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Articles 451 - 480 of 509

Full-Text Articles in Electrical and Computer Engineering

Thermal Management Of Algan-Gan Hfets On Sapphire Using Flip-Chip Bonding With Epoxy Underfill, Jie Sun, H. Fatima, Alexei Koudymov, A. Chitnis, X. Hu, H.-M Wang, J. Zhang, Grigory Simin, J. Yang, Asif Khan Jun 2003

Thermal Management Of Algan-Gan Hfets On Sapphire Using Flip-Chip Bonding With Epoxy Underfill, Jie Sun, H. Fatima, Alexei Koudymov, A. Chitnis, X. Hu, H.-M Wang, J. Zhang, Grigory Simin, J. Yang, Asif Khan

Faculty Publications

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy,underfill. They also show that in sapphire-based flip-chip mounted devices the heat …

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Rapid Prototyping Of Digital Controls For Power Electronics, Antonello Monti, Enrico Santi, Roger A. Dougal, Marco Riva May 2003

Rapid Prototyping Of Digital Controls For Power Electronics, Antonello Monti, Enrico Santi, Roger A. Dougal, Marco Riva

Faculty Publications

The process for designing digital controls for power electronics is typically quite convoluted and affords many opportunities for errors to occur. We present here a new and complete, method for rapid prototyping of digital controls that allows rapid realization of new designs. The approach uses a collection of tools that include both software (the virtual test bed (VTB) and Matlab/Simulink) and hardware (dSpace DSP). An example application of the methodology completes the discussion.

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Algan/Gan Heterostructure Field-Effect Transistors On Single-Crystal Bulk Aln, X. Hu, J. Deng, N. Pala, R. Gaska, M. S. Shur, C. Q. Chen, J. Yang, Grigory Simin, M. A. Khan, J. C. Rojo, L. J. Schowalter Feb 2003

Algan/Gan Heterostructure Field-Effect Transistors On Single-Crystal Bulk Aln, X. Hu, J. Deng, N. Pala, R. Gaska, M. S. Shur, C. Q. Chen, J. Yang, Grigory Simin, M. A. Khan, J. C. Rojo, L. J. Schowalter

Faculty Publications

We report on the performance of AlGaN/GaN/AlN heterostructurefield-effect transistors(HFETs) grown over slightly-off c-axis, single-crystal, bulk AlN substrates. Dc and rf characteristics of these devices were comparable to HFETs grown on semi-insulating SiC. The obtained results demonstrate that bulk AlN substrates are suitable for fabricating high-power microwave AlGaN/GaN transistors.

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Algan/Gan Hemts - Operation In The K-Band And Above, Ioulia P. Smorchkova, M. Wojtowicz, Rajinder Sandhu, R. Tsai, M. Barsky, C. Namba, P.-S. Liu, R. Dia, Minhdao Truong, D. Ko, J. Wang, H. Wange, Asif Khan Feb 2003

Algan/Gan Hemts - Operation In The K-Band And Above, Ioulia P. Smorchkova, M. Wojtowicz, Rajinder Sandhu, R. Tsai, M. Barsky, C. Namba, P.-S. Liu, R. Dia, Minhdao Truong, D. Ko, J. Wang, H. Wange, Asif Khan

Faculty Publications

We report on the power and microwave noise performance of AlGan/GaN high electron-mobility transistors (HEMTs) at frequencies f > 18 GHz (K- and Ka-bands). At 20 GHz, a record continuous-wave output power of 1.6 W has been achieved on an eight-finger 500-mum total gate-periphery device. At 29 GHz, a 120-mum gate-periphery device showed a pulsed output density of 1.6 W/mm with an associated gain of 6.7 dB and power-added efficiency of 26%. Minimum noise figure of 1.5 dB has been achieved on a 0.2 mum x 200 mum device at 26 GHz. The data demonstrate the viability of AlGan/GaN HEMTs for …

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Time-Resolved Electroluminescence Of Algan-Based Light-Emitting Diodes With Emission At 285 Nm, M. Shatalov, A. Chitnis, V. Mandavilli, R. Pachipulusu, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan, G. Tamulaitis, A. Sereika, I. Yilmaz, M. S. Shur, R. Gaska Jan 2003

