Modeling of parasitic capacitances in deep submicrometer conventional and high-K dielectric MOS transistors

In deep submicrometer MOSFETs the device performance is limited by the parasitic capacitance and resistance. Hence a circuit model is needed to treat these effects correctly. In this work, we have developed circuit models for the parasitic capacitances in conventional and high-K gate dielectric MOS transistors by taking into account the presence of source/drain contact plugs. The accuracy of the model is tested by comparing the modeled results with the results obtained from three-dimensional (3-D) Monte-Carlo simulations and two-dimensional (2-D) device simulations over a wide range of channel length and oxide thickness. The model is also used to study the dependence of parasitic capacitance on gate length, gate electrode thickness, gate oxide thickness, gate dielectric constant, and spacer width.     

Computer analysis of the double-diffused MOS transistor for integrated circuits

The effective length of an MOS transistor can be made narrow by using double diffusion similar to a bipolar transistor. Computations were conducted for an n-channel double-diffused transistor with different surface concentrations, channel lengths, channel gradients, surface-states densities, and substrate concentrations. A shorter channel length and a higher surface-state density, e.g.langle1, 1, 1ranglecrystal, gave a higher drain current and transconductance. The maximum transconductance in many cases occurs at low gate voltages. The computations indicate that a gain-bandwidth product in the gigahertz range can be expected when the graded channel region is less than 1 µm. The difference between an n-type substrate and a p-type substrate is not substantial. The analysis is also useful in predicting the performance of any integrated logic circuit using the diffused enhancement transistor as the active switch and a depletion-mode transistor (without a diffused channel) as the load device. The computation indicates that satisfactory performance can be obtained using a load device with the same geometry and an ON voltage of only a fraction of a volt, This revelation indicates that double-diffused channel MOS transistors not only give higher speed but also smaller chip area for integrated circuits and a lower supply voltage (hence less power dissipation).     

Experimental characterization of most''s scaled down to the 1 μm level

Fundamental MOS device performances are experimentally analyzed for the projected three levels of scaled-down, silicon-gate devices envisioned in the next decade. The final third-level device having 20 nm thick gate oxide and 0.7 μm effective channel length will have vertical dimension only 0.35 times that of the present 3 μm lithography level. Principal device characteristics discussed are threshold voltage, source to drain breakdown voltage, and effective carrier mobility under practical applied voltage conditions, mainly for dynamic MOS memory operation.

It is found that breakdown voltage reduction is the main obstacle hindering down-scaling, and also that the mobility lowering in the shorter channel length region reduces the merits of down-scaling. MOS device performances for the coming 1 μm geometry level LSI's under practical operation conditions are discussed on the basis of the experimental results obtained.     

Large-signal linearity of scaled MOS transistors

The tendency toward linearity between saturated drain current and gate-to-source voltage exhibited by small-dimension MOS transistors is explored from the standpoint of possible exploitation in analog MOS circuits. Nonlinearity is calculated using a simple MOS model which includes the high field dependence of inversion-layer carrier mobility. The nonlinearity for devices with a wide range of channel lengths and gate dielectric thicknesses was measured and is compared to results from the model. Some problems associated with the use of short-channel MOS transistors in analog circuits are discussed.     

Analytical modelling of threshold voltage for underlap Fully Depleted Silicon-On-Insulator MOSFET

In this article, surface-potential-based analytical threshold voltage model for underlap Fully Depleted Silicon-On-Insulator MOSFET (underlap-SOI) is developed by solving two-dimensional Poisson equation. The gate underlap at source/drain (S/D) has different boundary conditions as compared to channel region under the gate dielectric that divide the whole channel into three regions. It leads us to derive the new surface potential model for three different channel regions, i.e. the region under the gate dielectric and two gate underlap regions at S/D. The effects of underlap length, channel length, body thickness, channel doping concentration, metal gate work function and gate dielectric constant on threshold voltage have been included in our model. The threshold voltage dependence on different device parameters has been studied using analytical model and simulations. The closeness between the simulation results and model results show that the analytical model accurately calculate the threshold voltage values for large range of device parameters.     

