Narrow-Width SOI Devices: The Role of Quantum–Mechanical Size Quantization Effect and Unintentional Doping on the Device Operation (original) (raw)
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Role of quantization effects in the operation of ultrasmall MOSFETs and SOI device structures
Microelectronic Engineering, 2002
The continued scaling of devices towards the ultimate limit of 50-nm MOSFET by the year 2007 necessitates the use of higher substrate doping densities in both conventional devices and in the alternative device technologies. The higher substrate doping density, on the other hand, gives rise to pronounced space quantization effects that must be taken into account when modeling these novel device structures. One way tö include space quantization is via solution of the Schrodinger equation coupled to conventional drift-diffusion, hydrodynamic or Monte Carlo particle-based simulators. An alternative way is to use the recently proposed effective potential approach. In this work, we apply the effective potential approach when modeling a conventional 50-nm MOSFET device and an SOI device structure. For the SOI device we also utilize the Landauer's approach to calculate the current and estimate the device threshold voltage increase due to the lateral quantization.
MONTE-CARLO SIMULATION OF ULTRA-THIN FILM SILICON-ON-INSULATOR MOSFETs
State-of-the-Art devices are approaching to the performance limit of traditional MOSFET as the critical dimensions are shrunk. Ultrathin fully depleted Silicon-on-Insulator transistors and multigate devices based on SOI technology are the best candidates to become a standard solution to overcome the problems arising from such aggressive scaling. Moreover, the flexibility of SOI wafers and processes allows the use of different channel materials, substrate orientations and layer thicknesses to enhance the performance of CMOS circuits. From the point of view of simulation, these devices pose a significant challenge. Simulations tools have to include quantum effects in the whole structure to correctly describe the behavior of these devices. The Multi-Subband Monte Carlo (MSB-MC) approach constitutes today's most accurate method for the study of nanodevices with important applications to SOI devices. After reviewing the main basis of MSB-MC method, we have applied it to answer important questions which remain open regarding ultimate SOI devices. In the first part of the chapter we present a thorough study of the impact of different Buried OXide (BOX) configurations on the scaling of extremely thin fully depleted SOI devices using a Multi-Subband Ensemble Monte Carlo simulator (MS-EMC). Standard thick BOX, ultra thin BOX (UTBOX) and UTBOX with ground plane (UTBOX+GP) solutions have been considered in order to check their influence on short channel effects (SCEs). The simulations show that the main limiting factor for downscaling is the DIBL and the UTBOX+GP configuration is the only valid one to downscale SGSOI transistors beyond 20 nm channel length keeping the silicon slab thickness above the theoretical limit of 5 nm, where thickness variability and mobility reduction would play an important role. In the second part, we have used the multisubband Ensemble Monte Carlo simulator to study the electron transport in ultrashort DGSOI devices with different confinement and transport directions. Our simulation results show that transport effective mass, and subband redistribution are the main factors that affect drift and scattering processes and, therefore, the general performance of DGSOI devices when orientation is changed Keywords: Silicon-on-Insulator; scaling; short-channel effects; Multisubband Ensemble Monte Carlo; multigate transistor; quantum effects.
Quantum Effects in SOI Devices
Quantum effects have been reported to play an important role in the operation of narrow width SOI devices, in which the carriers experience a two dimensional confinement in a square quantum well at the semiconductor-oxide interface. This results not only in a significant increase in the threshold voltage but also in its pronounced channel width dependency. Typical method to simulate these effects is a simultaneous solution of the Schrödinger and Poisson equations, which can be a very time consuming procedure. An alternative way is to use the recently developed effective potential approach that takes into account the natural nonzero size of an electron wave packet in the quantized system. In this work, we have applied the effective potential approach in a recently proposed SOI device structure to quantify these effects. In a second effort we utilize the Landauer's formalism to calculate the on-state current quantum mechanically and estimate the increase in device threshold voltage due to the lateral quantization.