Time-Resolved Electroluminescence Of Algan-Based Light-Emitting Diodes With Emission At 285 Nm, M. Shatalov, A. Chitnis, V. Mandavilli, R. Pachipulusu, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan, G. Tamulaitis, A. Sereika, I. Yilmaz, M. S. Shur, R. Gaska

Faculty Publications

We present a study on the time evolution of the electroluminescence(EL)spectra of AlGaN-based deep ultraviolet light-emitting diodes(LEDs) under pulsed current pumping. The ELspectra peaks at 285 nm and 330 nm are found to result from recombination involving band-to-band and free carriers to deep acceptor level transitions. The 330 nm long-wavelength transitions to deep acceptor levels in the p-AlGaN layer as well as the nonradiative processes significantly influence the LED internal quantum efficiency.

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Milliwatt Power Deep Ultraviolet Light-Emitting Diodes Over Sapphire With Emission At 278 Nm, J. P. Zhang, A. Chitnis, V. Adivarahan, S. Wu, V. Mandavilli, R. Pachipulusu, M. Shatalov, Grigory Simin, J. W. Yang, M. Asif Khan Dec 2002

Milliwatt Power Deep Ultraviolet Light-Emitting Diodes Over Sapphire With Emission At 278 Nm, J. P. Zhang, A. Chitnis, V. Adivarahan, S. Wu, V. Mandavilli, R. Pachipulusu, M. Shatalov, Grigory Simin, J. W. Yang, M. Asif Khan

Faculty Publications

We report on AlGaN multiple-quantum-well (MQW)-based deep ultraviolet light-emitting diodes over sapphire with peak emission at 278 nm. A new buffer layer growth process was used to reduce the number of defects and hence the nonradiative recombination. The improved material quality and carrier confinement resulted in pulsed powers as high as 3 mW at 278 nm and a significantly reduced deep-level-assisted long-wavelength emission.

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Polarization Effects In Photoluminescence Of C- And M-Plane Gan/Algan Multiple Quantum Wells, E. Kuokstis, C. Q. Chen, M. E. Gaevski, W. H. Sun, J. W. Yang, Grigory Simin, M. Asif Khan, H. P. Maruska, D. W. Hill, M. C. Chou, J. J. Gallagher, B. Chai Nov 2002

Polarization Effects In Photoluminescence Of C- And M-Plane Gan/Algan Multiple Quantum Wells, E. Kuokstis, C. Q. Chen, M. E. Gaevski, W. H. Sun, J. W. Yang, Grigory Simin, M. Asif Khan, H. P. Maruska, D. W. Hill, M. C. Chou, J. J. Gallagher, B. Chai

Faculty Publications

Polarizationeffects have been studied in GaN/AlGaN multiple quantum wells(MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence(PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [11̄00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs.Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. …

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Gan Homoepitaxy On Freestanding (11̄00) Oriented Gan Substrates, C. Q. Chen, M. E. Gaevski, W. H. Sun, E. Kuokstis, J. P. Zhang, R. S. Q. Fareed, H. M. Wang, J. W. Yang, Grigory Simin, M. A. Khan, Herbert-Paul Maruska, David W. Hill, Mitch M. C. Chou, Bruce Chai Oct 2002

Gan Homoepitaxy On Freestanding (11̄00) Oriented Gan Substrates, C. Q. Chen, M. E. Gaevski, W. H. Sun, E. Kuokstis, J. P. Zhang, R. S. Q. Fareed, H. M. Wang, J. W. Yang, Grigory Simin, M. A. Khan, Herbert-Paul Maruska, David W. Hill, Mitch M. C. Chou, Bruce Chai

Faculty Publications

We report homoepitaxialGaNgrowth on freestanding (11̄00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition.Scanning electron microscopy,atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (11̄00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (11̄00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, …

