Statistical study of the effect of interface charge fluctuations in HEMTs using a 3D simulator

The intrinsic parameter fluctuations associated with the discreteness of charge and matter become an important factor when the semiconductor devices are scaled to nanometre dimensions. The interface charge in the recess regions of high electron mobility transistors (HEMTs) has a considerable effect on the overall device performance. We have employed a 3D parallel drift-diffusion device simulator to study the impact of interface charge fluctuations on the I-V characteristics of nanometre HEMTs. For this purpose, two devices have been analysed, a 120 nm gate length pseudomorphic HEMT with an In_0.2Ga_0.8As channel and a 50 nm gate length InP HEMT with an In_0.7Ga_0.3As channel.     

Pseudo-two-dimensional Poisson equation for the modeling of field-effect transistors

A modified Poisson equation able to take into account the influence of gates and a δ-doping in FETs and HEMTs is proposed. This equation can be solved self-consistently together with 1D transport equations along inhomogeneous transistor channels like those of the hydrodynamic or drift-diffusion approximations or with a Monte Carlo simulator used to describe carrier transport in n⁺nn⁺ structures.     

Electrothermal Monte Carlo Simulations of InGaAs/AlGaAs HEMTs

Results from the application of our electrothermal simulator to n-type 0.15 μm gate In_0.15Ga_0.85As-Al_0.28Ga_0.72As HEMT structures are presented. The simulator involves an iterative procedure which alternately solves the Heat Diffusion Equation (HDE) and executes a Monte Carlo electronic transport algorithm. The net thermal flux generated during each Monte Carlo stage, calculated from the net rate of phonon emission, is fed into the thermal solution; the resulting temperature map is then used in the following Monte Carlo iteration. The HDE is solved through application of a novel analytical thermal resistance matrix technique which allows calculation of temperatures solely within the region of interest while including the large-scale boundary conditions. A novel charge injection scheme is applied for the treatment of side ohmic contacts, which avoids anomalous generation of thermal flux in adjacent regions. The characteristic ‘thermal droop’ is found in the I-V characteristics of the simulated device. Associated temperature distributions are shown to be spatially non-uniform with peak values and spatial locations dependent upon bias and the length of the containing die. Electron drift velocities and energies along the HEMT channel exhibit the largest shift on the inclusion of thermal self-consistency below the drain end of the gate, not at the location of the temperature peak.     

Efficient optimization‐oriented design methodology of high‐order 3‐D filters using 2‐D and 3‐D electromagnetic simulators

This paper proposes a computationally highly efficient interface between two‐dimensional (2‐D) and three‐dimensional (3‐D) electromagnetic (EM) simulators for the optimization‐oriented design of high‐order 3‐D filters. In a first step, the novel optimization‐oriented design methodology aligns the 3‐D EM simulator response with the 2‐D EM simulator response of a low‐order 3‐D filter by using an inverse linear space mapping optimization technique. Then, a second mapping performs a calibration with the optimal 2‐D and 3‐D design parameters obtained from the first mapping. The optimization of high‐order filters is carried out using only the efficient 2‐D EM simulator, and the calibration equations directly give the design parameters of the 3‐D filter. The potential and the effectiveness of the proposed optimization‐oriented design methodology are demonstrated through the design of C‐band 3‐D evanescent rectangular waveguide bandpass filters with increasing orders from three to eight. Copyright © 2014 John Wiley & Sons, Ltd.     

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al_0.3Ga_0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.     

Impact of high‐κ gate dielectric and other physical parameters on the electrostatics and threshold voltage of long channel gate‐all‐around nanowire transistor

