G.R. Brandes

Slovak Academy of Sciences, Bratislava, Bratislavsky Kraj, Slovakia

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Publications (30)37.99 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 μm gate-length and 100 μm gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak gm of 92 mS/mm. The self-aligned devices exhibit an extrinsic fT of 39 GHz and an fMAX of 130 GHz, whereas non-self-aligned HEMTs show an fT of 15 GHz and an fMAX of 35 GHz.
    International Journal of High Speed Electronics and Systems 11/2011; 14(03).
  • Junghui Song, Wu Lu, Jeffrey S. Flynn, George R. Brandes
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    ABSTRACT: Schottky diodes on AlGaN/GaN heterostructures with Pt catalytic metal are fabricated and characterized from 200 to 800 °C for H2 sensing. Over this large range of temperature, the forward current of Schottky diodes increases with exposure to H2 gas, which is attributed to the reduction of Schottky barrier heights resulting from hydrogen absorption in the catalytic metal. The results indicate that AlGaN/GaN heterostructure Schottky diodes are capable of high-temperature operation for H2 sensing up to 800 °C. As temperature increases, the hydrogen detection sensitivity of Pt–AlGaN/GaN Schottky diodes improves due to the more effective H2 dissociation. When the testing ambient is changed from N2 to 5% H2/95% N2, the value of the Schottky barrier height decreases by 11 and 120 meV at 200 and 800 °C, respectively.
    Applied Physics Letters 09/2005; 87(13):133501-133501-3. · 3.52 Impact Factor
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    ABSTRACT: We discuss the growth and characterization of homoepitaxial GaN layers and AlGaN/GaN high electron mobility transistor (HEMT) structures grown by plasma-assisted molecular beam epitaxy (MBE) on freestanding n-GaN substrates. The GaN substrates were fabricated by hydride vapor phase epitaxy and exhibit low dislocation densities of ∼107 cm−2. The best MBE-grown homoepitaxial epilayers on these substrates were grown in the gallium droplet regime. Root-mean-square roughnesses of these layers were 3.5–4.0 Å over 5×5 μm2 regions. AlGaN/GaN HEMT structures were grown on these substrates and exhibit room-temperature Hall mobilities of 1920 cm2/V s at an electron sheet density of 0.9×1013 cm−2. Electrical isolation of the two-dimensional electron gas from the conductive substrate was accomplished using a Be:GaN buffer. HEMT devices were photolithographically defined and DC and RF device characteristics were measured. Off-state breakdown voltages of 90 V, saturated drain currents of nearly 700 mA/mm, and gate leakage currents of 0.07 mA/mm were observed on unpassivated devices. Preliminary results on RF performance and device reliability are presented and discussed.
    Journal of Crystal Growth 07/2005; · 1.55 Impact Factor
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    ABSTRACT: We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n -GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 Å over a 20×20 μ m <sup>2</sup> area. High resolution x-ray diffractometry indicates that the AlGaN peak is ∼20 % narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60×60 μ m <sup>2</sup> Hall test patterns defined photolithographically across the surface of the 10×10 mm <sup>2</sup> sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380 Ω/ ◻ and 0.94×10<sup>13</sup> cm <sup>-2</sup> , respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900 cm <sup>2</sup>/ V s . In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2005; · 1.36 Impact Factor
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    ABSTRACT: Subpicosecond time-resolved photoluminescence (TRPL) has been used to compare the room temperature carrier dynamics in Al0.1Ga0.9N/Al0.3Ga0.7N multiple quantum well (MQW) structures simultaneously deposited on a high quality free standing HVPE GaN substrate (dislocation density 1 × 107cm-2) and 1 mum MOCVD GaN template on sapphire. The PL lifetime of 500 ps in the MQW on GaN substrate is about 5 times longer than that for the MQW on GaN template, with a concomitant increase in CW PL intensity. This behavior is attributed primarily to an increase in nonradiative lifetime associated with a 100 times reduction in dislocation density in the GaN substrate. The observation that the PL lifetime in the MQW falls short of the 900 ps dominant decay time in the GaN substrate may be indicative of generation of additional defects and dislocations due to substrate surface preparation, strain relaxation, and nonoptimal growth temperature associated with the difference in heating of the thin GaN template on sapphire and the thick GaN substrate. An extended PL rise time of greater than 20 ps for the MQW emission when above barrier pumping is employed implies that both wells and barriers are of high quality.
    physica status solidi (c) 05/2005; 2(7):2332-2336.