Threshold voltage-minimum gate length trade-off in buried channelPMOS devices for scaled supply voltage CMOS technologies

The trade-off between threshold voltage (V_th) and the minimum gate length (L_min) is discussed for optimizing the performance of buried channel PMOS transistors for low voltage/low power high-speed digital CMOS circuits. In a low supply voltage CMOS technology it is desirable to scale V_th and L_min for improved circuit performance. However, these two parameters cannot be scaled independently due to the channel punch-through effect. Statistical process/device modeling, split lot experiments, circuit simulations, and measurements are performed to optimize the PMOS transistor current drive and CMOS circuit speed. We show that trading PMOS transistor V_th for a smaller L_min results in faster circuits for low supply voltage (3.3 to 1.8 V) n⁺-polysilicon gate CMOS technology, Circuit simulation and measurements are performed in this study. Approximate empirical expressions are given for the optimum buried channel PMOS transistor V _th for minimizing CMOS circuit speed for cases involving: (1) constant capacitive load and (2) load capacitance proportional to MOS gate capacitance. The results of the numerical exercise are applied to the centering of device parameters of a 0.5 μm 3.3 V CMOS technology that (a) matches the speed of our 0.5 μm 5 V CMOS technology, and (b) achieves good performance down to 1.8 V power supply. For this process the optimum PMOS transistor V_th (absolute value) is approximately 0.85-0.90 V     

Optimum design of n+-n-double-diffused drain MOSFET to reduce hot-carrier emission

Channel electric field reduction using an n⁺-n^-double-diffused drain MOS transistor to suppress hot-carrier emission is investigated. The double-diffused structure consists of a deep low-concentration P region and a shallow high-concentration As region. The channel electric field strongly depends on such process and device parameters as the length of the n^-diffusion region, drain junction depth, gate oxide thickness, gate length, applied voltage, and P implant energy. The optimum condition for a double-diffused structure is determined based on those parameter dependences of the channel electric field. The results of the optimum drain impurity profile to give the minimum channel electric field are obtained when the maximum lateral electric field is located at the boundary between the P region and the As region. The hot-carrier immunity of MOSFET and test circuits are improved by two orders of magnitude and one order of magnitude, respectively, under the optimum conditions.     

Leakage current mechanisms and leakage reduction techniques in deep-submicrometer CMOS circuits

High leakage current in deep-submicrometer regimes is becoming a significant contributor to power dissipation of CMOS circuits as threshold voltage, channel length, and gate oxide thickness are reduced. Consequently, the identification and modeling of different leakage components is very important for estimation and reduction of leakage power, especially for low-power applications. This paper reviews various transistor intrinsic leakage mechanisms, including weak inversion, drain-induced barrier lowering, gate-induced drain leakage, and gate oxide tunneling. Channel engineering techniques including retrograde well and halo doping are explained as means to manage short-channel effects for continuous scaling of CMOS devices. Finally, the paper explores different circuit techniques to reduce the leakage power consumption.     

MOSFET output characteristics after oxide breakdown

The performance of nMOSFETs after the gate oxide (SiO₂) dielectric breakdown (BD) has been studied. Different BD hardness, BD path locations along the channel and device aspect ratios have been considered. The results show that the BD of the gate oxide affects the overall I_D-V_DS characteristics and that the BD impact depends on BD hardness and location and device geometry. To describe the post-BD data, a simple BD MOSFET model has been used, which accounts for the after BD additional gate current and drain current effects. The model is able to fit all the observed post-BD behaviours and can be easily included in a circuit simulator, to evaluate the impact of device BD on the post-BD performance of circuits.     

Transistor scaling with constant subthreshold leakage

A model for scaling transistors with constant subthreshold leakage is presented. In contrast with other scaling theories, the scaling formulation presented does not necessarily lead to transistors with long channel characteristics. Instead, the transistor is scaled only to enhance circuit performance while meeting circuit specifications-in this case, subthreshold leakage current. The model is critically dependent upon the drain induced barrier lowering effect which has been evaluated as a function of channel length, gate thickness, and channel doping. The effect is found to vary asL^{-m}wherem = 1.2-1.4.     

