A non-quasi-static model for nanowire gate-all-around tunneling field-effect transistors

Nanowires with gate-all-around(GAA) structures are widely considered as the most promising candidate for 3-nm technology with the best ability of suppressing the short channel effects,and tunneling field effect transistors(TFETs)based on GAA structures also present improved performance.In this paper,a non-quasi-static(NQS) device model is developed for nanowire GAA TFETs.The model can predict the transient current and capacitance varying with operation frequency,which is beyond the ability of the quasi-static(QS) model published before.Excellent agreements between the model results and numerical simulations are obtained.Moreover,the NQS model is derived from the published QS model including the current-voltage(I-V) and capacitance-voltage(C-V) characteristics.Therefore,the NQS model is compatible with the QS model for giving comprehensive understanding of GAA TFETs and would be helpful for further study of TFET circuits based on nanowire GAA structure.

An analytic model for gate-all-around silicon nanowire tunneling field effect transistors

An analytical model of gate-all-around （GAA） silicon nanowire tunneling field effect transistors （NW-TFETs） is developted based on the surface potential solutions in the channel direction and considering the band to band tunneling （BTBT） efficiency. The three-dimensional Poisson equation is solved to obtain the surface potential distributions in the partition regions along the channel direction for the NW-TFET, and a tunneling current model using Kane＇s expression is developed. The validity of the developed model is shown by the good agreement between the model predictions and the TCAD simulation results.

Comparison of tunneling currents in graphene nanoribbon tunnel field effect transistors calculated using Dirac-like equation and Schrodinger’s equation

The tunneling current in a graphene nanoribbon tunnel field effect transistor(GNR-TFET) has been quantum mechanically modeled. The tunneling current in the GNR-TFET was compared based on calculations of the Dirac-like equation and Schrodinger’s equation. To calculate the electron transmittance, a numerical approach-namely the transfer matrix method(TMM)-was employed and the Launder formula was used to compute the tunneling current. The results suggest that the tunneling currents that were calculated using both equations have similar characteristics for the same parameters, even though they have different values. The tunneling currents that were calculated by applying the Dirac-like equation were lower than those calculated using Schrodinger’s equation.

Tunneling field effect transistors based on in-plane and vertical layered phosphorus heterostructures

Tunneling field effect transistors（TFETs） based on two-dimensional materials are promising contenders to the traditional metal oxide semiconductor field effect transistor, mainly due to potential applications in low power devices. Here,we investigate the TFETs based on two different integration types： in-plane and vertical heterostructures composed of two kinds of layered phosphorous（β-P and δ-P） by ab initio quantum transport simulations. NDR effects have been observed in both in-plane and vertical heterostructures, and the effects become significant with the highest peak-to-valley ratio（PVR）when the intrinsic region length is near zero. Compared with the in-plane TFET based on β-P and δ-P, better performance with a higher on/off current ratio of - 10-6 and a steeper subthreshold swing（SS） of - 23 mV/dec is achieved in the vertical TFET. Such differences in the NDR effects, on/off current ratio and SS are attributed to the distinct interaction nature of theβ-P and δ-P layers in the in-plane and vertical heterostructures.

Characteristics of cylindrical surrounding-gate GaAs_xSb_(1-x)/In_yGa_(1-y)As heterojunction tunneling field-effect transistors

A Ⅲ-Ⅴ heterojunction tunneling field-effect transistor(TFET) can enhance the on-state current effectively,and GaAsSb/InGaAs heterojunction exhibits better performance with the adjustable band alignment by modulating the alloy composition.In this paper,the performance of the cylindrical surrounding-gate GaAsSb/InGaAs heterojunction TFET with gate-drain underlap is investigated by numerical simulation.We validate that reducing drain doping concentration and increasing gate-drain underlap could be effective ways to reduce the off-state current and subthreshold swing(SS),while increasing source doping concentration and adjusting the composition of GaAsSbInGaAs can improve the on-state current.In addition,the resonant TFET based on GaAsSb/InGaAs is also studied,and the result shows that the minimum and average of SS reach 11 mV/decade and 20 mV/decade for five decades of drain current,respectively,and is much superior to the conventional TFET.

