R. Gaska

KTH Royal Institute of Technology, Stockholm, Stockholm, Sweden

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Publications (364)562.08 Total impact

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    ABSTRACT: We report on GaN microwave varactors with capacitively coupled isolated electrodes. Metal-insulator-semiconductor (MIS) varactors have 4–5 orders of magni-tude lower leakage currents than similar devices with Schottky contacts. Low leakage capacitively coupled contacts enable microwave switches and power limiters with low loss, high RF power and excellent temperature stability up to at least 200 °C. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2014;
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    ABSTRACT: The results on design, fabrication, and characterization of a hot-electron bolometer (HEB) based on the low-mobility two-dimensional electron gas (2DEG) in a AlGaN/GaN heterostructures show that our HEBs have high coupling to incident THz radiation due to the Drude absorption. Significant heating by THz radiation is realized due to the small value of the electron heat capacity. A low contact resistance achieved in our devices ensures that the THz radiation couples primarily to the 2DEG. Due to the small electron momentum relaxation time, the real part of the 2DEG sensor impedance is ~50–100 Ω, which provides a good impedance match between the sensor and antenna. Currently the room temperature responsivity of our devices reaches ~ 0.04 A/W at 2.55 THz along with a noise equivalent power ~ 5 nW/Hz1/2 at room temperature.
    Journal of Physics Conference Series 03/2014; 486(1):012028.
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    ABSTRACT: The influence of carrier localization on photoluminescence efficiency droop and stimulated emission is studied in AlGaN multiple quantum wells with different strength of carrier localization. We observe that carrier delocalization at low temperatures predominantly enhances the nonradiative recombination and causes the droop, while the main effect of the delocalization at elevated temperatures is enhancement of PL efficiency due to increasing contribution of bimolecular recombination of free carriers. When the carrier thermal energy exceeds the dispersion of the potential fluctuations causing the carrier localization, the droop is caused by stimulated carrier recombination.
    Optics Express 02/2014; 22(S2). · 3.55 Impact Factor
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    ABSTRACT: We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.
    Semiconductor Science and Technology 01/2014; 29(8). · 1.92 Impact Factor
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    ABSTRACT: The novel HFET design using perforated channel region under the gate reduces drain and source parasitic resistances due to the current spreading effect in the source-gate and gate–drain regions. Demonstrated results for AlGaN/GaN HFET show that the $R_{rm ON}C_{G}$ time constant reduces around two times using simple and robust perforated channel device processing. These results are especially important for new generations of power switching transistors.
    IEEE Electron Device Letters 01/2014; 35(4):449-451. · 2.79 Impact Factor
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    ABSTRACT: Photoluminescence studies of carrier dynamics in AlGaN epilayers with different degrees of carrier localization and densities of nonradiative recombination centers show that the prevailing droop mechanism in AlGaN epilayers with strong carrier localization and comparatively high density of nonradiative recombination centers is enhanced nonradiative recombination due to the carrier delocalization at elevated carrier density. The photoluminescence was investigated under quasi-steady-state excitation in the temperature range from 8 to 300 K. The results proved that the onset of this droop effect is below the threshold for the droop due to high-density effects in the epilayers, such as carrier heating, phase space filling, nonradiative Auger recombination, and stimulated emission.
    Applied Physics Letters 07/2013; 103(1). · 3.79 Impact Factor
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    ABSTRACT: The growing complexity and functionality of modern microwave systems put increasing demands on the performance and reliability of RF control devices. Low channel sheet resistance in the on-state and small leakage currents in the off-state, extremely high breakdown voltages, chemical inertness and planar structure make III-nitride heterostructure-based devices uniquely suitable for the next-generation switching devices. The AlInN-based devices and devices using low-conducting layers for controlling surface field and surface charges are expected to further boost the performance. This paper presents a review of III-nitride control devices based on heterostructure field-effect transistors (HFETs), insulated gate HFETs (MISHFETs) and novel capacitively coupled contact and frequency configurable electronics technologies.
    Semiconductor Science and Technology 06/2013; 28(7):074008. · 1.92 Impact Factor
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    ABSTRACT: Low-irradiance, long-term UV treatment using DUV-LEDs extended the cold storage shelf life of strawberries up to 2-fold as based on weight, moisture content, anthocyanin concentration, soluble solids, titratable acidity, visible damage, and mold growth.