Extraction of gate dependent source/drain resistance and effective channel length in MOS devices at 77 K

A new extraction technique for obtaining the parasitic source/drain resistance and the effective channel length of an MOS device at 77 K is presented. Unlike previous methods, both parameters are assumed to vary with the gate voltage. This results in positive and physically meaningful results at any temperature. Simulation results show that, in non-LDD devices, the source/drain resistance decreases and the effective channel length increases with gate bias, indicating that the gate dependence of both parameters is inherent to MOS devices.<>     

A DC model for asymmetric trapezoidal gate MOSFET's in stronginversion

Asymmetric trapezoidal gate (ATG) MOSFET is an innovative device having a structure of a relatively narrow drain-side width in order to reduce parasitic effects for enhancing device performance. In this paper, we develop a DC model for ATG MOSFET's. We use a charge-based approach to explore the asymmetric feature between source and drain of ATG MOSFET's, and obtain analytic formulae for threshold voltage, body effect, drain current, and channel length modulation effect in linear and saturation regions for both forward and reverse modes of operations. The model provides a physical analysis of the ATG structure, shows good agreement with measurement data, and is useful in circuit simulation with ATG devices     

A JMOS transistor fabricated with 100-Å low-pressure nitrided-oxide gate dielectric

A Junction MOS (JMOS) transistor is proposed to offer increased performance over conventionally scaled NMOS devices as the gate dielectric thickness is reduced. The design, fabrication, and characterization of the JMOS device with a 100-Å gate dielectric is presented. Conventionally scaled NMOS and JMOS devices with gate lengths down to 1 µm are compared. The JMOS devices show a 25- percent increase in channel electron mobility and a 15-percent increase in drain current for equivalent gate drives with minimal adverse short-channel effects.     

Choice of power-supply voltage for half-micrometer and lowersubmicrometer CMOS devices

The tradeoff between circuit performance and reliability is theoretically and experimentally examined in detail, down to half-micrometer and lower submicrometer gate lengths, taking into account high-field effects on MOSFETs. Some guidelines for optimum power-supply voltage and process/device parameters for half-micrometer and lower submicrometer CMOS devices are proposed in order to maintain MOS device reliability and achieve high circuit performance. It is shown that power-supply voltage must be reduced to maintain reliability and improved performance and that the optimum voltage reduction follows the square root of the design rule. Trends for scaling down power-supply voltage have been experimentally verified by results obtained from measurements on CMOS devices over a wide range of gate oxide thickness (7-45 nm) and gate lengths (0.3-2.0 μm)     

A methodology for the implementation of MOSFETs with a high-k dielectric gate material on the design of 90 nm technology circuits

Up to date, MOSFETs have been made through well established techniques that use SiO₂ as the gate dielectric and the related design issues are well established. The need to scale down device dimensions allowed researchers to seek for alternative materials, in order to replace SiO₂ as the gate dielectric. The implementation of such MOS devices in memory or logic circuits needs to take into account the effects that the use of the new gate dielectrics has on parameters such as the threshold voltage and the drain current. Hence, parameters such as the high dielectric constant values, extra oxide charges and process related defects at the physical level must be taken into account during the device design. As far as circuit applications are concerned, these changes may substantially affect the required performance. This paper presents and provides proposals about the issue of replacing commonly used parameters of the MOSFET modelling with new parameters, in which the presence of a gate dielectric with different properties from those of SiO₂ is taken into account. A stepwise procedure is described for the new device design. Moreover, a case study is presented which examines a memory circuit built up by such new technology devices. In particular, this paper presents and analyses the design of a DRAM cell made up of MOSFETs with an alternative gate dielectric. The 90 nm technology and the BSIM4 model equations are used to derive the single MOSFET behaviour and subsequently the DRAM circuit performance. The results are analysed and compared to those obtained from conventional SiO₂ devices. A cell layout is provided and the DRAM circuit characteristics are also presented.     

