Yu-Sheng Wu

Comparison of 4T and 6T FinFET SRAM Cells for Subthreshold Operation Considering Variability—A Model-Based Approach

This paper investigates the cell stability of recently introduced four-transistor (4T) and conventional six-transistor (6T) fin-shaped field-effect transistor static random access memory (SRAM) cells operating in a subthreshold region using an efficient model-based approach to consider the impact of device variations. Compared with the 6T cell, this paper indicates that 4T SRAM cells exhibit a better nominal READ static noise margin (RSNM) because of the reduced READ disturb. For 4T cells, the nearly ideal values of Vwrite,0 and Vwriet,1 guarantee the positive nominal WRITE static noise margin (WSNM) for selected cells. For half-selected cells on the selected bit line, a sufficient margin is observed between WRITE time (for selected cells) and WRITE disturb (for half-selected cells). Using the established model-based approach, the variability of subthreshold 6T and 4T SRAM cells is assessed with 1000 samples. Our results indicate that the 4T driverless cell with a larger μRSNM and a slightly worse σ-RSNM shows a comparable μ/σ ratio in RSNM with the 6T cell. Further more, for a given cell area, 4T SRAM cells using relaxed device dimensions with reduced σ-RSNM can outperform the 6T cell. For WRITE operation, 4T SRAM cells exhibit a superior WSNM, whereas the design margin between WRITE time and WRITE disturb needs to be carefully examined to ensure an adequate margin considering device variability.

Investigation of Cell Stability and Write Ability of FinFET Subthreshold SRAM Using Analytical SNM Model

In this paper, the static noise margin (SNM) of FinFET static random access memory (SRAM) cells operating in the subthreshold region was investigated using an analytical solution of 3-D Poissons equation. An analytical SNM model for subthreshold FinFET SRAM was demonstrated and validated by 3-D technology computer-aided design (TCAD) mixed-mode simulations. When compared with bulk SRAM, the standard 6T FinFET cell showed larger nominal READ SNM (RSNM), better variability immunity, and lesser temperature sensitivity of cell stability. Furthermore, examination of the stabilities of several novel independently controlled gate FinFET SRAM cells by using the proposed SNM model showed significant nominal RSNM improvements in these novel cells. However, the write ability is found to be degraded, which thus becomes an important concern for certain configurations in the subthreshold region. The result obtained indicates that the READ/WRITE word line voltage control technique is more effective than transistor sizing in improving the stability and write ability of the FinFET subthreshold SRAM. Furthermore, the impacts of process-induced variations on cell stability were also assessed. When compared with RSNM, it was found that WRITE SNM is more susceptible to process variations. While 6T is not a viable candidate for subthreshold SRAM, and 8T/10T cells must be used in bulk CMOS, the present analysis established the potential of 6T FinFET cells for subthreshold SRAM applications.

Analytical Quantum-Confinement Model for Short-Channel Gate-All-Around MOSFETs Under Subthreshold Region

This paper presents an analytical model for quantum-confinement effects in short-channel gate-all-around (GAA) MOSFETs under the subthreshold region. Our analytical model accurately predicts the impact of short-channel effects and doping concentration on the quantum-confinement effects. This scalable quantum-confinement model is crucial to the ultrascaled GAA MOSFET design.

Sensitivity of Gate-All-Around Nanowire MOSFETs to Process Variations—A Comparison With Multigate MOSFETs

This paper investigates the sensitivity of gate-all-around (GAA) nanowire (NW) to process variations compared with multigate devices using analytical solutions of Poissons equation verified with device simulation. GAA NW and multigate devices with both heavily doped and lightly doped channels have been examined regarding their immunity to process-induced variations and dopant number fluctuation. Our study indicates that the lightly doped GAA NW has the smallest threshold voltage (V th) dispersion caused by process variations and dopant number fluctuation. Specifically, the GAA NW shows better immunity to channel thickness variation than multigate devices because of its inherently superior surrounding gate structure. For heavily doped devices, dopant number fluctuation may become the dominant factor in the determination of overall V th variation. The V th dispersion of GAA NW may therefore be larger than that of multigate MOSFETs because of its larger surface-to-volume ratio.

Impact of Quantum Confinement on Short-Channel Effects for Ultrathin-Body Germanium-on-Insulator MOSFETs

This letter investigates the impact of quantum confinement (QC) on the short-channel effect (SCE) of ultrathin-body (UTB) and thin-buried-oxide germanium-on-insulator (GeOI) MOSFETs using an analytical solution of Schrödinger equation verified with TCAD simulation. Our study indicates that, although the QC effect increases the threshold voltage (V_th) roll-off when the channel thickness (T_ch) is larger than a critical value (T_ch,crit), it may decrease the V_th roll-off of GeOI MOSFETs when the T_ch is smaller than T_ch,crit. Since Ge and Si channels exhibit different degrees of confinement and T_ch,crit, the impact of QC must be considered when one-to-one comparisons between UTB GeOI and Si-on-insulator MOSFETs regarding the SCE are made.