Si–Ge based vertical tunnel field-effect transistor of junction-less structure with improved sensitivity using dielectric modulation for biosensing applications

A dielectric modulation strategy for gate oxide material that enhances the sensing performance of biosensors in junction-less vertical tunnel field effect transistors(TFETs)is reported.The junction-less technique,in which metals with specific work functions are deposited on the source region to modulate the channel conductivity,is used to provide the necessary doping for the proper functioning of the device.TCAD simulation studies of the proposed structure and junction structure have been compared,and showed an enhanced rectification of 10^(4) times.The proposed structure is designed to have a nanocavity of length 10 nm on the left-and right-hand sides of the fixed gate dielectric,which improves the biosensor capture area,and hence the sensitivity.By considering neutral and charged biomolecules with different dielectric constants,TCAD simulation studies were compared for their sensitivities.The off-state current IOFFcan be used as a suitable sensing parameter because it has been observed that the proposed sensor exhibits a significant variation in drain current.Additionally,it has been investigated how positively and negatively charged biomolecules affect the drain current and threshold voltage.To explore the device performance when the nanogaps are fully filled,half filled and unevenly filled,extensive TCAD simulations have been run.The proposed TFET structure is further benchmarked to other structures to show its better sensing capabilities.

Design and Analysis of Graphene Based Tunnel Field Effect Transistor with Various Ambipolar Reducing Techniques

The fundamental advantages of carbon-based graphene material,such as its high tunnelling probability,symmetric band structure(linear dependence of the energy band on the wave direction),low effective mass,and characteristics of its 2D atomic layers,are the main focus of this research work.The impact of channel thickness,gate under-lap,asymmetric source/drain doping method,workfunction of gate contact,and High-K material on Graphene-based Tunnel Field Effect Transistor(TFET)is analyzed with 20 nm technology.Physical modelling and electrical characteristic performance have been simulated using the Atlas device simulator of SILVACO TCAD with user-defined material syntax for the newly included graphene material in comparison to silicon carbide(SiC).The simulation results in significant suppression of ambipolar current to voltage characteristics of TFET and modelled device exhibits a significant improvement in subthreshold swing(0.0159 V/decade),the ratio of Ion/Ioff(1000),and threshold voltage(-0.2 V with highly doped p-type source and 0.2 V with highly doped n-type drain)with power supply of 0.5 V,which make it useful for low power digital applications.

Influence of trap-assisted tunneling on trap-assisted tunneling current in double gate tunnel field-effect transistor

Trap-assisted tunneling（TAT） has attracted more and more attention, because it seriously affects the sub-threshold characteristic of tunnel field-effect transistor（TFET）. In this paper, we assess subthreshold performance of double gate TFET（DG-TFET） through a band-to-band tunneling（BTBT） model, including phonon-assisted scattering and acoustic surface phonons scattering. Interface state density profile（D_（it）） and the trap level are included in the simulation to analyze their effects on TAT current and the mechanism of gate leakage current.

Ge/Si heterojunction L-shape tunnel field-effect transistors with hetero-gate-dielectric

A Ge/Si heterojunction L-shaped tunnel field-effect transistor combined with hetero-gate-dielectric （GHL-TFET） is proposed and investigated by TCAD simulation. Current-voltage characteristics, energy-band diagrams, and the distri- bution of the band-to-band tunneling （BTBT） generation rate of GHL-TFET are analyzed. In addition, the effect of the vertical channel width on the ON-current is studied and the thickness of the gate dielectric is optimized for better suppression of ambipolar current. Moreover, analog/RF figure-of-merits of GHL-TFET are also investigated in terms of the cut-off frequency and gain bandwidth production. Simulation results indicate that the ON-current of GHL-TFET is increased by about three orders of magnitude compared with that of the conventional L-shaped TFET. Besides, the introduction of the hetero-gate-dielectric not only suppresses the ambipolar current effectively but also improves the analog/RF performance drastically. It is demonstrated that the maximum cut-off frequency of GHL-TFET is about 160 GHz, which is 20 times higher than that of the conventional L-shaped TFET.

Two-dimensional threshold voltage model of a nanoscale silicon-on-insulator tunneling field-effect transistor

The tunneling field-effect transistor（TFET） is a potential candidate for the post-CMOS era.In this paper,a threshold voltage model is developed for this new kind of device.First,two-dimensional（2D） models are used to describe the distributions of potential and electric field in the channel and two depletion regions.Then based on the physical definition of threshold voltage for the nanoscale TFET,the threshold voltage model is developed.The accuracy of the proposed model is verified by comparing the calculated results with the 2D device simulation data.It has been demonstrated that the effects of varying the device parameters can easily be investigated using the model presented in this paper.This threshold voltage model provides a valuable reference to TFET device design,simulation,and fabrication.

