Transport In Inversion Layers Research Articles

Evidence of Sub-Band Modulated Transport in Planar Fully Depleted Silicon-on-Insulator MOSFETs

Modulation of the sub-band electron population in the inversion channel of 10-nm planar fully depleted silicon-on-insulator MOSFETs is evidenced by the bias dependence of inversion layer transport parameters. Two distinct inversion-layer electron species were detected by magnetic-field-dependent magnetoresistance measurements and high-resolution mobility spectrum analysis. According to self-consistent Poisson-Schrodinger calculations, these species correspond to carriers in distinct sub-bands within the Si channel region. The mobility peak of the carrier with the highest sheet density occurs under gate bias conditions that result in a minimum perpendicular effective electric field.

Hole effective mass in silicon inversion layers with different substrate orientations and channel directions

We explore the possibility to define an effective mass parameter to describe hole transport in inversion layers in bulk MOSFETs and silicon-on-insulator devices. To do so, we employ an accurate and computationally efficient self-consistent simulator based on the six-band k·p model. The valence band structure is computed for different substrate orientations and silicon layer thicknesses and is then characterized through the calculation of different effective masses taking account of the channel direction. The effective masses for quantization and density of states are extracted from the computed energy levels and subband populations, respectively. For the transport mass, a weighted averaging procedure is introduced and justified by comparing the results with hole mobility from experiments and simulations.

Inversion layer electron transport in 4H‐SiC metal–oxide–semiconductor field‐effect transistors

AbstractSilicon carbide is the only compound semiconductor with silicon dioxide as its native oxide and can form metal–oxide–semiconductor field‐effect transistors. In this article, we review the inversion layer electron transport properties of 4H‐SiC/SiO2. The inversion layer electron transport theories developed for the Si/SiO2 system have been reviewed to highlight the behaviour of inversion layer electrons at an ideal interface. The 4H‐SiC/SiO2 interface, is currently far from ideal and so we discuss the inversion layer electron transport for several techniques of oxidation of the (0001) 4H‐SiC including nitridation of thermal oxides and sodium enhanced oxidation of 4H‐SiC. Inversion layer mobility of nitrided oxides is dominated by Coulomb scattering at low surface electric field values and surface roughness scattering at high surface electric field values. In sodium enhanced oxidation samples no trapped charge is found at the interface close to the conduction band edge, but the temperature dependence of mobility shows that the samples are not limited by phonon scattering and therefore not ideal. We also review the results of inversion layer transport on alternate faces of 4H‐SiC. We conclude highlighting the future work remaining in the field and the behaviour of inversion layer electron mobility we expect to see in an ideal 4H‐SiC/SiO2 interface. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A 2D non-parabolic six-moments model

In order to describe carrier transport in inversion layers we have developed a two-dimensional non-parabolic macroscopic transport model up to the sixth order. To model the transport parameters with as few simplifying assumptions as possible, we apply an extraction technique from Subband Monte Carlo simulations followed by an interpolation within these Monte Carlo tables through the whole inversion layer. Important effects like surface-roughness scattering as well as quantization are inherently considered in the Subband Monte Carlo data, which are used to model higher-order mobilities as well as the macroscopic relaxation times as a function of the effective field and the carrier temperature. The parameters are compared with the results obtained from models using bulk Monte Carlo data, where neither surface roughness nor quantization are considered. The models are applied to a UTB SOI-MOSFET and their predictions are discussed for different gate lengths.

Current transport models for nanoscale semiconductor devices

Abstract Due to the rapid decrease in device dimensions the well-established TCAD tools are pushed to the limits of their applicability. Since conventional MOSFETs are already operating in the sub-100 nm range, new physical effects and principles begin to determine the transport characteristics, and the validity of conventional current transport models is in question. The drift-diffusion model, which has enjoyed a remarkable success due to its relative simplicity, numerical robustness, and the ability to perform two- and three-dimensional simulations on large unstructured meshes, must be generalized to include hot-carrier and classical non-local effects. This motivated the development of higher order moments transport models such as the hydrodynamic, the energy-transport, and the six-moments models. After the introduction of stress for device performance enhancement the demand for accurate carrier mobility calculations based on full-band Monte Carlo algorithms has significantly increased, since they allow calibration of phenomenological mobility models and thus justify closure relations for higher order moments equations. The transport models based on the semi-classical Boltzmann transport equation already contain information which can only be obtained from quantum-mechanical consideration. These are the band structure, expressions for the scattering rates, and the Pauli exclusion principle reflecting the Fermi statistics of carriers. With scaling continuing, other quantum-mechanical effects begin to affect transport properties. Quantum confinement in the direction orthogonal to transport in inversion layers makes the energy spectrum discrete. For sufficiently long channels, however, the carrier motion in transport direction can still be treated semi-classically, and development of transport models based on a set of subband Boltzmann equations is possible. A useful approximation to mimic the quantum-mechanical carrier concentration profile is to introduce an effective potential into otherwise classical transport models. Transport calculations can then be carried out using conventional TCAD tools providing accurate and timely results. However, when modeling transport in ultra-scaled structures with only a few subbands occupied the full subband method must be applied. Parallel to the search for new technological solutions for MOSFET scaling, the development of conceptually new devices and architectures is becoming increasingly important. New nanoelectronic structures, such as carbon nanotubes, nanowires, and even molecules, are considered to be prominent candidates for the post-CMOS era. At this small device size the geometrical spread of the carrier wave packet in transport direction can no longer be ignored. When the device size becomes shorter than the phase coherence length, the complete information about carrier dynamics inside the device including the phase of the wave function is needed and one has to resort to a full quantum-mechanical description including scattering. Transport in advanced nanodevices is determined by the interplay between coherent propagation and scattering. Numerical methods for dissipative quantum transport based on the non-equilibrium Green’s function formalism, the Liouville/von-Neumann equation for the density matrix, and the kinetic equation for the Wigner function are attaining relevance.