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    ABSTRACT: The performance of unpassivated GaN/AlGaN/GaN/SiC HEMTs with intentionally undoped and doped barrier structures are extensively studied. No drain current dispersion between DC and 50 ns pulses on doped devices and less than 10% dispersion on undoped ones is observed. The full (100%) current recovery on undoped sample was measured in 1 µs. The drain current extrapolated from 2 GHz large signal measurements corresponds to the measured static drain current confirming negligible current dispersion of our devices. Insignificant (<5%) degradation in overall device performance parameters (IDs, gm, fT, fmax, Pout) on both undoped and doped structures after 12-hour-long bias stress was obtained. These results documents that suitable device performances can be obtained also on unpassivated GaN-based HEMTs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2005; 2(7):2676 - 2679.
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    Junghui Song, W. Lu, Jeffrey S. Flynn, George R. Brandes
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    ABSTRACT: Schottky diodes on AlGaN/GaN heterostructures with Pt, IrPt, and PdAg catalytic metals are fabricated and characterized from 200°C to 800°C for H2 sensing. Over this large range of temperature, the forward current of all the diodes increases with exposure to H2 gas, which is attributed to Schottky barrier height reduction caused by the atomic hydrogen absorption on the metal–oxide interface. The results indicate that AlGaN/GaN heterostructure Schottky diodes are capable of high-temperature H2 sensor operation up to 800°C. As temperature increases, the hydrogen detection sensitivity of Pt and IrPt diodes improves due to the more effective H2 dissociation. However, the sensitivity of PdAg diodes degrades with the increase of temperature due to thermal instability of PdAg. At a range of temperature from 200°C to 300°C, PdAg diodes exhibit significant higher sensitivity compared with Pt and IrPt diodes. IrPt and Pt diodes show higher sensitivity at temperatures above 400°C.
    Solid-State Electronics 01/2005; 49(8):1330-1334. · 1.48 Impact Factor
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    ABSTRACT: Free-standing highly resistive Fe -doped GaN layers grown by hydride vapor phase epitaxy were characterized by temperature-dependent conductivity and Hall effect measurements. Samples with a room-temperature resistivity of 1.6×10<sup>7</sup>–6×10<sup>8</sup> Ω cm and a Hall mobility of ∼330 cm <sup>2</sup> V <sup>-1</sup> s <sup>-1</sup> showed simple band conduction with the mobility power x=-1.5 and an activation energy 0.58–0.60 eV , which can be attributed to a Fe acceptor. Samples with a lower mobility, ≤10 cm <sup>2</sup> V <sup>-1</sup> s <sup>-1</sup> , exhibited an increase of the mobility with temperature. Here, the conduction seems to be strongly influenced by potential barriers at inhomogeneities, with an activation energy of 0.21 eV and a barrier height of 0.14–0.18 eV . The activation energy 0.36 and 0.40 eV , evaluated from the resistivity measurements, does not correspond to that of the Fe acceptor.
    Applied Physics Letters 01/2005; · 3.52 Impact Factor
  • Junghui Song, J.S. Flynn, G.R. Brandes, Wu Lu
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    ABSTRACT: Schottky diodes on AlGaN/GaN heterostructures with Pt, IrPt, and PdAg catalytic metals are fabricated and characterized from 200°C to 800°C for H<sub>2</sub> sensing. Over this large range of temperatures, the forward current of all the diodes increases with exposure to H<sub>2</sub> gas, which is attributed to Schottky barrier height reduction caused by hydrogen absorption in the catalytic metals. The results indicate that AlGaN/GaN heterostructure Schottky diodes are capable of high temperature H<sub>2</sub> sensor operation up to 800°C. As temperature increases, the hydrogen detection sensitivity of Pt and IrPt diodes improves because H<sub>2</sub> dissociation is more effective. However, the sensitivity of PdAg diodes degrades with an increase of temperature due to thermal instability of PdAg. At a range of temperatures from 200°C to 300°C, PdAg diodes exhibit significant higher sensitivity compared with Pt and IrPt diodes. Above 400°C, IrPt and Pt diodes show higher sensitivity.