All-quantum simulation of an ultra-small SOI MOSFET
Proc. SPIE, 2008
The all-quantum program for 3D simulation of an ultra-thin body SOI MOSFET is overviewed. It is based on Landauer-Buttiker approach to calculate current. The necessary transmission coefficients are acquired from the selfconsistent solution of Schrödinger equation. The latter is stabilized with the help of expanding the wave function over the modes of transversal quantization inside the transistor channel. The program also contains a domain for onedimensional classical ballistics intended for calculation of the initial state for subsequent all-quantum simulation. This is a significant point of our approach as the straightforward procedure of the self-consistent solution of Schrödinger equation from the very beginning is diverging or, at least, extremely time-consuming. The main goal of all-quantum simulation is to clarify the impact of interference on charged impurities and quantum reflection from self-consistent potential on I-V curve reproducibility for different randomly doped transistors in a circuit. The 10nm technology node tri-gate (wrapped channel) structure with 2nm silicon body was used in simulation. 20 discrete impurities were dispersed by the source and drain contacts to imitate the same doping. The most important feature we demonstrate is a smoothness of I-V curves in spite of beforehand apprehension. The next peculiarity we came across was that the current spanned within 10% for different discrete impurity realizations. These results manifest that the reproducibility of nanotransistors could be fairly good to make ultra-large integrated circuits still feasible. We have also made a comparison with simulations based on drift-diffusion model.
Quantum effects are known to occur in the channel region of MOSFET devices, in which the carriers are confined in a triangular potential well at the semiconductor-oxide interface. Typically, these effects are quantified by a simultaneous solution of the Schrödinger and Poisson equations, which can be a very time consuming procedure if it needs to be incorporated in realistic device simulations. We have developed a simple and very efficient approach of approximating quantum effects by using an effective potential that takes into account the natural non-zero size of an electron wave packet in the quantized system. The benefits of the effective potential approach are that it eliminates the need for a full solution to the Schrödinger equation, thus leading to low additional computational cost. In this paper, the approach is applied in the investigation of the role of quantum-mechanical space-quantization effects in the operation of 0.1 µm MOSFET device and recently proposed SOI device structure.
Quantum simulations of an ultrashort channel single-gated n-MOSFET on SOI
IEEE Transactions on Electron Devices, 2002
We present quantum mechanical simulations of a single-gated ultrashort channel MOSFET on silicon-on-insulator (SOI). Ballistic transport is assumed, in order to investigate ideal device performance. In particular, the electrical characteristics and the dependence on the SOI body thickness variation and doping of source and drain is elaborated. The results show that excellent performance can be achieved for devices with channel lengths down to 15 nm for a single-gated device layout. The influence of the SOI-film roughness is investigated with an SOI body thickness down to 2.5 nm. Extremely high transconductances far in excess of today's state-of-the-art devices can be expected if the doping level in source and drain is chosen appropriately. We give the relevant design rules for the fabrication of such devices.
Physica B: Condensed Matter, 2002
In this work, the effective potential is employed to account for the quantum mechanical effects of charge setback and elevated ground-state energy in the inversion layer of fully depleted (FD) SOI MOSFETs. We use the effective potential along with a three-dimensional Poisson solver and a Monte Carlo transport kernel to illustrate these quantum mechanical effects on the output characteristics of the transistor. It is demonstrated that the inclusion of such effects has a significant influence on the threshold voltage, carrier energy, and drive current of the device. r
Threshold voltage calculation in ultrathin-film soi mosfets using the effective potential
IEEE Transactions On Nanotechnology, 2003
Abshncf-The success of the effective potential method of including quantum confinement effects in simulations of MOSFETs is based on the ability to calculate ahead of time the extent of the Gaussian wavepacket used to describe the electron. In the calculation of the Gaussian, the inversion layer is assumed to form in a triangular potential well, from which a suitable standard deviation can he obtained. The situation in an ultra-thin SO1 MOSFET is slightly different, in that the potential well has a triaugnlar bottom, but there is a significant contribution to the Confinement from the rectangular barriers formed by the gate oxide and the buried oxide (BOX). For this more complex potential well, it is of interest to determine the range of applicability of the constant standard deviation effective potential model. In this work, we include this effective potential model in 3D Monte Carlo calculations of the threshold voltage of ultra-thin SO1 MOSFETs. We find that the effective potential recovers the expected trend in threshold voltage shift with shrinking silicon thickness, down to a thickness of approximately 3 nm.
A Study of the Threshold Voltage Variations for Ultrathin Body Double Gate SOI MOSFETs
2004
Silicon on insulator (SOI) devices have been of great interest in these years. In this paper, simulation with density-gradient transport model is performed to examine the variation of threshold voltage (VTH) for double gate SOI MOSFETs. Different thickness of silicon (Si) film, oxide thickness, channel length and doping concentration are considered in this work. According to the numerical study, both