Boundary layer flow and heat transfer over a permeable shrinking sheet with partial slip

The steady, laminar flow of an incompressible viscous fluid over a shrinking permeable sheet is investigated. The governing partial differential equations are transformed into ordinary differential equations using similarity transformation, before being solved numerically by the shooting method. The features of the flow and heat transfer characteristics for different values of the slip parameter and Prandtl number are analyzed and discussed. The results indicate that both the skin friction coefficient and the heat transfer rate at the surface increase as the slip parameter increases

The High-Electron Mobility Transistor at 30: Impressive Accomplishments and Exciting Prospects

2010 marked the 30th anniversary of the High-Electron Mobility Transistor (HEMT). The HEMT represented a triumph for the, at the time, relatively new concept of bandgap engineering and nascent molecular beam epitaxy technology. The HEMT showcased the outstanding electron transport characteristics of two-dimensional electron gas (2DEG) systems in III-V compound semiconductors. In the last 30 years, HEMTs have been demonstrated in several material systems, most notably AlGaAs/GaAs and AlGaN/GaN. Their uniqueness in terms of noise, power and high frequency operation has propelled HEMTs to gain insertion in a variety of systems where they provide critical performance value. 2DEG systems have also been a boon in solid-state physics where new and often bizarre phenomena have been discovered. As we look forward, HEMTs are uniquely positioned to expand the reach of electronics in communications, signal processing, electrical power management and imaging. Some of the most exciting prospects in the near future for HEMT-like devices are those of GaN for high voltage power management and III-V CMOS to give a new lease on life to Moore’s Law. This paper briefly reviews some highlights of HEMT development in the last 30 years in engineering and science. It also speculates about potential future applications

Design and Fabrication of InP High Electron Mobility Transistors for Cryogenic Low-Noise Amplifiers

High electron mobility transistor (InP HEMT) cryogenic low noise amplifiers (LNAs) have made significant improvements in noise and gain following decades of development. Applications are found from radio astronomy to quantum computing. The noise figure for the best InP HEMT cryogenic LNA, however, is still almost one order of magnitude higher than for a quantum-noise limited amplifier. This motivates further studies to understand the physical mechanisms limiting noise reduction in the InP HEMT. In this thesis, 100-nm gate-length InP HEMTs were developed for probing the intrinsic channel noise in the transistor. Electrochemical etching was found to strongly deteriorate the gate recess etch. This was mitigated by modifying the InP HEMT fabrication scheme to a recess-first process. A comparison of two different device passivation methods, atomic layer deposition of Al2O3 and plasma enhanced chemical vapor deposition of SixNy, did not reveal any significant difference in neither gain nor noise for the InP HEMT cryogenic LNA. Channel noise of the InP HEMT was investigated by varying the spacer thickness from 1 to 7 nm in the InAlAs-InGaAs heterostructure. It was found that the optimum spacer thickness was 5 nm for lowest noise temperature in a 4-8 GHz three-stage hybrid LNA at 5 K. This was 2 nm thicker than previously reported spacer thickness used for a similar state-of-the-art InP HEMT cryogenic LNA. The 5 nm spacer InP HEMT LNA minimum average noise temperature was determined to 1.4 K. The channel noise dependence on spacer thickness for the cryogenic InP HEMT was explained by a real-space transfer mechanism associated with the injection of a minor fraction of hot electrons from channel to barrier. Finally, the subthreshold swing of the InP HEMT at 5 K was observed to exhibit a correlation with the noise temperature in the cryogenic LNA. This suggests that the subthreshold swing serves as an indicator of the amount of carrier fluctuations in the InP HEMT channel giving rise to noise

Terahertz and Sub-Terahertz Tunable Resonant Detectors Based on Excitation of Two Dimensional Plasmons in InGaAs/InP HEMTs