    Sensors, 2004. Proceedings of IEEE; 11/2004
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    ABSTRACT: An AlGaN/GaN high electron mobility transistor (HEMT) structure has been grown by plasma-assisted molecular beam epitaxy (MBE) on a free-standing hydride vapour phase epitaxy-grown GaN substrate with a threading dislocation density of ∼8×10<sup>6</sup> cm<sup>-2</sup>. A room temperature Hall mobility of 1920 cm<sup>2</sup>/V s with a sheet carrier density of 0.91×10<sup>13</sup> cm<sup>-2</sup> was measured. This is the highest room temperature electron mobility reported for an MBE-grown AlGaN/GaN structure. HEMTs fabricated on this material displayed excellent pinch-off, low gate leakage currents, and an off-state breakdown of 90 V.
    Electronics Letters 10/2004; · 1.04 Impact Factor
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    ABSTRACT: Self-aligned T-gate AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated on a sapphire substrate using a thin Ti/Al/Ti/Au ohmic layer. To suppress the gate leakage current, the ohmic contact annealing was performed in a furnace. The self-aligned HEMTs with 0.25 μm gate length and 100 μm width exhibit good pinch-off characteristics, a transconductance of 146 mS/mm, an extrinsic unity current gain cutoff frequency of 38 GHz and a maximum oscillation frequency of 130 GHz.
    Electronics Letters 10/2004; · 1.04 Impact Factor
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    ABSTRACT: High power microwave AlGaN-GaN high electron-mobility transistors (HEMTs) on free-standing GaN substrates are demonstrated for the first time. Measured gate leakage was -2.2 μA/mm at -20 V and -10 μA/mm at -45 V gate bias. When operated at a drain bias of 50 V, devices showed a record continuous-wave output power density of 9.4 W/mm at 10 GHz with an associated power-added efficiency of 40%. Long-term stability of device RF operation was also examined. Under room conditions, devices driven at 25 V and 3-dB gain compression remained stable in 200 h, degrading only by 0.18 dB in output power. Such results illustrate the potential of GaN substrate technology in supporting reliable, high performance AlGaN-GaN HEMTs for microwave power applications.
    IEEE Electron Device Letters 10/2004; · 2.79 Impact Factor
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    ABSTRACT: Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 μm gate-length and 100 μm gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak gm of 92 mS/mm. The self-aligned devices exhibit an extrinsic f<sub>T</sub> of 39 GHz and an f<sub>MAX</sub> of 130 GHz, whereas non-self-aligned HEMTs show an f<sub>T</sub> of 15 GHz and an f<sub>MAX</sub> of 35 GHz.
    High Performance Devices, 2004. Proceedings. IEEE Lester Eastman Conference on; 09/2004
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    ABSTRACT: The performance of unpassivated AlGaN/GaN/SiC HEMTs prepared on different MOVPE grown layer structures is reported. An overall improvement of device characteristics using doped structures in comparison to undoped counterpart is observed. This can be demonstrated by I<sub>DS</sub> of 0.86 and 1.33 A/mm, g<sub>m</sub> of 220 and 273 mS/mm, f<sub>T</sub> of 33 and 43 GHz and f<sub>max</sub> of 54 and 61 GHz for 0.3 μm gate length devices on undoped and doped structures, respectively. The DC/pulsed I-V characteristics as well as power measurements show insignificant RF dispersion of HEMTs on doped structures. These results underline the advantage of doped layer structures for preparation of high-performance AlGaN/GaN HEMTs.