High‐κ gate‐all‐around structure counters the Short Channel Effect (SCEs) mostly providing excellent off‐state performance, whereas high mobility III–V channel ensures better on‐state performance, rendering III–V nanowire GAAFET a potential candidate for replacing the current FinFETs in microchips. In this paper, a 2D simulator for the III–V GAAFET based on self‐consistent solution of Schrodinger–Poisson equation is proposed. Using this simulator, capacitance–voltage profile and threshold voltage are characterized, which reveal that gate dielectric constant (κ) and oxide thickness do not affect threshold voltage significantly at lower channel doping. Moreover, change in alloy composition of In_xGa_1‐xAs, channel doping, and cross‐sectional area has trivial effects on the inversion capacitance although threshold voltage can be shifted by the former two. Although, channel material also affects the threshold voltage, most sharp change in threshold voltage is observed with change in fin width of the channel (0.005 V/nm for above 10 nm fin width and 0.064 V/nm for sub‐10 nm fin width). Simulation suggests that for lower channel doping below 10²³ m⁻³, fin width variation affects the threshold voltage most. Whereas when the doping is higher than 10²³ m⁻³, both the thickness and dielectric constant of the oxide material have strong effects on threshold voltage (0.05 V/nm oxide thickness and 0.01 V/per unit change in κ). Copyright © 2014 John Wiley & Sons, Ltd.     

Code for the 3D Simulation of Nanoscale Semiconductor Devices, Including Drift-Diffusion and Ballistic Transport in 1D and 2D Subbands, and 3D Tunneling

We present a three-dimensional device simulator, suitable for the study of a wide range of nanoscale devices, in which quantum confinement and carrier transport are taken into account. In particular, depending on the confinement, the 1D, 2D or 3D Schrödinger equation with density functional theory in the local density approximation is coupled with the Poisson equation in the three-dimensional domain. Continuity equation in the ballistic and in the drift-diffusion regime are also solved assuming separation of the subbands.     

Atomistic effect of delta doping layer in a 50 nm InP HEMT

Fluctuations caused by discreteness of charge will play an important role when devices are scaled to gate lengths approaching nanometer dimensions. In this paper, we use a 3D drift-diffusion simulator to study an influence of discrete random dopant charges in the delta doping layer of a 50 nm gate length InP high electron mobility transistor.     

Structural and electrical properties of low resistance Pt/Pd/Au contact on p-GaN

We have investigated electrical and structural properties of Pt/Pd/Au ohmic contact on p-type GaN:Mg (2.5 × 10¹⁷ cm⁻³) using Auger electron spectroscopy (AES) and glancing angle x-ray diffraction (GXRD) analysis. It was shown that the specific contact resistivity improved with increasing annealing temperature. The annealing of the contact at 600^∘C for 2 min in flowing N₂ atmosphere resulted in a specific contact resistivity of 3.1 × 10⁻⁵ Ω cm². Both GXRD and AES depth profile results show that Ga₃Pt₅, Ga₂Pd₅, and Au₇Ga₂ phases are formed at the interface region between metal and GaN when annealed at temperatures 600^∘C. Possible explanation is suggested to describe the annealing dependence of the specific contact resistivity of the Pt/Pd/Au contacts.     

Effect of quantum dots on InAs/GaAs p-i-p quantum dots infrared photodetectors

ABSTRACT

In this paper, the InAs/GaAs p-i-p quantum dots infrared photodetectors (QDIPs) were successfully demonstrated by Apsys software. It consists of Al_0.3Ga_0.7As/GaAs structure to reduce dark current and InAs quantum dots (QDs) embedded in In_0.15Ga_0.85As as an active layer. The effect of structure parameters of InAs QDs on the dark current, photocurrent of the device and SNR (signal to noise) is discussed respectively, including different QDs density, the number of QD layer, GaAs thickness between QDs layers and Al_0.3Ga_0.7As, and GaAs thickness between two the QD layers.     

Efficient three‐dimensional parallel simulations of PHEMTs

An efficient 3D semiconductor device simulator is presented for a memory distributed multiprocessor environment using the drift–diffusion (D–D) approach for carrier transport. The current continuity equation and the Poisson equation, required to be solved iteratively in the D–D approach, are discretized using a finite element method (FEM) on an unstructured tetrahedral mesh. Parallel algorithms are employed to speed up the solution. The simulator has been applied to study a pseudomorphic high electron mobility transistor (PHEMT). We have carried out a careful calibration against experimental I–V characteristics of the 120 nm PHEMT achieving an excellent agreement. A simplification of the device buffer, which effectively reduces the mesh size, is investigated in order to speed up the simulations. The 3D device FEM simulator has achieved almost a linear parallel scalability for up to eight processors. Copyright © 2005 John Wiley & Sons, Ltd.     