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Low-Temperature Operation Of Alfan Single-Quantum-Well Light-Emitting Diodes With Deep Ultraviolet Emission At 285 Nm, A. Chitnis, R. Pachipulusu, V. Mandavilli, M. Shatalov, E. Kuokstis, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan Oct 2002

Low-Temperature Operation Of Alfan Single-Quantum-Well Light-Emitting Diodes With Deep Ultraviolet Emission At 285 Nm, A. Chitnis, R. Pachipulusu, V. Mandavilli, M. Shatalov, E. Kuokstis, J. P. Zhang, V. Adivarahan, S. Wu, Grigory Simin, M. Asif Khan

Faculty Publications

We present a study of the electrical and optical characteristics of 285 nm emission deep ultraviolet light-emitting diodes(LED) at temperatures from 10 to 300 K. At low bias, our data show the tunneling carrier transport to be the dominant conduction mechanism. The room-temperature performance is shown to be limited mostly by poor electron confinement in the active region and a pronounced deep level assisted recombination but not by the hole injection into the active region. At temperatures below 100 K, the electroluminescence peak intensity increases by more than one order of magnitude indicating that with a proper device design and …

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Near-Band-Edge Photoluminescence Of Wurtzite-Type Aln, E. Kuokstis, J. Zhang, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. Shur, C. Rojo, L. Schowalter Sep 2002

Near-Band-Edge Photoluminescence Of Wurtzite-Type Aln, E. Kuokstis, J. Zhang, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. Shur, C. Rojo, L. Schowalter

Faculty Publications

Temperature-dependentphotoluminescence(PL)measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron–hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron–hole plasma expansion peculiarities.

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Low-Loss High Power Rf Switching Using Multifinger Algan/Gan Moshfets, Alexei Koudymov, Xuhong Hu, Kirill Simin, Grigory Simin, Mohammod Ali, Jinwei Yang, M. Asif Khan Aug 2002

Low-Loss High Power Rf Switching Using Multifinger Algan/Gan Moshfets, Alexei Koudymov, Xuhong Hu, Kirill Simin, Grigory Simin, Mohammod Ali, Jinwei Yang, M. Asif Khan

Faculty Publications

We demonstrate a novel RF switch based on a multifinger AlGan/GaN MOSHFET. Record high saturation current and breakdown voltage, extremely low gate leakage current and low gate capacitance of the III-N MOSHFETs make them excellent active elements for RF switching. Using a single element test circuit with 1-mm wide multifinger MOSHFET we achieved 0.27 dB insertion loss and more than 40 dB isolation. These parameters can be further improved by impedance matching and by using submicron gate devices. The maximum switching power extrapolated from the results for 1a/mm 100 mum wide device exceeds 40 W for a 1-mm wide 2-a/mm …

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Nonresonant Detection Of Terahertz Radiation In Field Effect Transistors, W. Knap, V. Kachorovskii, Y. Deng, S. Rumyantsev, J.-Q. Lü, R. Gaska, M. S. Shur, Grigory Simin, X. Hu, M. Asif Khan, C. A. Saylor, L. C. Brunel Jun 2002

Nonresonant Detection Of Terahertz Radiation In Field Effect Transistors, W. Knap, V. Kachorovskii, Y. Deng, S. Rumyantsev, J.-Q. Lü, R. Gaska, M. S. Shur, Grigory Simin, X. Hu, M. Asif Khan, C. A. Saylor, L. C. Brunel

Faculty Publications

We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructurefield effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.

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Dynamic Multiphysics Model For Solar Array, Shengyi Liu, Roger A. Dougal Jun 2002

Dynamic Multiphysics Model For Solar Array, Shengyi Liu, Roger A. Dougal

Faculty Publications

An approach to model the solar cell system with coupled multiphysics equations (photovoltaic, electro-thermal, direct heating and cooling processes) within the context of the resistive-companion method in the Virtual Test Bed computational environment is presented. Appropriate across and through variables are defined for the thermal terminal of the system so that temperature is properly represented as a state variable, rather than as a parameter of the system. This allows enforcement of the system power conservation through all terminals, and allows simultaneous solutions for both the electrical potentials and the system temperature. The thermal port built accordingly can be used for …