    CLEO: Applications and Technology; 06/2013
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    ABSTRACT: Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructure have been examined by time-resolved photoluminescence (PL), transient photoreflectance and scanning near-field optical spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited carrier transfer from AlInN to GaN via sub-band edge states. Near-field PL scans and photoreflectance data show that the diameter of the localization sites and the distance between them are well below 100 nm. Majority of these states is assigned to In clusters, in which the valence band has a higher energy than the valence band in a uniform AlInN alloy. It is likely that the carrier transport through the sub-band edge states proceeds via high conductivity channels involving extended defects. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 05/2013; 10(5):853-856.
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    ABSTRACT: We report on the low-loss microwave switch monolithic microwave integrated circuit (MMIC) built using voltage-controlled capacitors formed by the two Schottky contacts deposited over an AlGaN/GaN heterostructure. A symmetrical structure of the varactor allows for either positive or negative control voltage. A small electrode and a spacing size of 2 μm ensure low on-impedance. The fabricated series-shunt switch MMIC shows 0.8-dB insertion loss and 28-dB isolation at 18 GHz and the maximum linear power of 34 dBm at 10 GHz and 38 dBm at 18 GHz (extrapolated from 2- to 10-GHz data). The device does not require contact alignment or annealing; it is robust, simple to fabricate, and well suited for MMICs.
    IEEE Electron Device Letters 02/2013; 34(2):208-210. · 2.79 Impact Factor
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    ABSTRACT: Deep UV LED structures with UV transparent design, reflective p-electrodes and die encapsulation exhibited external quantum efficiency above 11%. Progress in material growth and device fabrication will be discussed along with issues further limiting efficiency.
    Lasers and Electro-Optics Pacific Rim (CLEO-PR), 2013 Conference on; 01/2013
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    R. Gaska, G. Simin, M. Shur
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    ABSTRACT: Materials properties of nitride semiconductors make them superior candidates for power electronics applications. Materials quality, strongly non-uniform field distribution, variations of parameters from device to device, and reliability problems have to be improved or resolved. New designs, especially suitable to nitride devices, include the use of Low Conducting Layers, “composite channel structures, and novel contacts. All these improvement still have to be validated and improved to make the dream of nitride power electronics to be competitive with Si and SiC power transistors.
    Energytech, 2013 IEEE; 01/2013
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    ABSTRACT: We report a novel type of RF component using two-terminal III-Nitride heterostructure varactor with capacitively-coupled contacts (C3) which can be used as a RF switch as well as a power limiter(PL) or other control device type. This C3 varactor consists of two electrodes deposited on top of an AIGaN/GaN heterostructure forming capacitors between the electrode and the 2DEG channel. C3 electrodes allow for efficient RF signal injection into the 2DEG channel with low equivalent impedance at high frequencies. The C3 varactor has simple planar structure fully compatible with MMICs. The GaN C3 varactors have significant advantages over GaN heterostructure field-effect transistors (HFETs) as they allow for shorter channel, do not have gates or ohmic contacts and hence the fabrication is alignment- and anneal - free, they do not consume DC current and provide the DC block. The fabricated SPST C3 switch exhibits 0.8 dB insertion loss and 27 dB isolation at 18 GHz. The maximum switching power extrapolated from 2 GHz data is around 38 dBm. The fabricated varactor PLs show 0.2-0.7 dB loss and limiting powers in the range from 17 to 40 dBm. The C3 PL shows superior performance compared to other known types, e.g. Schottky diode PLs. The fabricated C3 PLs and switches showed the output power variations within 0.5 dB during 100 hours 24 dBm CW stress. They also demonstrated as low as 0.5 dB loss degradation at 2000 C as compared to the room temperature performance.
    Electronic Components and Technology Conference (ECTC), 2013 IEEE 63rd; 01/2013
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    ABSTRACT: Laser pulses with duration much shorter than the effective carrier lifetime cause permanent photoluminescence (PL) quenching and enhancement of PL decay rate in bare-faceted and capped AlGaN epilayers and multiple quantum wells at pulse energies about an order of magnitude lower than those causing the surface to melt and degrade. In contrast, GaN epilayers exhibit no photomodification in the same excitation intensity range. PL spectra and decay kinetics show that lattice heating is not responsible for the observed changes in AlGaN, which result from the formation of nonradiative recombination centers via recombination-enhanced defect reactions occurring at high nonequilibrium carrier densities.
    Journal of Applied Physics 01/2013; 114(20):203103-203103-5. · 2.21 Impact Factor