Plasmons can be generated in the two dimensional electron gas (2DEG) of grating-gated high electron mobility transistors (HEMTs). The grating-gate serves dual purposes, namely to provide the required wavevector to compensate for the momentum mismatch between the free-space radiation and 2D-plasmons, and to tune the 2DEG sheet charge density. Since the plasmon frequency at a given wavevector depends on the sheet charge density, a gate bias can shift the plasmon resonance. In some cases, plasmon generation results in a resonant change in channel conductance which allows a properly designed grating-gated HEMT to be used as a voltage-tunable resonant detector or filter. Such devices may find applications as chip-scale tunable detectors in airborne multispectral detection and target tracking. Reported here are investigations of InGaAs/InP-based HEMT devices for potential tunable resonant sub-THz and THz detectors. The HEMTs were fabricated from a commercial double-quantum well HEMT wafer by depositing source, drain, and semi-transparent gate contacts using standard photolithography processes. Devices were fabricated with metalized transmission gratings with multiple periods and duty cycles. For sub-THz devices, grating period and duty cycle were chosen to be 9 ?m and 22%, respectively; while they were chosen to be 0.5 ?m and 80% for the THz device. The gratings were fabricated on top of the gate region with dimensions of 250 ?m x 195 ?m. The resonant photoresponse of the larger grating-period HEMT was investigated in the sub-THz frequency range of around 100 GHz. The free space radiation was generated by an ultra-stable Backward Wave Oscillator (BWO) and utilized in either frequency modulation (FM), or amplitude modulation (AM) experiments. The photoresponse was measured at 4K sample temperature as the voltage drop across a load resistor connected to the drain while constant source-drain voltages of different values, VSD, were applied. The dependence of such optoelectrical effect to polarization of the incident light, and applied VSD is studied. The results of AM and FM measurements are compared and found to be in agreement with the calculations of the 2D-plasmon absorption theory, however, a nonlinear behavior is observed in the amplitude and the line-shape of the photoresponse for AM experiments. For detection application, the minimum noise-equivalent-power (NEP) of the detector was determined to be 235 and 113 pW/Hz1/2 for FM and AM experiments, respectively. The maximum responsivity of the detector was also estimated to be ~ 200 V/W for the two experiments. The far-IR transmission spectra of the device with nanometer scale period was measured at 4 K sample temperature for different applied gate voltages to investigate the excitation of 2D-plasmon modes. Such plasmon resonances were observed, but their gate bias dependence agreed poorly with expectations

Advanced III-V / Si nano-scale transistors and contacts: Modeling and analysis

The exponential miniaturization of Si CMOS technology has been a key to the electronics revolution. However, the continuous downscaling of the gate length becomes the biggest challenge to maintain higher speed, lower power, and better electrostatic integrity for each following generation. Hence, novel devices and better channel materials than Si are considered to improve the metal-oxide-semiconductor field-effect transistors (MOSFETs) device performance. III-V compound semiconductors and multi-gate structures are being considered as promising candidates in the next CMOS technology. III-V and Si nano-scale transistors in different architectures are investigated (1) to compare the performance between InGaAs of III-V compound semiconductors and strained-Si in planar FETs and triple-gate non-planar FinFETs. (2) to demonstrate whether or not these technologies are viable alternatives to Si and conventional planar FETs. The simulation results indicate that III-V FETs do not outperform Si FETs in the ballistic transport regime, and triple-gate FinFETs surely represent the best architecture for sub-15nm gate contacts, independently from the choice of channel material. ^ This work also proves that the contact resistance becomes a limiting factor of device performance as it takes larger fraction of the total on-state resistance. Hence, contact resistance must be reduced to meet the next ITRS requirements. However, from a modeling point of view, the understanding of the contacts still remains limited due to its size and multiple associated scattering effects, while the intrinsic device performance can be projected. Therefore, a precise theoretical modeling is required to advance optimized contact design to improve overall device performance. In this work, various factors of the contact resistances are investigated within realistic contact-to-channel structure of III-V quantum well field-effect transistors (QWFET). The key finding is that the contact-to-channel resistance is mainly caused by structural reasons: 1) barriers between multiple layers in the contact region 2) Schottky barrier between metal and contact pad. These two barriers work as bottleneck of the system conductance. The extracted contact resistance matches with the experimental value. The approximation of contact resistance from quantum transport simulation can be very useful to guide better contact designs of the future technology nodes. ^ The theoretical modeling of these nano-scale devices demands a proper treatment of quantum effects such as the energy-level quantization caused by strong quantum confinement of electrons and band structure non-parabolicity. 2-D and 3-D quantum transport simulator that solves non-equilibrium Green\u27s functions (NEGF) transport and Poisson equations self-consistently within a real-space effective mass approximation. The sp3d5s* empirical tight-binding method is employed to include non-parabolicity to obtain more accurate effective masses in confined nano-structures. The accomplishment of this work would aid in designing, engineering and manufacturing nano-scale devices, as well as next-generation microchips and other electronics with nano-scale features