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Time-Resolved Photoluminescence Of Quaternary Alingan-Based Multiple Quantum Wells, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, G. G. Sim, P. W. Yu May 2002

Time-Resolved Photoluminescence Of Quaternary Alingan-Based Multiple Quantum Wells, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, G. G. Sim, P. W. Yu

Faculty Publications

Time-resolvedphotoluminescence(PL)dynamics has been studied in AlInGaN/AlInGaN multiple quantum wells(MQWs) grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) procedure. The PL decay kinetics was found to be sensitive to the emission energy and temperature. The PL decay time increases with decreasing emission energy, which is a characteristic of localized carrier/exciton recombination due to alloy fluctuations. Its temperature dependence shows radiative recombination to be the dominant process at low temperatures, indicating a high quality of PMOCVD grown quaternary AlInGaN MQWs and establishing them as promising structures for the active region of deep ultraviolet light emitting diodes.

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Luminescence Mechanisms In Quaternary AlXInYGa1-X-YN Materials, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan May 2002

Luminescence Mechanisms In Quaternary AlXInYGa1-X-YN Materials, Mee-Yi Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan

Faculty Publications

Low-temperature photoluminescence investigations have been carried out in the quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by pulsedmetalorganic chemical-vapor deposition (PMOCVD). With increasing excitation power density, the emission peaks in both AlInGaN epilayers and MQWs show a strong blueshift and theirlinewidths increase. The luminescence of the samples grown by PMOCVD is attributed to recombination of carriers/excitons localized at band-tail states. We also demonstrate theluminescence properties of AlInGaN and AlGaN materials grown by a pulsed atomic-layerepitaxy and conventional MOCVD, respectively.

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Crack-Free Thick Algan Grown On Sapphire Using Aln/Algan Superlattices For Strain Management, J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan May 2002

Crack-Free Thick Algan Grown On Sapphire Using Aln/Algan Superlattices For Strain Management, J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, Grigory Simin, M. Asif Khan

Faculty Publications

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n+-AlGaNlayers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al0.2Ga0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.

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Maximum Current In Nitride-Based Heterostructure Field-Effect Transistors, A. Koudymov, H. Fatima, Grigory Simin, J. Yang, M. Asif Khan, A. Tarakji, X. Hu, M. S. Shur, R. Gaska Apr 2002

Maximum Current In Nitride-Based Heterostructure Field-Effect Transistors, A. Koudymov, H. Fatima, Grigory Simin, J. Yang, M. Asif Khan, A. Tarakji, X. Hu, M. S. Shur, R. Gaska

Faculty Publications

We present experimental and modeling results on the gate-length dependence of the maximum current that can be achieved in GaN-based heterostructurefield-effect transistors(HFETs) and metal–oxide–semiconductor HFETs (MOSHFETs). Our results show that the factor limiting the maximum current in the HFETs is the forward gate leakage current. In the MOSHFETs, the gate leakage current is suppressed and the overflow of the two dimensional electron gas into the AlGaN barrier region becomes the most important factor limiting the maximum current. Therefore, the maximum current is substantially higher in MOSHFETs than in HFETs. The measured maximum current increases with a decrease in the gate …

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Two Mechanisms Of Blueshift Of Edge Emission In Ingan-Based Epilayers And Multiple Quantum Wells, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur Feb 2002

Two Mechanisms Of Blueshift Of Edge Emission In Ingan-Based Epilayers And Multiple Quantum Wells, E. Kuokstis, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur

Faculty Publications

We present the results of a comparative photoluminescence(PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells(MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm2 up to 1 MW/cm2. In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms …

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Localization Of Carriers And Polarization Effects In Quaternary Alingan Multiple Quantum Wells, E. Kuokstis, J. Zhang, M.-Y. Ryu, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur Dec 2001

Localization Of Carriers And Polarization Effects In Quaternary Alingan Multiple Quantum Wells, E. Kuokstis, J. Zhang, M.-Y. Ryu, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur

Faculty Publications

We report on observing a long-wavelength band in low-temperature photoluminescence(PL)spectrum of quaternary Al0.22In0.02Ga0.76N/Al0.38In0.01Ga0.61N multiple quantum wells(MQWs), which were grown over sapphire substrates by a pulsed atomic-layer epitaxy technique. By comparing the excitation-power density and temperature dependence of the PLspectra of MQWs and bulk quaternary AlInGaN layers, we show this emission band to arise from the carrier and/or exciton localization at the quantum well interface disorders. PL data for other radiative transitions in MQWs indicate that excitation-dependent spectra position is determined by screening of the built-in electric field.

