Modeling current transport in ultra-scaled field-effect transistors

An overview of models used for the simulation of current transport in nanoelectronic devices within the framework of TCAD applications is presented. Modern enhancements of semiclassical transport models based on microscopic theories as well as quantum-mechanical methods used to describe coherent and dissipative quantum transport are specifically addressed. This comprises the incorporation of quantum corrections and tunneling models up to dedicated quantum-mechanical simulators, and mixed approaches which are capable of accounting for both, quantum interference and scattering. Specific TCAD requirements are discussed from an engineer’s perspective and an outlook on future research directions is given.     

Quantum mechanical effects on noise properties of nanoelectronic devices: application to Monte Carlo simulation

A discussion about the quantum mechanical effects on noise properties of ballistic (phase-coherent) nanoscale devices is presented. It is shown that quantum noise can be understood in terms of quantum trajectories. This interpretation provides a simple and intuitive explanation of the origin of quantum noise that can be very salutary for nanoelectronic engineers. In particular, an injection model is presented that, coupled with a standard Monte Carlo algorithm, provides an accurate modeling of quantum noise. As a test, the standard results of noise in tunneling junction devices are reproduced within this approach.     

Combined two-band models of resonant tunneling diodes

A satisfactory agreement between calculated voltage-current characteristics of GaAs/AlAs and Si/SiGe heterostructure resonant tunneling diodes and experimental data was obtained by using combined two-band models based on semiclassical and quantum-mechanical approaches. A high sensitivity of the characteristics of GaAs/AlAs-based devices to factors such as the transverse wave vector, changes of the heterostructure??s X-conduction band minimum, the surface charge density on heterointerfaces, and G-X intervalley scattering, is shown.     

Inelastic scattering effects on carrier relaxation in quantum well-based hot electron structures

We calculate the inelastic scattering rate of “hot” electrons injected into doped quantum well structures by including on an equal footing, both the electron-electron (Coulomb) and the electron-polar optical phonon (Fröhlich) interaction effects in the theory within the dynamical random-phase-approximation. Our theory includes in a consistent diagrammatic approximation effects of quantum statistics, dynamical screening and phonon renormalization (i.e. the so-called “plasmon-phonon coupling” effect). Our results, obtained for the two-dimensional, the quasi-two-dimensional and the three-dimensional models, are appropriate for a number of transport and optical experiments including ballistic hot-electron transport, resonant tunneling, femtosecond carrier relaxation, and, planar parallel transport.     

考虑源漏隧穿的DG MOSFET弹道输运及其模拟

在经典弹道输运模型中引入源漏隧穿 (S/ D tunneling) ,采用 WKB方法计算载流子源漏隧穿几率 ,对薄硅层(硅层厚度为 1nm) DG(dual gate) MOSFETs的器件特性进行了模拟 .模拟结果表明当沟道长度为 10 nm时 ,源漏隧穿电流在关态电流中占 2 5 % ,在开态电流中占 5 % .随着沟道长度进一步减小 ,源漏隧穿比例进一步增大 .因此 ,模拟必须包括源漏隧穿 .     

MC simulation of strained-Si MOSFET with full-band structure and quantum correction

A new two-dimensional full-band Monte Carlo simulator, "Monte Carlo University of Texas" (MCUT) is introduced and described in this paper. MCUT combines some of the best features of semiclassical MC device simulation including full-band structure and flexibility of scattering processes, with generality of material composition and the ability to address degeneracy breaking among energy valleys and the associated effects on scattering and transport due to quantum confinement and strain effects. The latter capability derives from extension of a prior crystal-momentum-independent self-consistent Poisson-Schro/spl uml/dinger-based quantum corrected potential, to a valley dependent quantum correction via, in part, a new modeling concept of "effective strain" within the full-band structure code. Low field mobility simulation results for large tensile strained-Si channel nMOSFETs and unstrained-Si channel nMOSFETs device are compared with other simulation methods and experimental data to demonstrate the effectiveness of the approach, and the abilities to simulate high-field transport and transport in devices of a few 10s of nanometer channel lengths are briefly demonstrated.     

Influence of Elastic and Inelastic Phonon Scattering on the Drive Current of Quasi-Ballistic MOSFETs

In this paper, we study the influence of elastic and inelastic phonon scattering on the drive current of Si MOSFETs under quasi-ballistic transport. Inelastic phonon emission involving energy relaxation helps achieve ballistic current, even in the presence of scattering, if the channel length is scaled down to the 10-nm scale. This result agrees with Natori's previous predictions. However, for longer channel devices, inelastic phonon emission degrades the drain current due to space charge effects caused by charge accumulation. We also demonstrate that source-end potential engineering to electrically reduce the bottleneck barrier length can result in a ballistic current even in longer channel devices.      