Optimization of ambipolar current and analog/RF performance for T-shaped tunnel field-effect transistor with gate dielectric spacer

A new T-shaped tunnel field-effect transistor(TTFET) with gate dielectric spacer(GDS) structure is proposed in this paper. To further studied the effects of GDS structure on the TTFET, detailed device characteristics such as current-voltage relationships, energy band diagrams, band-to-band tunneling(BTBT) rate and the magnitude of the electric field are investigated by using TCAD simulation. It is found that compared with conventional TTFET and TTFET with gate-drain overlap(GDO) structure, GDS-TTFET not only has the minimum ambipolar current but also can suppress the ambipolar current under a more extensive bias range. Furthermore, the analog/RF performances of GDS-TTFET are also investigated in terms of transconductance, gate-source capacitance, gate-drain capacitance, cutoff frequency, and gain bandwidth production. By inserting a low-κ spacer layer between the gate electrode and the gate dielectric, the GDS structure can effectively reduce parasitic capacitances between the gate and the source/drain, which leads to better performance in term of cutoff frequency and gain bandwidth production. Finally, the thickness of the gate dielectric spacer is optimized for better ambipolar current suppression and improved analog/RF performance.

Double-gate-all-around tunnel field-effect transistor

In this work, a double-gate-all-around tunneling field-effect transistor is proposed. The performance of the novel device is studied by numerical simulation. The results show that with a thinner body and an additional core gate, the novel device achieves a steeper subthreshold slope, less susceptibility to the short channel effect, higher on-state current, and larger on/off current ratio than the traditional gate-all-around tunneling field-effect transistor. The excellent performance makes the proposed structure more attractive to further dimension scaling.

Heteromaterial-gate line tunnel field-effect transistor based on Si/Ge heterojunction

A Si/Ge heterojunction line tunnel field-effect transistor （LTFET） with a symmetric heteromaterial gate is proposed. Compared to single-material-gate LTFETs, the heteromaterial gate LTFET shows an off-state leakage current that is three orders of magnitude lower, and steeper subthreshold characteristics, without degradation in the on-state current. We reveal that these improvements are due to the induced local potential barrier, which arises from the energy-band profile modulation effect. Based on this novel structure, the impacts of the physical parameters of the gap region between the pocket and the drain, including the work-function mismatch between the pocket gate and the gap gate, the type of dopant, and the doping concentration, on the device performance are investigated. Simulation and theoretical calculation results indicate that the gap gate material and n-type doping level in the gap region should be optimized simultaneously to make this region fully depleted for further suppression of the off-state leakage current.

Quantum confinement effects and source-to-drain tunneling in ultra-scaled double-gate silicon n-MOSFETs

By using the linear combination of bulk band （LCBB） method incorporated with the top of the barrier splitting （TBS） model, we present a comprehensive study on the quantum confinement effects and the source-to-drain tunneling in the ultra-scaled double-gate （DG） metal-oxide semiconductor field-effect transistors （MOSFETs）. A critical body thickness value of 5 nm is found, below which severe valley splittings among different X valleys for the occupied charge density and the current contributions occur in ultra-thin silicon body structures. It is also found that the tunneling current could be nearly 100% with an ultra-scaled channel length. Different from the previous simulation results, it is found that the source-to-drain tunneling could be effectively suppressed in the ultra-thin body thickness （2.0 nm and below） by the quantum confinement and the tunneling could be suppressed down to below 5% when the channel length approaches 16 nm regardless of the body thickness.

Ambipolar performance improvement of the C-shaped pocket TFET with dual metal gate and gate–drain underlap

Dual-metal gate and gate–drain underlap designs are introduced to reduce the ambipolar current of the device based on the C-shaped pocket TFET(CSP-TFET).The effects of gate work function and gate–drain underlap length on the DC characteristics and analog/RF performance of CSP-TFET devices,such as the on-state current(I_(on)),ambipolar current(I_(amb)),transconductance(g_(m)),cut-off frequency(f_(T))and gain–bandwidth product(GBP),are analyzed and compared in this work.Also,a combination of both the dual-metal gate and gate–drain underlap designs has been proposed for the C-shaped pocket dual metal underlap TFET(CSP-DMUN-TFET),which contains a C-shaped pocket area that significantly increases the on-state current of the device;this combination design substantially reduces the ambipolar current.The results show that the CSP-DMUN-TFET demonstrates an excellent performance,including high I_(on)(9.03×10^(-4)A/μm),high I_(on)/I_(off)(~10^(11)),low SS_(avg)(~13 mV/dec),and low I_(amb)(2.15×10^(-17)A/μm).The CSP-DMUN-TFET has the capability to fully suppress ambipolar currents while maintaining high on-state currents,making it a potential replacement in the next generation of semiconductor devices.

