Hirohisa Taguchi

Analysis of Deviation of Threshold Voltage from Hole Accumulation Model at High Excitation

The energy state, concentration, and potential energy for both electrons and holes in the channel of InAlAs/InGaAs high electron mobility transistors (HEMTs) were studied using the theory based on the local density functional method. The numerical result shows that the potential profile changes from the triangle to the square well as the sheet concentration of holes accumulated in the source region ( ps) increases above the sheet concentration of the two-dimensional electron gas (2DEG) because the same amount of electrons as holes was injected from the source to maintain the charge neutrality in the channel. As a result, the quasi-Fermi energy increased and the potential energy of electrons in the channel approached that of the square-well potential, the former led to an increase in the threshold voltage (VTH). The overlap integral between the wavefunctions of an electron and a hole was estimated as a function of the channel thickness (Lz) and was shown to decrease with increasing Lz. A detailed theory concerning the relation between the VTH shift and ps was developed and compared with the experimental results. In this theory, the recombination of holes with 2DEG was taken into account, on the assumption that the dominant process was due to the non-radiative Auger recombination mechanism.

Frequency Dependence of Drain Conductance due to Hole Accumulation in InAlAs/InGaAs High Electron Mobility Transistors

The frequency dependencies of the drain conductance (Gd) and the responsivity of InAlAs/InGaAs high electron mobility transistors were investigated using a network analyzer. The Gd exhibited the same frequency dependence as responsivity under 1.3-µm-wavelength laser illumination, indicating that the frequency dependence of Gd arises from the recombination of holes that have accumulated in the source region with the two-dimensional electron gas (2DEG). The holes were generated in the drain region by impact ionization under the strong electric field and drifted toward the source region. The drain conductance due to impact ionization can be expressed as Lorentz frequency dependence with f3dB being the 3-dB-down frequency. The minority carrier (hole) lifetime, τ, was estimated using the relation 1/2 πf3dB. The frequency dependence of Gd at several drain-to-source and gate-to-source voltages was investigated. The recombination lifetime for a system in which both electrons and holes co-exist was theoretically estimated, taking account of self-consistent solutions of both the Schrodinger and Poisson equations and the nonradiative Auger recombination mechanism. The experimental and theoretically estimated results clearly show that the frequency dependence of Gd is caused by the accumulation of holes in the source region and their recombination with the 2DEG. Based on the experimental and theoretical results, the distribution of holes in the channel and the multiplication factor due to the impact ionization are discussed in details.

Dependence of Carrier Lifetime of InAlAs/InGaAs High-Electron-Mobility Transistors on Gate-to-Source Voltage

Recently, we have investigated the frequency dependence of the drain conductance Gd of high-electron-mobility transistors (HEMTs) employing an InAlAs/InGaAs material system that is lattice-matched to an InP substrate in detail and showed that, on the basis of theoretical considerations, it is due to the accumulation of holes in the source region and their recombination with two-dimensional electron gas (2DEG) in the source region. To better understand this carrier recombination model, we have investigated in detail the dependence of the carrier lifetime τ on the gate-to-source voltage VGS and on the effective drain-to-gate voltage VDG,eff applied to the side of the gate-to-drain path. Experimental results for the VGS dependence of the carrier lifetime τ not only confirm that the recombination of holes accumulating in the source region with 2DEG is dominated by Auger recombination but also show that the gradients of τ become less steep at a VGS larger than 0.4 V. A theoretical study of this phenomenon revealed that excess electrons penetrate in the δ-doped layer with increasing VGS and are captured by the ionized donors distributed δ-doped layer. As a result, the electrical field in the δ-doped layer decreases with increasing VGS and then the surface potential drops.

Ultrafast Optical Response of InAlAs/InAs/InGaAs Pseudomorphic High Electron Mobility Transistors

High-electron-mobility transistors with a pseudomorphically strained InAs channel (InAs-PHEMTs) have excellent electron transport properties and a high electron density, which are due to their large conduction band discontinuity. In this work, we show the dependence of optical response on drain-to-source voltage (VDS) for InAs-PHEMTs and clarify the physical mechanism for the response time. The experimental results can be explained successfully using two different lifetimes, one dominated by the time required for a hole to transit from the channel to the source region under the channel field and the other dominated by Auger recombination. To numerically understand the optical response, we estimate minority carrier lifetime using the Auger recombination theory. The theoretical result agrees well with the experimental result.