Physics of Hole Transport in Strained Silicon MOSFET Inversion Layers

A comprehensive quantum anisotropic transport model for holes was used to study silicon PMOS inversion layer transport under arbitrary stress. The anisotropic band structures of bulk silicon and silicon under field confinement as a twodimensional quantum gas are computed using the pseudopotential method and a six-band stress-dependent k.p Hamiltonian. Anisotropic scattering is included in the momentum-dependent scattering rate calculation. Mobility is obtained from the Kubo-Greenwood formula at low lateral field and from the fullband Monte Carlo simulation at high lateral field. Using these methods, a comprehensive study has been performed for both uniaxial and biaxial stresses. The results are compared with device bending data and piezoresistance data for uniaxial stress, and device data from strained Si channel on relaxed SiGe substrate devices for biaxial tensile stress. All comparisons show a very good agreement with simulation. It is found that the hole band structure is dominated by 12 wings, where mechanical stress, as well as the vertical field under certain stress conditions, can alter the energies of the few lowest hole subbands, changing the transport effective mass, density-of-states, and scattering rates, and thus affecting the mobility

Magnetoresistance of Si [formula omitted] MOSFETs with high concentration of electrons

We present an experimental study of electron transport in inversion layers of high-mobility Si (0 0 1) samples with occupied excited subbands. The second series of oscillations, observed in addition to the main series of Shubnikov–de Hass oscillations, is tentatively attributed to the occupation of a subband associated with the E 0′ level. Besides, a strong negative magnetoresistance and nonlinear field dependence of the Hall resistance accompany the novel oscillations at high carrier concentrations. The heating of the two-dimensional electron layers leads to suppression of the observed anomalies.

Magnetotransport with two occupied subbands in a Si(100) inversion layer

We have studied electron transport in inversion layers of high-mobility Si(100) samples. At high electron concentrations and temperatures below 4.2 K, two series of Shubnikov-de Haas (SdH) oscillations have been observed. The temperature damping of the second series oscillations indicates that the second occupied subband belongs to the first energy level of the fourfold-degenerate ladder 0′. Samples with two occupied subbands exhibit a strong anomalous negative magnetoresistance, reaching ≈ 25% of a zero field value at B = 12 T. The decrease of resistance is more pronounced for lower temperatures and higher electron concentrations. We explain this behaviour by an increase of the second subband mobility due to the freezing-out of the scattering of 0′ electrons. Based on the measured periods of SdH oscillations, we conclude that the electrons are distributed inhomogeneously beneath the sample gate.

Transmission matrix approach for electron transport in inversion layers

An efficient “transmission matrix” (TM) approach is proposed for including quantization effects in the treatment of electron transport in inversion layers. For each input mode of the momentum space, the probabilities of the output modes and average times are computed and used to follow the electron motion in time. The TM approach makes it possible to treat electron transport with improved numerical efficiency while maintaining the accuracy of the Monte Carlo (MC) approach. Examples with a constant electric field are considered to demonstrate the TM approach and it is shown that the results obtained by the TM and MC techniques are in excellent agreement with each other. The TM approach reduces the computation time by a factor of 6 or more. Integration of the TM approach in device analysis is discussed.

Inversion layer transport and properties of oxides on InAs

Inversion layer transport in p-type InAs at 77 K using Van der Pauw samples has been investigated with anodic oxides and sputtered SiO2 as the gate insulators. It has been found that there is a correlation between the surface transport measurements, the fixed charge in the oxide, and possibly the insulating properties of the oxide. An attempt is made to develop a model to interrelate these measurements. It is found that fixed oxide charge appears to be the dominant mechanism in limiting the mobility at low surface carrier concentrations, while at higher concentrations surface roughness is the dominant mechanism.

Application of effective-medium theory to inversion layers

Two-dimensional effective-medium theory is applied to the macroscopic-inhomogeneity model for transport in inversion layers near the threshold of conduction. The predicted behaviour resembles that which has been observed provided that reported Hall measurements have been made at carrier concentrations above the percolation threshold. The theory predicts a dramatic change in behaviour below the percolation threshold and extension of Hall measurements to lower carrier concentrations would provide an acid test of the validity of the model.

V. The Hall effect in impurity bands and inversion layers

Abstract Results are presented on transport processes when carriers are localized in impurity bands and inversion layers. For impurity bands, the evidence of conduction by excitation to a mobility edge is reviewed, and contrasted with the Hall effect which shows the Hall mobility to be thermally activated. Evidence from the literature is presented showing how magnetic-field-induced localization of carriers can give rise to a Hall effect close to that expected for excitation to a mobility edge. Some points on transport in inversion layers are summarized, in particular the variation of the temperature dependence of conductance on sample preparation treatment. It is hoped that this will clarify the literature on these, often referred, sample variations. Results are presented showing how it is possible to obtain a transport process in which 1/R H is activated or one in which μH is activated. Where 1/R H is activated, delocalization by impact ionization is demonstrated. It is concluded that the mechanism deter...

Inversion Layer Transport and the Radiation Hardness of the Si-SiO2 Interface

A brief review is presented on low temperature transport in Si inversion layers when carriers are localized. The characteristics of transport in this regime are dependent on the post-oxidation annealing treatment received by the interface. The previously proposed model of compensating positive and negative interfacial charge pairs, in which the negative centres act as hole traps, is discussed. It is suggested that the cause of the radiation hardness of an interface is a reduced concentration of the pairs and the annealing treatments which achieve this are an exercise in interface engineering.