    Electronics Letters 02/2004; · 1.04 Impact Factor
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    ABSTRACT: The electrical characteristics of delta doped HEMTs were measured using C–V, Lehighton sheet resistance and 300°K Hall mobility techniques. Spacer layer thickness and dopant concentration were found to have a significant impact on the electron mobility and electron gas carrier density in the 2DEG. The product μ-Ns of delta doped HEMTs can be optimized by varying the position of the delta doped layer and the dopant concentration to develop HEMT devices with high transconductance. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 11/2003;
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    ABSTRACT: The dislocation density in freestanding gallium nitride wafers was measured using atomic force microscope (AFM) imaging of chemical mechanically polished (CMP) samples and with transmission electron microscopy (TEM). Etch pits were introduced after chemical mechanical polishing of the gallium side of the GaN wafer and the pits easily imaged with the AFM. TEM was also utilized to measure the dislocation density. Good agreement was found between the dislocation density measured by TEM and the etch pit density measured by AFM, demonstrating that chemical mechanical polish of the GaN(0001) surface decorates threading dislocations and that the AFM technique provides a reasonably accurate and convenient measure of the dislocation density. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 11/2003;
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    ABSTRACT: In this study, quasi-enhancement mode AlGaN/GaN HEMT devices with the gate length of 1-μm are fabricated using a conventional method (i.e. without gate-recessing process) and the direct current and radio frequency characteristics of these devices are investigated. The threshold voltages are in the range of −0.3 to −0.5 V. The devices exhibit a maximum drain current of 280 mA/mm at a gate bias of 2 V. The pinch off voltage is about −1.0 V. At the gate bias of 1.5 V and the drain bias of 6 V, the devices exhibit an extrinsic transconductance of 140 mS/mm, a unity current gain cutoff frequency (fT) of 4.3 GHz, and a maximum oscillation frequency (fMAX) of 13.3 GHz, respectively.
    Solid-State Electronics 11/2003; 47(11):2081-2084. · 1.48 Impact Factor
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    ABSTRACT: Semi-insulating freestanding GaN substrates were produced by hydride vapor phase epitaxy using intentionally introduced iron impurity atoms to compensate residual donors in GaN. Variable temperature resistivity measurements determined the resistivity of an iron-doped GaN sample to be ∼3 × 105 Ω cm at 250 °C. The activation energy of the carrier was 0.51 eV and room temperature resistivity was determined to be ∼2 × 109 Ω cm at room temperature by linear fitting and extrapolation to room temperature. Near-infrared photoluminescence at 1.6 K exhibited sharp emission at 1.3 eV, associated with the 4T1(G) →6A1(S) internal transition of the Fe3+ charge state. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (a) 10/2003; 200(1):18 - 21. · 1.21 Impact Factor
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    ABSTRACT: The influence of annealed ohmic contact metals on the polarisation of the AlGaN barrier layer has been investigated by the Schottky contacts on the AlGaN/GaN HFET structure. The analysed result shows that annealed ohmic contact metals weaken the polarisation of the AlGaN barrier layer. When ohmic contact metals are close to Schottky contact metals, the weakened polarisation decreases the 2DEG sheet carrier concentration in the channel.
    Electronics Letters 10/2003; · 1.04 Impact Factor
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    ABSTRACT: Ir, Ni, and Re Schottky contacts on strained Al0.25Ga0.75N/GaN heterostructures are characterized using capacitance–voltage (C–V) and I–V techniques. Based on the measured C–V characteristics, two dimensional electron gas sheet carrier concentrations at the AlGaN/GaN interface and barrier heights of Ir, Ni, and Re Schottky contacts are calculated. The barrier heights of 1.12, 1.27, and 1.68 eV are obtained for Ir, Ni, and Re Schottky contacts, respectively. The results show that the barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures are strongly dependent on the metal work functions. However, contrary to Schottky contacts on bulk AlGaN or GaN, the barrier height on strained AlGaN/GaN heterostructures is lower for a Schottky contact with a higher metal work function. This is attributed to the stronger wave function coupling between electrons in the Schottky metal and surface donor electrons. The I–V characteristics for Ir, Ni, and Re Schottky contacts confirm the results obtained by C–V characteristics. © 2003 American Institute of Physics.
    Applied Physics Letters 06/2003; 82(24):4364-4366. · 3.52 Impact Factor

Publication Stats

311 Citations
37.99 Total Impact Points

Institutions

  • 2005
    • Slovak Academy of Sciences
      • Institute of Electrical Engineering
      Bratislava, Bratislavsky Kraj, Slovakia
  • 2003
    • The Ohio State University
      • Department of Electrical and Computer Engineering
      Columbus, OH, United States