Simulation and optimization of HEMTs

We have developed a simulation system for nanoscale high-electron mobility transistors, in which the self-consistent solution of Poisson and Schr\"odinger equations is obtained with the finite element method. We solve the exact set of nonlinear differential equations to obtain electron wave function, electric potential distribution, electron density, Fermi surface energy and current density distribution in the whole body of the device. For more precision, local dependence of carrier mobility on the electric field distribution is considered. We furthermore compare the simulation to a recent experimental measurement and observe perfect agreement. We also propose a graded channel design to improve the transconductance and thereby the threshold frequency of the device.Comment: 8 pages, 19 figure

Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems

Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical Nanoscienc

Carrier transport in high-speed photodetectors based on two-dimensional-gas

Monolithically integrated high-speed photodetectors are important components in fiber communications and optoelectronic integrated circuits (OEIC) with metal-semiconductor-metal (MSM) photodetectors (PD) being the device of choice due to the its high overall performance and technology compatibility with integrated circuits (IC). The speed of conventional top illuminated MSM-PD is limited by the transit time of photogenerated carriers. The concept of internal vertical field, developed in the MSM intrinsic absorption region by the two-dimensional-gas along the heterojunction, is proposed and implemented for the purpose of facilitating carrier transport, hence improving the transit time limitations. The time response of a two-dimensional-electron-gas (2-DEG) based photodetector suggests an enhanced electron transport but a long tail due to the slow holes. We have designed and fabricated two-dimensional-hole-gas (2-DHG) based MSM photodetector to investigate hole transport in the vertical field MSM photodetector. Simulation of charge carrier transport, verifies experimentally observed behavior, which manifests the enhanced hole transport benefit from the vertical field.In addition, the 2-DHG based MSM structure device shows excellent capacitance-voltage (C-V) characteristic making it an excellent candidate for applications in odd-order high frequency multipliers. The high Cmax/Cmin ratio of 113 and high sensitivity of 35 are one of the best results reported. In addition, optoelectronic measurements demonstrate the slope of the C-V relationship can be modulated by the intensity of the incident optical power. A model describing the source of the C-V results is proposed along with the simulation results verifying the observed C-V behavior. In order to produce a complete picture of charge transport and collection, we developed a program using Ensemble-Monte-Carlo (EMC) method incorporating the electron-electron scattering in the 2-DEG confined by AlGaAs/GaAs heterojunction. The result reveals an energy thermalization time of tens of femto-second in the 2-DEG, which suggest the 2D gas has the potential to collect the photogenerated carriers.Based on all previous experimental results and analysis, a 2-DEG/2-DHG combined structure has been proposed on GaAs substrate. The design, taking advantage of the vertical field and fast thermalization time in the confined 2D gas, results in a wide bandwidth, high external quantum efficiency for vertically illuminated MSM device.Ph.D., Electrical Engineering -- Drexel University, 200

Ensemble Monte Carlo Based Simulation Analysis of GaN HEMTs for High-Power Microwave Device Applications

The high electron mobility transistors (HEMTs) fabricated using wide-bandgap semiconductors show promise as high-gain, low-noise devices with superior frequency response. The structure and operation principle of HEMT are first briefly discussed. The distinguishing and unique properties of GaN are reviewed and compared with those of GaAs. Calculations of the electronic mobility and drift velocity have been carried out for bulk GaN based on a Monte Carlo approach, which serves as a validity check for the simulation model. By taking account of polarization effects, degeneracy and interface roughness scattering, important microwave performance measures such as the dynamic range, harmonic distortion and inter-modulation characteristics are fully studied. Monte Carlo based calculations of the large-signal nonlinear response characteristics of GaN-AlGaN HEMTs with particular emphasis on intermodulation distortion (IMD) have been performed. The nonlinear electrical transport is treated on first principles, including all scattering mechanisms. Both memory and distributed effects are built into the model. The results demonstrate an optimal operating point for low intermodulation distortion (IMD) at reasonably large output power due to the exist of a minima in the IMD curve. Dependence of the nonlinear characteristics on the barrier mole fraction “x” is also demonstrated and analyzed. High-temperature predictions of the IMD have also been made by carrying out the simulations at 600 K. Due to a relative suppression of interface roughness scattering, an increase in dynamic range with temperature is predicted. Finally, towards the end of the research, real-space transfer (RST) phenomena are included in the Monte Carlo simulator to accurately describe the electron transport behavior in HEMTs. The RST is shown to affect the velocity overshoot and inter-modulation distortion behavior and to lead to enhanced substrate leakage current as well as lowered overall performance speed. The potential for drain current compression has also been examined through simulations. Comparisons with and without RST have been performed based on Monte Carlo simulations. Results show that the velocity, IMD and dynamic range are all affected by the applied bias, temperature, internal electric field and gate length characteristics

The development of sub-25 nm III-V High Electron Mobility Transistors

High Electron Mobility Transistors (HEMTs) are crucially important devices in microwave circuit applications. As the technology has matured, new applications have arisen, particularly at millimetre-wave and sub-millimetre wave frequencies. There now exists great demand for low-visibility, security and medical imaging in addition to telecommunications applications operating at frequencies well above 100 GHz. These new applications have driven demand for high frequency, low noise device operation; key areas in which HEMTs excel. As a consequence, there is growing incentive to explore the ultimate performance available from such devices. As with all FETs, the key to HEMT performance optimisation is the reduction of gate length, whilst optimally scaling the rest of the device and minimising parasitic extrinsic influences on device performance. Although HEMTs have been under development for many years, key performance metrics have latterly slowed in their evolution, largely due to the difficulty of fabricating devices at increasingly nanometric gate lengths and maintaining satisfactory scaling and device performance. At Glasgow, the world-leading 50 nm HEMT process developed in 2003 had not since been improved in the intervening five years. This work describes the fabrication of sub-25 nm HEMTs in a robust and repeatable manner by the use of advanced processing techniques: in particular, electron beam lithography and reactive ion etching. This thesis describes firstly the development of robust gate lithography for sub-25 nm patterning, and its incorporation into a complete device process flow. Secondly, processes and techniques for the optimisation of the complete device are described. This work has led to the successful fabrication of functional 22 nm HEMTs and the development of 10 nm scale gate pattern transfer: simultaneously some of the shortest gate length devices reported and amongst the smallest scale structures ever lithographically defined on III-V substrates. The first successful fabrication of implant-isolated planar high-indium HEMTs is also reported amongst other novel secondary processes