Investigation of Electrostatic Integrity for Ultrathin-Body Germanium-On-Nothing MOSFET

This paper examines the electrostatic integrity of ultrathin-body (UTB) germanium-on-nothing (GeON) MOSFET using theoretically calculated subthreshold swing from the analytical solution of Poissons equation. Our results indicate that UTB GeON MOSFETs with the ratio of channel length ( Lg) to channel thickness ( Tch) around 4 can show comparable subthreshold swing to that of the silicon-on-nothing counterparts. The impact of buried insulator (BI) thickness ( TBI) and BI permittivity on the electrostatic integrity of the UTB germanium channel devices are also examined.

Impact of Quantum Confinement on Subthreshold Swing and Electrostatic Integrity of Ultra-Thin-Body GeOI and InGaAs-OI n-MOSFETs

This paper investigates the electrostatic integrity (EI) of ultra-thin-body (UTB) germanium-on-insulator (GeOI) and InGaAs-OI n-MOSFETs considering quantum confinement (QC) using a derived analytical solution of Schrödinger equation verified with TCAD numerical simulation. Although the electron conduction path of the high-mobility channel device can be far from the frontgate interface due to high channel permittivity, our study indicates that the quantum confinement effect can move the carrier centroid toward the frontgate and, therefore, improve the subthreshold swing (SS) of the UTB device. Since InGaAs, Ge, and Si channels exhibit different degrees of quantum confinement due to different quantization effective mass, the impact of quantum confinement has to be considered when one-to-one comparisons among UTB InGaAs-OI, GeOI, and SOI MOSFETs regarding the subthreshold swing and electrostatic integrity are made.

Static Noise Margin of Ultrathin-Body SOI Subthreshold SRAM Cells—An Assessment Based on Analytical Solutions of Poisson's Equation

This paper investigates the static noise margin (SNM) of ultrathin-body (UTB) SOI SRAM 6T/8T cells operating in the subthreshold region using analytical solutions of Poissons equation validated with TCAD simulations. An analytical framework to calculate the SNM for UTB SOI SRAMs operating in the subthreshold region is presented. Our results indicate that for improving both read SNM (RSNM) and write SNM (WSNM), the back-gating technique is more effective in the subthreshold region than in the superthreshold region. The 6T UTB SOI subthreshold SRAM cell with the back-gating technique by increasing the strength of the pull-up transistors and decreasing the strength of the pass-gate transistors shows comparable RSNM with the 10T bulk subthreshold SRAM and an improvement in RSNM variation. Due to better electrostatic integrity, the back-gating technique (pull-up transistors with positive back-gate bias, pull-down/pass-gate transistors with negative back-gate bias) mitigates the 6T UTB SOI SRAM RSNM variation significantly with some improvement in RSNM. Increasing cell beta-ratio shows a limited improvement on RSNM and has no benefit on the SNM variability for the subthreshold operation. The UTB SOI 8T SRAM cell exhibits RSNM 2times larger than the 6T SRAM cell in the subthreshold region. Both negative bit-line voltage (VBL) and boosted word-line voltage (VWL) are more effective than lower cell supply voltage to improve WSNM, and negative VBL shows a larger improvement in WSNM than boosted VWL.

Investigation of scalability for Ge and InGaAs channel multi-gate NMOSFETs

Using a physical and predictive 2-D confinement model considering the impact of source/drain coupling on the potential well, this work investigates the scalability of Ge and InGaAs multi-gate NMOSFETs by exploring a wide design space with various aspect ratio (AR). Our study indicates that, for a given subthreshold swing, multi-gate devices with InGaAs channel are more scalable than the Ge counterpart because of the larger fin-width allowed. Since the quantum-confinement effect can improve the Vth roll-off, Tri-gate (AR=1) with significant 2-D confinement effect exhibits better Vth roll-off than FinFET (AR>;1). In addition, the InGaAs devices exhibit better Vth roll-off than the Ge devices.

A Closed-Form Quantum “Dark Space” Model for Predicting the Electrostatic Integrity of Germanium MOSFETs With High-

This paper provides a closed-form model of the “dark space (DS)” for Ge MOSFETs with high- k gate dielectrics. This model shows accurate dependences on barrier height, surface electric field, and quantization effective mass of the channel and gate dielectric. Our model predicts that the surface DS due to quantum confinement decreases with reverse substrate bias and increasing channel doping. Our model can be also used for devices with a steep retrograde doping profile. This physically accurate model will be crucial to the prediction of the subthreshold swing and electrostatic integrity of advanced Ge devices.