Multiscale simulation of MOS systems based on high-κ oxides

We report on a multiscale simulation approach that includes both macroscopic drift-diffusion current model and quantum tunneling model. The models are solved together in a self-consistent way inside a single simulation package. As an example, we study the subthreshold transfer characteristics of MOS transistors based on high-κ oxides. We compare the high-κ gates based on HfO₂ and ZrO₂ with a SiO₂ gate of the same equivalent thickness and show the effect of the tunneling current on transistor performance.     

Simulation study of an optimized current matching for In0.39Ga0.61N/In0.57Ga0.43N/In0.74Ga0.26N triple-junction solar cells

This paper deals with the design and optimization of a triple-junction (TJ) solar cell using indium gallium nitride (InGaN) material. Two tunnel diodes are used to ensure connection between the different subcells. A comprehensive study is performed by means of 2D numerical simulations to locate the best bandgap combination that leads to an optimized current matching. During the simulations, the doping concentration and the base thickness are considered as fitting parameters for the top and the middle subcells. The In_0.39Ga_0.61N/In_0.57Ga_0.43N/In_0.74Ga_0.26N bandgap combination is supposed to be 2.02 eV/1.52 eV/1.13 eV. A high short-circuit current density (13.313 mA/cm²) is achieved by assuming a base thickness of 1 µm for each subcell and a p/n doping ratio of 5?×?10¹⁸ cm^?3/5?×?10¹⁵ cm^?3 in the top cell, 1.5?×?10¹⁹ cm^?3/1.5?×?10¹⁶ cm^?3 in the middle cell, and 7.5?×?10¹⁸ cm^?3/7.5?×?10¹⁵ cm^?3 in the bottom cell. The optimized structure has an improved open-circuit voltage (2.877 V), fill factor (83%), and conversion efficiency (33.11%).

     

On the Breakdown of Universal Mobility Curves: A Brownian 3D Simulation Framework

We present an efficient simulation approach to study the universal mobility behaviour in Si MOS structures with random Si/SiO₂ interfaces. Our approach is based on 3D Brownian dynamics in devices with realistic Si/SiO₂ interfaces reconstructed from a Gaussian or exponential correlation function. The carrier-interface roughness scattering is treated ab-initio in our simulations and it results in correct velocity and real space distributions. The method is efficient and capable of 3D simulation of the interface roughness limited mobility in small MOSFETs in a statistical manner. After a careful calibration procedure, we reproduce the effective field dependence of interface mobility for _Bulk = 1100 cm²/Vs using a random interface with single atomic steps and a correlation length of 6 nm.     

Two-dimensional subthreshold current model for dual-material gate SOI nMOSFETs with single halo

A two-dimensional (2D) model for the subthreshold current in the dual-material gate (DMG) silicon-on- insulator (SOI) MOSFET with a single halo is presented. The model considers single halo doping in the channel near the source and a dual-material gate to derive the channel potential using the explicit solution of the 2D Poisson’s equation. Together with the conventional drift-diffusion theory, this results in the development of a subthreshold current model for the novel structure. Model verification is carried out using the 2D device simulator ISE. Excellent agreement is obtained between the calculations and the simulated results of the model. __________ Translated from Chinese Journal of Semiconductors, 2008, 29(4): 746–750 [译自 : 半导体学报]     

Electrical properties of Ga2O3-based dielectric thin films prepared by plasma enhanced atomic layer deposition (PEALD)

Ga₂O₃ and Ga₂O₃-TiO₂ (GTO) nano-mixed thin films were prepared by plasma enhanced atomic layer deposition with an alternating supply of reactant sources, [(CH₃)₂GaNH₂]₃, Ti(N(CH₃)₂)₄ and oxygen plasma. The uniform and smooth Ga₂O₃ and GTO thin films were successfully deposited. Excellent step coverage of these films was obtained by chemisorbed chemical reactions with oxygen plasma on the surface. The dielectric constant of GTO thin film definitely increased compared to Ga₂O₃ film, and the leakage currents of GTO films were comparable to Ga₂O₃ films. The leakage current density of a 40-nm-GTO film annealed at 600^∘C was approximately 1×10⁻⁷ A/cm² up to about 600 kV/cm.     