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Ultraviolet Light-Emitting Diodes At 340 Nm Using Quaternary Alingan Multiple Quantum Wells, V. Adivarahan, A. Chitnis, J. P. Zhang, M. Shatalov, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur Dec 2001

Ultraviolet Light-Emitting Diodes At 340 Nm Using Quaternary Alingan Multiple Quantum Wells, V. Adivarahan, A. Chitnis, J. P. Zhang, M. Shatalov, J. W. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. S. Shur

Faculty Publications

An ultraviolet light-emitting diode with peak emission wavelength at 340 nm is reported. The active layers of the device were comprised of quaternary AlInGaN/AlInGaN multiple quantum wells, which were deposited over sapphire substrates using a pulsed atomic-layer epitaxy process that allows precise control of the composition and thickness. A comparative study of devices over sapphire and SiC substrates was done to determine the influence of the epilayer design on the performance parameters and the role of substrate absorption.

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Si3N4/Algan/Gan-Metal-Insulator-Semiconductor Heterostructure Field-Effect Transistors, X. Hu, A. Koudymov, Grigory Simin, J. Yang, M. Asif Khan, A. Tarakji, M. S. Shur, R. Gaska Oct 2001

Si3N4/Algan/Gan-Metal-Insulator-Semiconductor Heterostructure Field-Effect Transistors, X. Hu, A. Koudymov, Grigory Simin, J. Yang, M. Asif Khan, A. Tarakji, M. S. Shur, R. Gaska

Faculty Publications

We report on a metal–insulator–semiconductor heterostructurefield-effect transistor (MISHFET) using Si3N4 film simultaneously for channel passivation and as a gate insulator. This design results in increased radio-frequency (rf) powers by reduction of the current collapse and it reduces the gate leakage currents by four orders of magnitude. A MISHFET room temperature gate current of about 90 pA/mm increases to only 1000 pA/mm at ambient temperature as high as 300 °C. Pulsed measurements show that unlike metal–oxide–semiconductor HFETs and regular HFETs, in a Si3N4 MISHFET, the gate voltage amplitude required for current collapse is much higher …

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Induced Strain Mechanism Of Current Collapse In Algan/Gan Heterostructure Field-Effect Transistors, Grigory Simin, A. Koudymov, A. Tarakji, X. Hu, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska Oct 2001

Induced Strain Mechanism Of Current Collapse In Algan/Gan Heterostructure Field-Effect Transistors, Grigory Simin, A. Koudymov, A. Tarakji, X. Hu, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska

Faculty Publications

Gated transmission line model pattern measurements of the transient current–voltage characteristics of AlGaN/GaN heterostructurefield-effect transistors(HFETs) and metal–oxide–semiconductor HFETs were made to develop a phenomenological model for current collapse. Our measurements show that, under pulsed gate bias, the current collapse results from increased source–gate and gate–drain resistances but not from the channel resistance under the gate. We propose a model linking this increase in series resistances (and, therefore, the current collapse) to a decrease in piezoelectriccharge resulting from the gate bias-induced nonuniform strain in the AlGaN barrier layer.