n型纳米非对称DG-TFET阈值电压特性研究

n型纳米非对称双栅隧穿场效应晶体管(DG-TFET)速度快、功耗低,在高速低功耗领域具有很好的应用前景,但其阈值电压的表征及其模型与常规MOSFET不同。在深入研究n型纳米非对称DG-TFET的阈值特性基础上,通过求解器件不同区域电场、电势的方法,建立了n型纳米非对称DG-TFET器件阈值电压数值模型,探讨了器件材料物理参数以及漏源电压对阈值电压的影响,通过与Silvaco Atlas的仿真结果比较,验证了模型的正确性。研究表明,n型纳米非对称DG-TFET的阈值电压分别随着栅介质层介电常数的增加、硅层厚度的减薄以及源漏电压的减小而减小,而栅长对其阈值电压的影响有限。该研究对纳米非对称DG-TFET的设计、仿真及制造有一定的参考价值。

双栅无掺杂隧穿晶体管特性提升仿真研究

对双栅无掺杂型隧穿晶体管(DopingLess TFET,DLTFET)进行了仿真研究。通过对传统无掺杂型器件的载流子分布、电流密度、电势分布及能带图等参数的深入研究,较为全面的了解了此类器件的工作原理。基于DGTFET器件的仿真研究成果,提出了使用两种功函数栅材料构成的隧穿栅(Ф_(TG)=4.0 eV)-控制栅(Ф_(CG)=4.6 eV)-辅助栅(Ф_(AG)=4.0 eV)及异质栅介质共同实现的新型DLTFET器件。含异质栅介质和3种栅材料双栅无掺杂隧穿场效应晶体管(Hetero gate oxide TMDG-DLTFET,HG-TMDG-DLTFET),可有效促进形成更为陡峭的“突变结”。通过仿真验证了该器件较传统DLTFET的有效隧穿分布面积更大,提升了隧穿电流,表现了很好的频率特性。当栅压为1.0 V且漏压为0.5 V时,该器件的开态电流为5.5×10^(−6)A,较传统DLTFET器件的开态电流5.9×10^(−10) A提高了4个数量级;当栅压增至1.5 V时,开态电流达到了2.3×10^(−5)A。

异质栅介质双栅隧穿场效应晶体管TCAD仿真研究

采用异质栅介质结构,加入高k介质Pocket区,对双栅隧穿场效应晶体管进行了改进。通过搭建不同模型,分析了栅介质的长度、介电常数以及Pocket区厚度等参数对器件开态电流Ion、双极性特性、亚阈值摆幅等的影响。TACD仿真结果表明,Ion随介电常数的增大可提升至5.17×10-5A/μm,且双极性电流也有极大的增幅,亚阈值摆幅因开态电流的改善降低至28.3 mV/dec,而异质栅介质的不同长度对器件性能并无明显影响;在双极性上,Pocket区厚度的增加使得栅漏隧穿宽度增大,双极性电流减小至1.0×10-17A/μm,抑制了双极性导通,同时对器件其他特性未产生明显影响。

MTJ和TFET中隧道电流的计算及研究进展

随着器件尺寸的缩小和集成度的不断提高,电子器件的发展正遭遇功耗以及尺寸极限等瓶颈。为突破这些瓶颈,许多新型的电子器件模型应运而生。其中重要的一类是基于量子力学隧道效应或者与隧道效应直接相关的器件。因此,严格计算隧道电流成为这些器件研究的基础。本文以磁隧道结和隧穿场效应管为例,介绍了隧道电流计算的理论基础以及一维到一维(1T1)、二维到二维(2T2)、二维到三维(2T3)、三维到二维(3T2)等各种模式的隧道电流的计算方法和研究进展。由于石墨烯具有许多独特性质,成为近年人们关注的明星材料。但石墨烯材料的采用也带来许多理论和实践中的问题,比如3T2隧穿的理论计算问题。本文对这些问题也进行了简单综述和讨论。

基于FDSOI的TFET和MOSFET总剂量效应仿真

对基于全耗尽绝缘体上硅(FDSOI)的隧穿场效应晶体管(TFET)器件和金属氧化物半导体场效应晶体管(MOSFET)器件进行了总剂量(TID)效应仿真,基于两种器件不同的工作原理,研究了总剂量效应对两种器件造成的电学影响,分析了辐照前后TFET和MOSFET的能带结构、载流子密度等关键因素的变化。仿真结果表明:两种器件在受到较大辐射剂量时(1 Mrad (Si)),TFET受辐射引起的固定电荷影响较小,仍能保持较好的开关特性、稳定的阈值电压;而MOSFET则受固定电荷的影响较大,出现了背部导电沟道,其关态电流增加了几个数量级,开关特性发生了严重退化,阈值电压也严重地向负电压偏移。此外,TFET的开态电流会随着辐照剂量的增加而减小,这与MOSFET的表现恰好相反。因此TFET比MOSFET有更好的抗总剂量效应能力。