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    ABSTRACT: Optical and electronic properties of mid-ultraviolet light emitting diodes with a two-step p-AlGaN region are measured at temperatures from 8 to 300 K. While the electroluminescence intensity increases down to a temperature lower than that reported in InGaN light emitting diodes (LEDs), there is still a collapse at cryogenic temperatures. Capacitance-voltage measurements across the same range of temperatures reveal an increasing depletion region with decreasing temperature indicating that hole injection is a significant factor in the temperature dependent behavior, as well as the cause of electroluminescence collapse.
    Applied Physics Letters 12/2012; 101(25). · 3.79 Impact Factor
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    ABSTRACT: We discuss physics, design, fabrication, performance, and selected applications of Deep Ultraviolet Light Emitting Diodes (DUV LEDs). Our analysis reveals the relative contributions of electrical injection, internal quantum efficiency, and light extraction efficiency to the overall DUV LED performance. Our calculations show that the reduction of the dislocation density at least below value of 2×108 1/cm3 is necessary for reaching high DUV LED efficiency. Better light extraction has been achieved using an innovative p-type transparent sub-contact layer and reflecting ohmic p-type contact resulting in nearly tripling DUV LED power. At high power dissipation, temperature rise might be significant, and we present data showing the power degradation with temperature increase and the results of the detailed 1D and 3D analysis of thermal impedance of DUV LEDs. As an example of DUV LED application, we report on microbial disinfection using 19 watt 275 nanometer DUV LED.
    International Journal of High Speed Electronics and Systems 12/2012; 21(01).
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    ABSTRACT: The emission efficiency droop and internal quantum efficiency (IQE) in AlGaN epilayers and heterostructures were investigated by studying photoluminescence intensity dependence on excitation power density at different temperatures in the range from 8 to 300 K in three AlGaN samples with similar Al content (33%-35%) and different strength of carrier localization: an epilayer and multiple quantum wells with well widths of 5.0 and 2.5 nm. It is shown that the phenomena leading to the efficiency droop strongly influence the photoluminescence intensity dependence on temperature and, therefore, affect the estimation of IQE based on this dependence. A procedure to optimize the determination of IQE is proposed.
    Applied Physics Letters 11/2012; 101(21). · 3.79 Impact Factor
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    ABSTRACT: Time-resolved photoreflectance (PR) in AlInN/GaN heterostructures was applied to study carrier dynamics at energies extending from the uniform AlInN alloy band gap to the band gap of GaN. PR at the AlInN band gap has been found to have subpicosecond decay. Such ultrafast carrier relaxation from the extended to the sub-band edge states implies that the localization sites are small and dense, most probably originating from the In-rich clusters. At energies below the AlInN band gap, a complicated energy dependence of the PR signal is attributed to the properties of the localized states and to the modulation of the interface electric field by photoexcitation.
    AIP Advances. 11/2012; 2(4).
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    ABSTRACT: The photoluminescence droop effect, i.e., the decrease in emission efficiency with increasing excitation intensity, is observed and studied in GaN epilayers with different carrier lifetimes. Spontaneous and stimulated emissions have been studied in the front-face and edge emission configurations. The onset of stimulated recombination occurs simultaneously with the droop onset in the front-face configuration and might be considered as an origin of the droop effect in GaN epilayers.
    Optics Express 11/2012; 20(23):25195-200. · 3.55 Impact Factor
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    ABSTRACT: Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20 mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates.
    Applied Physics Express 08/2012; 5(8):2101-. · 2.73 Impact Factor

Publication Stats

3k Citations
562.08 Total Impact Points


  • 2009
    • KTH Royal Institute of Technology
      • Department of Microelectronics and Applied Physics (MAP)
      Stockholm, Stockholm, Sweden
    • Florida International University
      • Department of Electrical and Computer Engineering
      Miami, Florida, United States
  • 2000–2009
    • University of South Carolina
      • Department of Electrical Engineering
      Columbia, SC, United States
    • Vilnius University
      • Radiofizikos katedra
      Vilnius, Vilniaus Apskritis, Lithuania
    • Rensselaer Polytechnic Institute
      • Department of Electrical, Computer, and Systems Engineering
      Troy, NY, United States
  • 2007
    • Columbia Power Technologies, Inc.
      Corvallis, Oregon, United States
  • 2005
    • Synopsys
      Mountain View, California, United States
  • 1997–1999
    • University of Toronto
      • Department of Electrical and Computer Engineering
      Toronto, Ontario, Canada
  • 1998
    • Texas Tech University
      • Department of Electrical and Computer Engineering
      Lubbock, TX, United States