Direct thermal-to-electric energy conversion material consisting of environmentally-benign Mg 2 Si synthesized using waste Si sludge and recycled Mg alloy

In order to perturb global warming and realize a sustainable global energy system, enhancements in the energy efficiency are required. One of the reliable technologies to reduce the greenhouse gas emissions and the consumption of fossil fuel that is attracting attention is thermoelectric technology, which can directly convert heat into electricity and consequently increase the energy conversion efficiency of power generation by combustion. Magnesium silicide (Mg 2 Si) has been identified as a promising advanced thermoelectric material operating in the temperature range from 500 to 800 K. Compared with other thermoelectric materials that operate in the same conversion temperature range, such as PbTe, TAGS (Ge-Te-Ag-Sb) and CoSb 3 , Mg 2 Si shows promising aspects, such as the abundance of its constituent elements in the earth’s crust and the non-toxicity of its processing by-products, resulting in freedom from care regarding prospective extended restriction on hazardous substances. Here we have tried to introduce reusing of industrial waste of Si sludge as a source material for Mg 2 Si, because the current product inversion rate of Si for semiconductor LSI devices remains at 25 to 30 %, while most of the remainder is disposed of as a waste; this is mainly discharged as sludge from grinding and polishing processes. It is possible that the reuse of this waste Si could be effective in both reducing the cost of source Si and in the reduction of industrial waste. On the other hand, recycled materials of standard lightweight magnesium alloys based on the Mg-Al-Zn-Mn system, such as AZ91 or AM50, were also introduced as a Mg source for Mg2Si synthesis. The concept of this work is a production of wasted heat recovery device using environmentally-benign Mg2Si by means of industrial waste or less pure recycled sources. The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S2σT/κ, of its constituent thermoelectric material, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. As a target for practical use, ZT value exceed unity, which gives about 10 % conversion efficiencies, is expected. So far, we succeeded to obtain the Mg 2 Si with ZT=1.08 using rather pure Si (99.999% : solar grade) and Mg (99.95%) sources. In this article, we report multifarious fabrication processes in order to realize ZT value as high as unity and the detailed thermoelectric properties concerning Mg 2 Si initiated from reused Si sludge and the recycled Mg-alloy sources. In conjunction, we will also discuss the practical output-power characteristics of the samples with the formation of Ni electrodes by monobloc sintering. A tentative generated power density from the wasted heat at 773 K was ˜2 KW/m 2 .

Spectroscopic Ellipsometry for the Characterization of the Morphology of Ultra-thin Thermal CVD Amorphous and Nanocrystalline Silicon Thin Films

Ultra-thin hydrogenated amorphous silicon thin films have been deposited by thermal chemical vapor deposition (CVD) to prepare smooth top surface of the films avoiding the ion bombardment. Rapid thermal oxidation of thermal CVD a-Si:H results in nanocrystalline dots in the ultra-thin silicon films. Spectroscopic ellipsometry (SE) and high resolution transmission electron microscopy (TEM) have been used to investigate the optical and structural properties of both ultra-thin a-Si:H and nanocrystalline silicon films. To analyze the ellipsometric data of ultra-thin a-Si:H films, a new parameterization i.e., the combination of Sellmeier law and four Lorentz peaks, has been successfully introduced. Width of the Lorentz peaks are directly related with the change of optical functions with the thickness of a-Si:H films. It has been certified that the dense Si matrix with smaller degree of disorder is formed when the thickness exceeds 8nm and the films with the thickness of less than 3.8 nm becomes voided. To interpret the ellipsometric data for nanocrystalline silicon films, three layer model (SiO 2 , poly-Si+a-Si+void and SiO 2 ) has been adapted. It is inferred from SE and TEM analyses that the size and the density of nanocrystalline dots can be controlled by the morphology of initial ultra-thin a-Si:H films and RTO conditions.