漏极散射机制对弹道和非弹道沟道应变硅二极管中电子输运特性的影响

在漏极区域，多重散射对于沟道区域有无散射的应变硅二极管中电子传输性能的影响进行了数值研究。使用应变和散射模型，对于非弹道（有散射）沟道硅二极管的性能与弹道（无散射）沟道的硅二极管的性能进行了比较研究。研究结果表明应变模型中的电子速度和电流的值比无应变模型的相应值更高，弹道沟道模型中的电子速度和电流的值比非弹道沟道模型中的相应值更高。使用应变和散射模型，漏极区域中的每个散射机制对于硅二极管性能的影响进行了分析。对于弹道沟道模型，结果表明谷间光学声子散射会提高器件的性能，而谷内声学声子散射会降低器件的性能。对于应变模型，结果表明较大应变硅能带分裂可以抑制谷间声子散射率。总而言之，为了提高纳米级弹道器件性能，对于漏极区域应变和散射机制模型的研究是很有必要的。     

Nonlinear electron properties of an InGaAs/InAlAs-based ballistic deflection transistor: Room temperature DC experiments and numerical simulations

We present a detailed experimental and numerical study of a novel device so-called ballistic deflection transistor (BDT). Based on InGaAs-InAlAs heterostructure on InP substrate, BDT utilizes a two dimensional electron gas (2DEG) supported by a gated microstructure to achieve nonlinear electron transport at room temperature. BDT channel is larger than the mean free path implying that electron transport is not purely ballistic in nature. However, the asymmetric geometrical deflection combined with the electron steering caused by the applied differential gate voltages ultimately results in an attractive nonlinear behavior of the BDT useful for the working of the large scale devices at room temperature. Device performance was studied by analyzing the effects of several modifications of the BDT geometry and biasing conditions, both experimentally and by numerical simulations.     

Phase-breaking effect on polaron transport in organic conjugated polymers

Despite intense investigations and many accepted viewpoints on theory and experiment, the coherent and incoherent carrier transport in organic semiconductors remains an unsettled topic due to the strong electron-phonon coupling. Based on the tight-binding Su-Schrieffer-Heeger (SSH) model combined with a non-adiabatic dynamics method, we study the effect of phase-breaking on polaron transport by introducing a group of phase-breaking factors into π-electron wave-functions in organic conjugated polymers. Two approaches are applied: the modification of the transfer integral and the phase-breaking addition to the wave-function. Within the former, it is found that a single site phase-breaking can trap a polaron. However, with a larger regular phase-breaking a polaron becomes more delocalized and lighter. Additionally, a group of disordered phase-breaking factors can make the polaron disperse in transport process. Within the latter approach, we show that the phase-breaking can render the delocalized state in valence band discrete and the state in the gap more localized. Consequently, the phase-breaking frequency and intensity can reduce the stability of a polaron. Overall, the phase-breaking in organic systems is the main factor that degrades the coherent transport and destroys the carrier stability.     

On the ballistic transport in nanometer-scaled DG MOSFETs

The scattering effects are studied in nanometer-scaled double-gate MOSFET using Monte Carlo simulation. The nonequilibrium transport in the channel is analyzed with the help of the spectroscopy of the number of scatterings experienced by electrons. We show that the number of ballistic electrons at the drain-end, even in terms of flux, is not the only relevant characteristic of ballistic transport. Then, the drive current in the 15-nm-long channel transistor generations should be very close to the value obtained in the ballistic limit even if all electrons are not ballistic. Additionally, most back-scattering events, which deteriorate the on current, take place in the first half of the channel and, in particular, in the first low field region. However, the contribution of the second half of the channel cannot be considered as negligible in any studied case i.e., for a channel length below 25 nm. Furthermore, the contribution of the second half of the channel tends to be more important as the channel length is reduced. So, in ultrashort-channel transistors, it becomes very difficult to extract a region of the channel, which itself determine the drive current I/sub on/.     

Detailed modeling of sub-100-nm MOSFETs based on Schro/spl uml/dinger DD per subband and experiments and evaluation of the performance gap to ballistic transport

We analyze in detail the requirements for the detailed physical modeling of nanoscale MOSFETs and show that Schro/spl uml/dinger drift-diffusion per subband simulations are adequate for the inverse modeling of bulk-Si MOSFETs with gate length down to 40 nm (channel length down to 26 nm) from their dc electrical characterization. We show that a proper treatment of quantum effects both in the channel and in the polysilicon gate through the direct solution of Schro/spl uml/dinger equation, and a transport model based on two-dimensional subbands are required for accurate and-after calibration-predictive modeling. The model is included in the NANOTCAD2D code (Curatola and Iannaccone, 2003). We also evaluate the performance gap to ballistic transport, by comparing the experiments with simulations based on a fully ballistic transport model on the devices structures extracted with the inverse modeling procedure.     