The surface charging effects in three‐dimensional simulation of the profiles of plasma‐etched nanostructures

Particles and fields represent two major modeling paradigms in pure and applied science at all. Particles typically exist in a spatial domain and they may interact with other particles or with field quantities defined on that domain. A field, on the other hand, defines a set of values on a region of space. In this paper, a methodology and some of the results for three‐dimensional (3D) simulations that includes both field and particle abstractions are presented. In our studies, charging damage to a semiconductor structure during plasma etching is simulated by using 3D level set profile evolution simulator. The surface potential profiles and electric field for the entire feature were generated by solving the Laplace equation using finite elements method. Calculations were performed in the case of simplified model of Ar⁺/CF₄ non‐equilibrium plasma etching of SiO₂. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermoelectric properties of Ca3-xEuxCo3.95Ga0.05O9+δ (0 ≤ x ≤ 0.10)

Polycrystalline samples of Ca_3–xEu_xCo_3.95Ga_0.05O_9+δ (x?=?0.00, 0.02 and 0.10) have been prepared by conventional solid-state synthesis and their thermoelectric properties measured at 25 K to 300 K. The XRD results revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower were simultaneously increased with increasing Eu³⁺ content. The total thermal conductivity decreased with increasing Eu³⁺ content. A maximum dimensionless figure of merit of 0.033 at 300 K was reached for Ca_2.9Eu_0.1Co_3.95Ga_0.05O_9+δ, which is about 27 % higher than that of the undoped sample. These results suggest that the Eu is an effective doping element for improving the thermoelectric properties of Ca₃Co_3.95Ga_0.05O_9+δ.     

Oscillator strengths of the intersubband electronic transitions in the multi-layered nano-antidots with hydrogenic impurity

In this study, we have obtained the exact solutions of the Schr?dinger equation for a multi-layered quantum antidot (MLQAD) within the effective mass approximation and dielectric continuum model for the spherical symmetry. The MLQAD is nano-structured semiconductor system that consists of a spherical core (e.g. Ga_1?x Al _x As) and a coated spherical shell (e.g. Ga_1?y Al _y As) as the whole anti-dot is embedded inside a bulk material (e.g. GaAs). The dependence of the electron energy spectrum and its radial probability density on nano-system radius are studied. The numeric calculations and analysis of oscillator strength of intersubband quantum transition from the ground state into two first allowed excited states at the varying radius, for both the finite and infinite confining potential (CP) as well as constant shell thickness, are performed. It is shown that, in particular, the binding energy and the oscillator strength of the hydrogenic impurity of a MLQAD behave differently from that of a single-layered quantum antidot (SLQAD). For a MLQAD with finite core and shell CPs, the state energies and the oscillator strengths of the impurity are found to be dependent on the shell thickness. At the large core radius and very small shell thickness, our results are closer to respective values for a SLQAD that previously reported.     

A Quantum Mechanical Approach for the Simulation of Si/SiO2 Interface Roughness Scattering in Silicon Nanowire Transistors

In this work, we present a quantum mechanical approach for the simulation of Si/SiO₂ interface roughness scattering in silicon nanowire transistors (SNWTs). The simulation domain is discretized with a three-dimensional (3D) finite element mesh, and the microscopic structure of the Si/SiO₂ interface roughness is directly implemented. The 3D Schrödinger equation with open boundary conditions is solved by the non-equilibrium Green’s function method together with the coupled mode space approach. The 3D electrostatics in the device is rigorously treated by solving a 3D Poisson equation with the finite element method. Although we mainly focus on computational techniques in this paper, the physics of SRS in SNWTs and its impact on the device characteristics are also briefly discussed.