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Indium-Silicon Co-Doping Of High-Aluminum-Content Algan For Solar Blind Photodetectors, V. Adivarahan, Grigory Simin, G. Tamulaitis, R. Srinivasan, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska Sep 2001

Indium-Silicon Co-Doping Of High-Aluminum-Content Algan For Solar Blind Photodetectors, V. Adivarahan, Grigory Simin, G. Tamulaitis, R. Srinivasan, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska

Faculty Publications

We report on an indium–silicon co-doping approach for high-Al-content AlGaN layers. Using this approach, very smooth crack-free n-type AlGaN films as thick as 0.5 μm with Al mole fraction up to 40% were grown over sapphire substrates. The maximum electron concentration in the layers, as determined by Hall measurements, was as high as 8×1017 cm−3 and the Hall mobility was up to 40 cm2/Vs. We used this doping technique to demonstrate solar-blind transparent Schottky barrierphotodetectors with the cut-off wavelength of 278 nm.

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Pulsed Atomic Layer Epitaxy Of Quaternary Alingan Layers, J. Zhang, E. Kuokstis, Q. Fareed, H. Wang, J. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. Shur Aug 2001

Pulsed Atomic Layer Epitaxy Of Quaternary Alingan Layers, J. Zhang, E. Kuokstis, Q. Fareed, H. Wang, J. Yang, Grigory Simin, M. Asif Khan, R. Gaska, M. Shur

Faculty Publications

In this letter, we report on a material deposition scheme for quaternary AlxInyGa1−x–yN layers using a pulsed atomic layer epitaxy (PALE) technique. The PALE approach allows accurate control of the quaternary layer composition and thickness by simply changing the number of aluminum,indium, and gallium pulses in a unit cell and the number of unit cell repeats. Using PALE, AlInGaN layers with Al mole fractions in excess of 40% and strong room-temperature photoluminescence peaks at 280 nm can easily be grown even at temperatures lower than 800 °C.

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Passivity-Based Control Of Saturated Induction Motors, Levent U. Gökdere, Marwan A. Simaan, Charles W. Brice Aug 2001

Passivity-Based Control Of Saturated Induction Motors, Levent U. Gökdere, Marwan A. Simaan, Charles W. Brice

Faculty Publications

A passivity-based controller, which takes into account saturation of the magnetic material in the main flux path of the induction motor, is developed to provide close tracking of time-varying speed and flux trajectories in the high magnetic saturation regions. The proposed passivity based controller is experimentally verified. Also, a comparison between the controllers based on the saturated and nonsaturated magnetics is presented to demonstrate the benefit of the controller based on the saturated magnetics

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Low Frequency Noise In Gan Metal Semiconductor And Metal Oxide Semiconductor Field Effect Transistors, S. L. Rumyantsev, N. Pala, M. S. Shur, R. Gaska, M. E. Levinshtein, M. Asif Khan, Grigory Simin, X. Hu, J. Yang Jul 2001

Low Frequency Noise In Gan Metal Semiconductor And Metal Oxide Semiconductor Field Effect Transistors, S. L. Rumyantsev, N. Pala, M. S. Shur, R. Gaska, M. E. Levinshtein, M. Asif Khan, Grigory Simin, X. Hu, J. Yang

Faculty Publications

The low frequency noise in GaNfield effect transistors has been studied as function of drain and gate biases. The noise dependence on the gate bias points out to the bulk origin of the low frequency noise. The Hooge parameter is found to be around 2×10−3 to 3×10−3.Temperature dependence of the noise reveals a weak contribution of generation–recombination noise at elevated temperatures.

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Very-Low-Specific-Resistance Pd/Ag/Au/Ti/Au Alloyed Ohmic Contact To P Gan For High-Current Devices, V. Adivarahan, A. Lunev, M. Asif Khan, J. Yang, Grigory Simin, M. S. Shur, R. Gaska Apr 2001

Very-Low-Specific-Resistance Pd/Ag/Au/Ti/Au Alloyed Ohmic Contact To P Gan For High-Current Devices, V. Adivarahan, A. Lunev, M. Asif Khan, J. Yang, Grigory Simin, M. S. Shur, R. Gaska

Faculty Publications

We report on Pd/Ag/Au/Ti/Au alloyed metallic contact to pGaN. An 800 °C anneal for 1 min in flowing nitrogen ambient produces an excellent ohmic contact with a specific contact resistivity close to 1×10−6 Ω cm2 and with good stability under high current operation conditions. This high-temperature anneal forms an alloy between Ag,Au, and pGaN resulting in a highly p-doped region at the interface. Using x-ray photoelectron spectroscopy and x-ray diffractionanalysis, we confirm that the contact formation mechanism is the metal intermixing and alloying with the semiconductor.