Controlling Phase-Coherent Electron Transport in III-Nitrides: Toward Room Temperature Negative Differential Resistance in AlGaN/GaN Double Barrier Structures

Resonant tunneling of electrons is important for the manufacture of high-speed electronic oscillators and the electron injection control in quantum cascade lasers. In this work, room temperature negative differential resistance (NDR) in AlGaN/GaN double barrier structure with AlN/GaN digital alloy (DA) barriers is demonstrated. The peak-to-valley current ratio (PVCR) ranges from 1.1 to 1.24 at room temperature and becomes 1.5 to 2.96 at low temperatures, whereas no NDR is observed in double barrier structures with conventional ternary AlGaN barriers. The room temperature NDR together with the high PVCR at low temperature is attributed to the suppression of alloy disorder scattering by introducing AlN/GaN DA barriers. This work presents the successful control of phase-coherent electron transport in III-nitride heterostructures and is expected to benefit the future design of nitride-based resonant tunneling structures and high-speed electronic devices.     

Analysis of subthreshold carrier transport for ultimate DGMOSFET

A novel transport model for the subthreshold mode of double-gate MOSFETs (DGMOSFETs) is proposed in this paper. The model enables the analysis of short-channel effects (SCEs) such as the subthreshold swing (SS), the threshold-voltage rolloff, and the drain-induced barrier lowering. The proposed model includes the effects of thermionic emission and the quantum tunneling of carriers through the source-drain barrier. An approximative solution of the two-dimensional Poisson equation is used for the distribution of the electric potential, and the Wentzel-Kramers-Brillouin approximation is used for the tunneling probability. The model is verified by comparing the SS with numerical simulations. The new model is used to investigate the subthreshold characteristics of a DGMOSFET having the gate length in the nanometer range with an ultrathin gate oxide and channel thickness. The SCEs degrade the subthreshold characteristics of DGMOSFETs when the gate length is reduced below 10 nm, and any design in the sub-10-nm-regime should include the effects of quantum tunneling.     

Modeling of 10-nm-scale ballistic MOSFET's

We have performed numerical modeling of nanoscale dual-gate ballistic n-MOSFET's with ultrathin undoped channel, taking into account the effects of quantum tunneling along the channel and through the gate oxide. The results show that transistors with channel length as small as 8 nm can exhibit either a transconductance up to 4000 mS/mm or gate modulation of current by more than 8 orders of magnitude, depending on the gate oxide thickness. These characteristics make the sub-10-nm devices potentially suitable for logic and memory applications, though their parameters are rather sensitive to size variations     

Local tunneling spectroscopy of silicon nanostructures

The recharging of many-hole and few-electron quantum dots under the conditions of the ballistic transport of single charge carriers inside self-assembled quantum well structures on a Si (100) surface are studied using local tunneling spectroscopy at high temperatures (up to room temperature). On the basis of measurements of the tunneling current-voltage characteristics observed during the transit of single charge carriers through charged quantum dots, the modes of the Coulomb blockade, Coulomb conductivity oscillations, and electronic shell formation are identified. The tunneling current-voltage characteristics also show the effect of quantum confinement and electron-electron interaction on the characteristics of single-carrier transport through silicon quantum wires containing weakly and strongly coupled quantum dots.     

A comparative study for quantum transport calculations of nanosized field-effect transistors

This article presents a comparative study on quantum mechanical frameworks between the widely used local Quasi-Fermi Level (QFL) model and a recently developed top of the barrier splitting (TBS) model. Both models are based on an atomistic quantum mechanical solver using the linear combination of bulk band method (LCBB). The QFL model uses the local Quasi-Fermi Level to represent the local equilibrium and calculate the occupied charge density as well as the current flow along the channel. The TBS model extracts scattering state information from the stationary solution of the system, then calculates the charge density as well as the ballistic and tunneling current. Using these two models, the 10 nm and 22 nm double-gate ultra-thin-body structures are simulated. Comparisons in occupied charge densities, self-consistent potentials as well as the I–V characteristics between these two models are presented. It is found that the QFL model significantly overestimate the subthreshold charge density inside the channel, as well as the current, while it works fine in the ON state of the device. It is also found that the results from both QFL and TBS models tend to coincide with each other as the drain bias approaching zero.     

n-Type Ge–SiGe Quantum Cascade Structure Utilizing Quantum Wells for Electrons in the L and Γ Valleys

In this letter, we propose an n-type vertical transition bound-to-continuum Ge-SiGe quantum cascade structure utilizing electronic quantum wells in the L and Gamma valleys of the Ge layers. The optical transition levels are located in the quantum wells in the L valley. Under a bias of 80 kV/cm, the carriers in the lower level are extracted by miniband transport and L -Y tunneling into the subband in the Gamma well of the next period. And then the electrons are injected into the upper level by ultrafast intervalley scattering, which not only effectively increases the tunneling rate and suppresses the thermal backfilling of electrons, but also enhances the injection efficiency of the upper level. The performance of the laser is discussed.