyze the impact of SWCNTs on the performance of ultrascaled digital very large scale integration (VLSI) design.

A circuit-compatible model of ballistic carbon nanotube field-effect transistors

Carbon nanotube field-effect transistors (CNFETs) are being extensively studied as possible successors to CMOS. Novel device structures have been fabricated and device simulators have been developed to estimate their performance in a sub-10-nm transistor era. This paper presents a novel method of circuit-compatible modeling of single-walled semiconducting CNFETs in their ultimate performance limit. For the first time, both the I-V and the C-V characteristics of the device have been efficiently modeled for circuit simulations. The model so developed has been used to simulate arithmetic and logic blocks using HSPICE.

Carbon-nanotube-based voltage-mode multiple-valued logic design

Multivalued logic has always attracted the attention of digital system and logic designers. However, the high-performance and low-power CMOS process, which has been developed over the last two decades, has traditionally assisted successful circuit implementation of binary logic. Consequently, in spite of its large potential multivalued logic design is seldom a circuit designers choice. This paper presents a novel method of multiple-valued logic design using carbon-nanotube field-effect transistors (CNFETs). The geometry-dependent threshold voltage of CNFETs has been effectively used to design a ternary logic family. We have developed a SPICE-compatible model of ballistic CNFETs that can account for varying geometries and operating conditions. SPICE simulations have been performed on the proposed logic gates, and the transfer characteristics as well as transient behavior have been extensively studied. Finally, a comparison in terms of power and performance of the ternary logic family vis-a/spl grave/-vis traditional complementary field-effect transistor binary logic family has been presented.

Leakage Power Analysis and Reduction for Nanoscale Circuits

Leakage current in the nanometer regime has become a significant portion of power dissipation in CMOS circuits as threshold voltage, channel length, and gate oxide thickness scale downward. Various techniques are available to reduce leakage power in high-performance systems

Accurate estimation of total leakage current in scaled CMOS logic circuits based on compact current modeling

Dramatic increase of subthreshold, gate and reverse biased junction band-to-band tunneling (BTBT) leakage in scaled devices, results in the drastic increase of total leakage power in a logic circuit. In this paper a methodology for accurate estimation of the total leakage in a logic circuit based on the compact modeling of the different leakage current in scaled devices has been developed. Current models have been developed based on the exact device geometry, 2-D doping profile and operating temperature. A circuit level model of junction BTBT leakage (which is unprecedented) has been developed. Simple models of the subthreshold current and the gate current have been presented. Here, for the first time, the impact of quantum mechanical behavior of substrate electrons, on the circuit leakage has been analyzed. Using the compact current model, a transistor has been modeled as a sum of current sources (SCS). The SCS transistors model has been used to estimate the total leakage in simple logic gates and complex logic circuits (designed with transistors of 25nm effective length) at the room and at the elevated temperatures.

Design space and scalability exploration of 1T-1STT MTJ memory arrays in the presence of variability and disturbances

This paper presents modeling and analysis of 1T-1MTJ STT RAM memory arrays under process variations and thermal disturbances. Bounds on the magnetic material design space for embedded applications are illustrated. Impact of relaxed timing/area and the effect of scaling for 1T-1MTJ bitcells have been evaluated.

Device optimization for digital subthreshold logic operation

Digital circuits operated in the subthreshold region (supply voltage less than the transistor threshold voltage) can have orders of magnitude power advantage over standard CMOS circuits for applications requiring ultralow power and medium frequency of operation. Although the implication of technology scaling on subthreshold operation is not obvious (since an obsolete technology node can deliver the same performance as a scaled technology in subthreshold), it has been shown that technology scaling helps to reduce the supply-voltage and, hence, the power consumption at iso-performance. It is possible to implement subthreshold logic circuits using the standard transistors that are designed primarily for ultra high performance super-threshold logic design. However, an Si MOSFET so optimized for performance in the super-threshold regime is not the best device to use in the subthreshold domain. We propose device designs apt for subthreshold operation. Results show that the optimized device improves the delay and power delay product (PDP) of an inverter chain by 44% and 51%, respectively, over the normal super-threshold device operated in the subthreshold region.

Carbon Nanotube Electronics: Design of High-Performance and Low-Power Digital Circuits

Scaling of silicon transistors continue in the sub 100-nm regime amidst severe roadblocks. Increased short-channel effects, rising leakage currents, severe process parameter variations are only a few of the overwhelming challenges that the device and circuit designers are faced with. In an attempt to alleviate the problems associated with the scaling of silicon transistors, researchers have began a quest for novel alternate materials in a post-Si nanoelectronics era. Of the different materials investigated so far, carbon nanotubes with their superior transport properties, excellent thermal conductivities and high current handling capacities have proved to be a potential heir to Si. This paper reviews the promise of carbon nanotube field-effect transistors as future devices for high-performance as well as low-power electronics.

Accurate estimation of total leakage in nanometer-scale bulk CMOS circuits based on device geometry and doping profile

Dramatic increase of subthreshold, gate and reverse biased junction band-to-band-tunneling (BTBT) leakage in scaled devices results in the drastic increase of total leakage power in a logic circuit. In this paper, a methodology for accurate estimation of the total leakage in a logic circuit based on the compact modeling of the different leakage current in nanoscaled bulk CMOS devices has been developed. Current models have been developed based on the device geometry, two-dimensional doping profile, and operating temperature. A circuit-level model of junction BTBT leakage has been developed. Simple models of the subthreshold current and the gate current have been presented. Also, the impact of quantum mechanical behavior of substrate electrons, on the circuit leakage has been analyzed. Using the compact current model, a transistor has been modeled as a sum of current sources (SCS). The SCS transistor model has been used to estimate the total leakage in simple logic gates and complex logic circuits (designed with transistors of 25-nm effective length) at room and elevated temperatures.