Threshold voltage predictions from MICROMOS: A 3-D MOS simulator

This paper provides predicted values of threshold voltage for the fully recessed MOS structure obtained from the 3-D code called MICROMOS. Uniform and implanted channel cases are both considered. Additionally, short channel, narrow width and small geometry effects are automatically included. Results for these conditions as well as variable oxide thickness and substrate doping are shown. All of the results obtained indicate the correct tendency of change for threshold voltage. Values for threshold voltage prediction are obtained from 3-D computer generated drain current vs gate voltage characteristics.     

Gate dielectric engineering of quarter sub micron AlGaN/GaN MISHFET: A new device architecture for improved transconductance and high cut-off frequency

In this paper analytical modeling for a novel three region gate dielectric engineered AlGaN/GaN Metal Insulator Semiconductor heterostructure field effect transistor (MISHFET) device architecture is presented which shows high transconductance and enhanced cut-off frequency at quarter micron gate lengths. Using a three region analysis along the horizontal direction in the gate dielectric region the expressions for transconductance and cut-off frequency of the device are obtained. It has been observed that using these gate dielectric schemes, improvements on device performance are observed over conventional MISHFET structures. Relative comparison of T and Γ-gate shaped structures is done with uniform gate dielectric profile and enhancement in microwave performance is observed. The proposed model is capable of modeling electrical characteristics like drain current, output conductance and threshold voltage of various other existent structures like uniform gate dielectric MISHFETs, HFETs and T-gate HFETs. The present model is based on closed form expression and does not involve any fitting parameter. The results obtained are compared with experimental data and show excellent agreement, thereby proving the validity of the model.     

In-process voltage stressing to increase reliability of MOS integrated circuits

Time-dependent dielectric breakdown of gate oxides is one of the principal failure mechanisms of MOS integrated circuits. Voltage stressing of completed devices, which has been used to screen oxide defects and to thereby increase product reliability, is less effective with scaled high-density MOS integrated circuits because of limitations in the voltage which can be applied. Inprocess voltage stressing of silicon wafers, prior to completion of wafer processing, offers a feasible technique for achieving an effective voltage screen. Several possible techniques for inprocess voltage stressing are described, and the advantages and limitations of these are outlined. Data are presented showing typical fast-ramp dielectric breakdown distributions for MOS transistor arrays with an oxide thickness of 35 and 50 nm. Time-dependent dielectric breakdown distribution data on devices from the same wafers indicate that with all MOS transistors of an integrated circuit connected in parallel, as in one type of inprocess voltage stressing, defective oxide sites can be screened in periods of time ranging from a few seconds to hours. Inprocess voltage stressing, by decreasing susceptibility of completed devices to time-dependent dielectric breakdown, can substantially increase MOS integrated circuit reliability.     

Low-Frequency Noise Partition of Asymmetric MOS Transistors Operating in Linear Regime

The low-frequency noise in asymmetric MOS transistors with graded channel doping from the source to the drain can be partitioned by assuming a series connection of two or more transistors along the device's channel length. The partition explains the noise overshoot at gate biases around the threshold voltage of the composite device. Expressions for the input-referred gate noise voltage are obtained and verified.      

Advanced model and analysis of series resistance for CMOS scalinginto nanometer regime. I. Theoretical derivation

An advanced series resistance model is developed to accurately predict source/drain (S/D) series resistance of complementary metal-oxide semiconductor (CMOS) in the nanometer regime. The series resistance is modeled by division into four resistance components named SDE-to-gate overlap, S/D extension, deep S/D, and silicide-diffusion contact resistance, considering the nonnegligible doping-dependent potential relationship in MOS accumulation region due to scaled supply voltage, current behavior related to heavily doped ultra-shallow source/drain extension (SDE) junction, polysilicon gate depletion effects (PDE), lateral and vertical doping gradient effect of SDE junction, silicide-diffusion contact structure, and high-κ dielectric sidewall. The proposed model well characterizes unique features of nanometer-scale CMOS and is useful for analyzing the effect of source/drain parameters on CMOS device scaling and optimization