A study of subterahertz HEMT monolithic oscillators

A detailed study of monolithic InP-based HEMT oscillators for subterahertz operation is presented. InAlAs/InGaAs HEMT's have been optimized for high frequency operation and showed very high maximum oscillation frequencies (f(sub max)) of 310 GHz using offset self-aligned gamma-gate technology. Power characteristics of HEMT oscillators are reported. An oscillation power of more than 10 mW was evaluated by large-signal analysis at 320 GHz using HEMT's with f(sub max) = 450 GHz, V(sub br) = 10 V and a gate width (W(sub g)) of 8 x 22.5 microns. Oscillator topology studies showed that complex feedback schemes such as dual and active feedback enhance the negative resistance. Push-push oscillator designs based on harmonic signal generation can finally be used to overcome the frequency barrier imposed by f(sub max)

Self‐consistent analysis of lattice‐matched and pseudomorphic quantum‐well emission transistors

A self‐consistent analysis of the quantum‐well emission transistor (QWET) is presented allowing an exact calculation of the device quantum properties. Poisson’s and Schrödinger’s equation are solved numerically using a finite‐difference method on a self‐consistent basis. Pseudomorphic AlGaAs/InGaAs designs with 15%–20% excess In are suggested for improving the device performance. Design with doping in various parts of the QWET are also studied. This analysis reveals that the device performance is less optimistic than previously predicted by analytic approaches. By introducing the pseudomorphic channel principle, while maintaining a reasonably low Al content for the gate and collector layers, it is, however, possible to obtain satisfactory performance. Optimum pseudomorphic designs showed high current driving capability (2×105 A/cm2), high transconductance (3S/mm) and small intrinsic delay time (2 ps).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70178/2/JAPIAU-69-4-2662-1.pd

Individual Analysts Earnings Forecasts: Evidence For Overreaction In The UK Stock Market

This paper presents an analysis of two forms of overreaction (generalized overreaction and overreaction to prior earnings changes) in analysts earnings forecasts for the UK stock market, using a sample of individual forecasts of earning per share from a British investment bank over the period 1989-2002. Given that previous UK empirical research over 1980s and mid 90s has provided limited and contradictory findings, we investigate whether and how overreaction of analysts forecasts varies across forecast horizons, firm size (small and large) and growth opportunities (high and low P/E ratio) in order to provide further and comparable evidence. Overall, our findings support the generalized overreaction hypothesis but reject the firm size effect, the overreaction for high P/E ratio companies and the higher overreaction regarding the forecasting horizon. Keywords: Overreaction, Underreaction, Analysts forecasts, forecast horizons, size effect, price/earnings ratio

A physics‐based fitting and extrapolation method for measured impact ionization coefficients in III‐V semiconductors

A general approach based on a physical model of impact ionization to fit and extrapolate measured ionization coefficients of electrons α and holes β in III‐V semiconductors is described. Materials being considered include GaAs, AlxGa1−xAs (x=0.1–0.4), InP, In0.53Ga0.47As, and In0.52Al0.48As. Expressions giving the correct dependencies are obtained at very large or small electric fields outside the range of most measurements while at the same time a reasonable fit is achieved for experimental data. The results of the proposed approach yielded a set of physical parameters, which can be coupled with the temperature‐dependence relationships in the model to predict impact ionization coefficients over a wide range of electric fields at different temperatures, and can be useful in calculations of temperature‐dependent avalanche breakdown voltages of electronic and optical devices.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70428/2/JAPIAU-72-2-531-1.pd

Material related issues and their characterization with a view to III-V heterojunction device optimization

The impact of various material choices, i.e. InAlAs/InGaAs, AlGaAs/GaAs, GaInP/GaAs on device characteristics is analyzed. Traps located at various regions of the device impact its performance and can be identified by device characterization such as gm, Rds dispersion and low-frequency noise. Reliability characteristics can be related to material modifications under stress, i.e. traps and dopant diffusion and degradation can be minimized by growth optimization. Process induced damage due to dry-etching can be minimized by proper selection of etching conditions. A strong interaction between material and device research is necessary for best results in optimizing III-V technology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30729/1/0000378.pd

Current status of heterojunction bipolar and high-electron mobility transistor technologies

The current status of Heterojunction Bipolar and High-Electron Mobility Transistor technology is reviewed. Applications include analog and digital circuits with focus on high power and optical communications for HBT's and millimeter-wave low-noise and power modules for HEMT's. Material choices, designs, technology, reliability issues and circuit demonstrations are addressed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29872/1/0000221.pd

Theoretical study of GaN growth: A Monte Carlo approach

An atomistic model consistent with a variety of experimental observations is developed for GaN growth by molecular‐beam epitaxy. The model is used in Monte Carlo simulation to study the impact of substrate temperature, Ga flux, and V/III (group‐V element to group‐III element) ratio on growth rate and growth front quality. The growth rate increases with the V/III ratio reaching a saturation value which is determined by the Ga flux. The quality of the growth front improves by using a smaller Ga flux for a fixed temperature and V/III ratio or by reducing the V/III ratio at a given temperature. A consideration of the growth kinetics suggests that GaN grown surfaces are likely to be Ga stabilized. These theoretically estimated trends are evidenced by two‐dimensional and three‐dimensional growth front contours evaluated under various growth conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71260/2/JAPIAU-76-6-3502-1.pd

GaN/air gap based micro-opto-electro-mechanical (MOEM) Fabry-PÉrot filters

Structural and optical properties of Fabry-PÉrot filters (FPFs) with GaN/air gap based distributed Bragg reflectors (DBRs) were studied. Reflectance of GaN/air gap DBRs on sapphire substrate was calculated from the standard transmission matrix method and results showed that 98% reflectance is achievable with only 3.5 pairs at a center wavelength of 450 nm. The thickness of the GaN layer and the first AlN layer was determined according to the deformation induced by the residual stress. In-plane strain corresponding to growth conditions and the thickness of the GaN epilayer was considered for this analysis. Optical tuning efficiency and spectral range were found to be 0.27 and 25 nm respectively for FPFs with GaN/air gap (322 nm/113 nm) based DBRs and a Λ 0 /2 air resonant cavity. The calculated pull-in voltage was 1.5 V. Crack free AlN grown on GaN by in-house MOCVD showed an etching rate of 0.2 nm/min. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/56100/1/2764_ftp.pd

Quantum state transfer in double‐quantum‐well devices

A Monte Carlo simulation of double‐quantum‐well (DQW) devices is presented in view of analyzing the quantum state transfer (QST) effect. Different structures, based on the AlGaAs/GaAs system, were simulated at 77 and 300 K and optimized in terms of electron transfer and device speed. The analysis revealed the dominant role of the impurity scattering for the QST. Different approaches were used for the optimization of QST devices and basic physical limitations were found in the electron transfer between the QWs. The maximum transfer of electrons from a high to a low mobility well was at best 20%. Negative differential resistance is hampered by the almost linear rather than threshold dependent relation of electron transfer on electric field. By optimizing the doping profile the operation frequency limit could be extended to 260 GHz. © 1994 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70987/2/JAPIAU-76-11-7428-1.pd