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Mechanism Of Radio-Frequency Current Collapse In Gan-Algan Field-Effect Transistors, A. Tarakji, Grigory Simin, N. Ilinskaya, X. Hu, A. Kumar, A. Koudymov, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska Apr 2001

Mechanism Of Radio-Frequency Current Collapse In Gan-Algan Field-Effect Transistors, A. Tarakji, Grigory Simin, N. Ilinskaya, X. Hu, A. Kumar, A. Koudymov, J. Yang, M. Asif Khan, M. S. Shur, R. Gaska

Faculty Publications

The mechanism of radio-frequency current collapse in GaN–AlGaN heterojunctionfield-effect transistors(HFETs) was investigated using a comparative study of HFET and metal–oxide–semiconductor HFET current–voltage (I–V) and transfer characteristics under dc and short-pulsed voltage biasing. Significant current collapse occurs when the gate voltage is pulsed, whereas under drain pulsing the I–V curves are close to those in steady-state conditions. Contrary to previous reports, we conclude that the transverse electric field across the wide-band-gap barrier layer separating the gate and the channel rather than the gate or surface leakage currents or high-field effects in the gate–drain spacing is responsible for the current collapse. We …

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Band-Edge Luminesce In Quaternary Alingan Light-Emitting Diodes, M. Shatalov, A. Chitnis, V. Adivarahan, A. Lunev, J. Zhang, J. W. Yang, Q. Fareed, Grigory Simin, A. Zakheim, M. Asif Khan, R. Gaska, M. S. Shur Feb 2001

Band-Edge Luminesce In Quaternary Alingan Light-Emitting Diodes, M. Shatalov, A. Chitnis, V. Adivarahan, A. Lunev, J. Zhang, J. W. Yang, Q. Fareed, Grigory Simin, A. Zakheim, M. Asif Khan, R. Gaska, M. S. Shur

Faculty Publications

Operation of InGaNmultiple-quantum-well(MQW)light-emitting diodes(LEDs) with quaternary AlInGaN barriers at room and elevated temperatures is reported. The devices outperform conventional GaN/InGaN MQWLEDs, especially at high pump currents. From the measurements of quantum efficiency and total emitted power under dc and pulsed pumping, we show the emission mechanism for quaternary barrier MQWs to be predominantly linked to band-to-band transitions. This is in contrast to localized state emission observed for conventional InGaN/InGaN and GaN/InGaN LEDs. The band-to-band recombination with an increased quantum-well depth improves the high-current performance of the quaternary barrier MQWLEDs, making them attractive for high-power solid-state lighting applications.

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Highly Doped Thin-Channel Gan-Metal-Semiconductor Field-Effect Transistors, R. Gaska, M. S. Shur, X. Hu, J. W. Yang, A. Tarakji, Grigory Simin, A. Khan, J. Deng, T. Werner, S. Rumyantsev, N. Pala Feb 2001

Highly Doped Thin-Channel Gan-Metal-Semiconductor Field-Effect Transistors, R. Gaska, M. S. Shur, X. Hu, J. W. Yang, A. Tarakji, Grigory Simin, A. Khan, J. Deng, T. Werner, S. Rumyantsev, N. Pala

Faculty Publications

We report on the influence of the channel doping on dc, high frequency, and noise performance of GaN metal–semiconductor field-effect transistors (MESFETs) grown on sapphire substrates. The devices with the channel thicknesses from 50 to 70 nm and doping levels up to 1.5×1018 cm−3 were investigated. An increase in the channel doping results in the improved dc characteristics, higher cutoff, and maximum oscillation frequencies, and reduced low frequency and microwave noise. The obtained results demonstrate that the dc and microwave performance characteristics of short-channel GaN MESFETs may be comparable to those for conventional AlGaN/GaN heterostructure FETs.

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