Fundamental Kinetics and Innovative Applications of Nonequilibrium Atomic Vibration in Thermal Energy Transport and Conversion.

All energy conversion inefficiencies begin with emission of resonant atomic motions, e.g., vibrations, and are declared as waste heat once these motions thermalize to equilibrium. The nonequilibrium energy occupancy of the vibrational modes can be targeted as a harvestable, low entropy energy source for direct conversion to electric energy. Since the lifetime of these resonant vibrations is short, special nanostructures are required with the appropriate tuning of the kinetics. These in turn require multiscale, multiphysics treatments. Atomic vibration is described with quasiparticle phonon in solid, and the optical phonon emission is dominant relaxation channel in semiconductors. These optical modes become over-occupied when their emission rate becomes larger than their decay rate, thus hindering energy relaxation and transport in devices. Effective removal of these phonons by drifting electrons is investigated by manipulating the electron distribution to have higher population in the low-energy states, thus allowing favorable phonon absorption. This is done through introduction, design and analysis of a heterobarrier conducting current, where the band gap is controlled by alloying, thus creating a spatial variation which is abrupt followed by a linear gradient (to ensure directed current). Self-consistent ensemble Monte Carlo simulations based on interaction kinetics between electron and phonon show that up to 19% of the phonon energy is converted to electric potential with an optimized GaAs/AlxGa1−xAs barrier structure over a range of current and electron densities, and this system is also verified through statistical entropy analysis. This direct energy conversion improves the device performance with lower operation temperature and enhances overall energy conversion efficiency. Through this study, the paradigm for harvesting the resonant atomic vibration is proposed, reversing the general role of phonon as only causing electric potential drop. Fundamentals pertaining to thermal energy transport and conversion are further explored by directly addressing the nonequilibria in phonon and molecular vibration. Enhancement of the laser cooling performance in molecular gas is examined by nonequilibrium interaction kinetics between molecules and photons. Thermal energy transport across interfaces and junctions is also studied, and decomposition of thermal interfacial resistance, atomic restructuring, and phonon wave features are addressed.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/98020/1/shinsha_1.pd

Accurate temperature measurements on semiconductor devices.

Self-heating can have a detrimental effect on the performance and reliability of high power microwave devices. In this work, the thermal performance of the gallium arsenide (GaAs) Gunn diode was studied. Infrared (IR) thermal microscopy was used to measure the peak operating temperature of the graded-gap structured device. Temperature measurements were experimentally validated using micro-thermocouple probing and compared to values obtained from a standard 1D thermal resistance model. Thermal analysis of the conventionally structured Gunn diode was also undertaken using high resolution micro-Raman temperature profiling, IR thermal microscopy and electro/thermal finite element modeling. The accuracy of conventional IR temperature measurements, made on semiconductor devices, was investigated in detail. Significant temperature errors were shown to occur in IR temperature measurements made on IR transparent semiconductors layers and low emissivity/highly reflective metals. A new technique, employing spherical carbon microparticles, was developed to improve the measurement accuracy on such surfaces. The new ‘IR microparticle’ technique can be used with existing IR microscopes and potentially removes the need to coat a device with a high emissivity layer, which causes damage and heat spreading

Investigation and suppression of semiconductor–oxide related defect states : from surface science to device tests

Many present challenges in semiconductor technology are related to utilizing solid structures with atomic scale dimensions and materials with higher charge carrier mobility and/or other readily controllable properties. These include many surface-related problems because the ratio of surface parts of devices to the whole material volume increases all the time in practical device structures. One of the major problems has been oxidation of semiconductor surfaces during the manufacturing of devices. This PhD work deals with the surface and oxide interface properties of different III–V semiconductors induced by the oxidation, the study of which is imperative in realizing devices with desired characteristics. The general goal has been in finding answers to these problematic issues on atomic scale, and whether they can be resolved with simple parameter control of a thermal oxidation treatment. Much of the work leans on a previous novel finding of crystalline oxide phases on indium-containing III–V semiconductor (100) surfaces. Various aspects of applicability of such a structure in real semiconductor devices are considered in this work. Common denominator in all of the experiments and studies is that the initial investigations were carried out in very controlled environment in ultrahigh-vacuum: detailed basics and initial characterizations were carried out with high resolution and precision surface science methods. In particular, this work has resulted in novel crystalline oxide phases observed on GaSb(100) and InSb(111)B semiconductor surfaces. They have been extensively discussed from an applied point of view as well as their fundamental characteristics, relating to their already previously studied counterpart, InSb(100). Furthermore, beneficial passivating characteristics of a stabilizing crystalline InOx interfacial layer beneath an Al2O3 and reasons behind such behavior are demonstrated for InGaAs IR detector device structure. This thesis provides background of semiconductors, their surfaces, interfaces, and semiconductor technology as appropriate, research methods utilized, and briefly summarizes the findings of the work