References of "Wolter, M"
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See detailPhotomixers fabricated on nitrogen-ion-implanted GaAs
Mikulics, M.; Marso, Michel UL; Cámara Mayorga, I. et al

in Applied Physics Letters (2005), 87(4), 41106-1-3

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion ... [more ▼]

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion concentration of 3 1012 cm−2. The resulting material exhibited 110 fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350 nm and 950 nm. In comparison to their counterparts photomixers fabricated on low-temperature-grown GaAs the N+-implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100 mA/W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N+-implanted GaAs the material of choice for efficient optoelectronic photomixers. [less ▲]

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See detailA novel two-color photodetector based on an InAlAs-InGaAs HEMT layer structure
Marso, Michel UL; Wolter, M.; Kordoš, P.

in IEEE Photonics Technology Letters (2004), 16 (2004)(11), 2541-2543

The spectral responsivity of an InAlAs–InGaAs metal–semiconductor–metal diode above a two-dimensional electron gas (2DEG) is investigated as a function of the applied bias. At low voltages, only the ... [more ▼]

The spectral responsivity of an InAlAs–InGaAs metal–semiconductor–metal diode above a two-dimensional electron gas (2DEG) is investigated as a function of the applied bias. At low voltages, only the InAlAs layer above the 2DEG contributes to the photocurrent, while the InGaAs channel layer is activated at higher bias. This results in a voltage-dependent spectral response of the photodetector. The ratio of the responsivities at 1300 and 850 nm changes from 0.03- at 1-V to 0.44- at 1.6-V bias. This property makes the device a candidate suitable to detect and to separate optical information originated both from the GaAs (850 nm) and in the InGaAs (1300, 1550 nm)-based optoelectronic technology. [less ▲]

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See detailPerformance of unpassivated AlGaN/GaN/SiC HEMTs after short-term electrical bias stress
Bernát, J.; Wolter, M.; Javorka, P. et al

in Solid-State Electronics (2004), 48((2004)), 1825-1828

Performance of unpassivated MOVPE grown AlGaN/GaN/SiC HEMTs under short-term electrical bias stress isreported. Insignificant degradation in DC and RF behaviour of prepared devices after 16 h stress at VD ... [more ▼]

Performance of unpassivated MOVPE grown AlGaN/GaN/SiC HEMTs under short-term electrical bias stress isreported. Insignificant degradation in DC and RF behaviour of prepared devices after 16 h stress at VD ¼ 21 V and VG ¼ 2:5 V is observed. This can be demonstrated by a maximum drain current of 1.03 and 1.01 A/mm and a peak extrinsic transconductance of 233 and 225 mS/mm before and after bias stress, respectively. Consecutive drain voltage sweeps indicates on low current collapse in samples investigated. The RF characteristics show a decrease of the fmax-tof T ratio from 2.23 to 2.15 and the saturated output power density measured at 2 GHz from 3.60 to 3.53 W/mm after the devices were stressed. This result indicates that reliable AlGaN/GaN/SiC HEMTs without passivation can be prepared. However, additional studies are needed to understand if the material structure and/or the device processing is responsible for low current collapse and RF dispersion in these devices. [less ▲]

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See detailMSM varactor diodes based on AlGaN/GaN/SiC HEMT layer structures
Marso, Michel UL; Bernát, J.; Javorka, P. et al

in Proc. 5th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’04 (2004)

The influence of a carrier supply layer on the electrical properties of MSM diodes based on AlGaN/GaN HEMT layer structures is investigated. The voltage dependence of the Schottky contact capacitance ... [more ▼]

The influence of a carrier supply layer on the electrical properties of MSM diodes based on AlGaN/GaN HEMT layer structures is investigated. The voltage dependence of the Schottky contact capacitance allows the use of the device as varactor diode with CMAX / CMIN ratios up to 17, tuneable by contact geometry. The carrier supply doping determines the sheet carrier density of the twodimensional electron gas channel, which is responsible for the transition voltage and the series resistance of the MSM diode. A device based on the highest doped layer structure shows a cutoff frequency as high as 65 GHz. [less ▲]

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See detailImpact of layer structure on performance of unpassivated AlGaN/GaN/SiC HEMTs,
Bernát, J.; Wolter, M.; Fox, A. et al

in Electronics Letters (2004), 40(1), 78-80

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 ... [more ▼]

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 IDS of 0.86 and 1.33 A=mm, gm of 220 and 273 mS=mm, fT of 33 and 43 GHz and fmax of 54 and 61 GHz for 0.3 mm 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. [less ▲]

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See detailInvestigation of buffer traps in an AlGaN/GaN/Si high electron mobility transistor by backgating current deep level transient spectroscopy
Marso, Michel UL; Wolter, M.; Javorka, P. et al

in Applied Physics Letters (2003), 82

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between ... [more ▼]

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 30 to 100 °C. With this method, known as backgating current deep level transient spectroscopy, majority carrier traps with activation energy of 200 meV as well as minority carrier traps at 370 meV are identified. The experiments are performed on completed HEMTs, allowing the investigation of the influence of device fabrication technology. [less ▲]

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See detailMaterial and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Proceedings of the MRS Fall Meeting, Boston 2002 (2003)

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed ... [more ▼]

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm-2 and μ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3−0.7 μm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ~16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 μm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices. [less ▲]

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See detailInvestigation of traps in AlGaN/GaN HEMTs on silicon substrate
Wolter, M.; Marso, Michel UL; P.Javorka, J. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2003), (7), 2360-2363

Deep levels in AlGaN/GaN heterostructures are known to be responsible for trapping processes like current collapse or dispersion. In order to investigate these processes we performed different ... [more ▼]

Deep levels in AlGaN/GaN heterostructures are known to be responsible for trapping processes like current collapse or dispersion. In order to investigate these processes we performed different measurements on HEMTs fabricated with heterostructures grown on silicon substrate. First by photoionization spectroscopy we found three different traps with activation energies of about 2.1 eV, 2.9 eV and 3.2 eV. Secondly, the temperature dependent relaxation of the drain current was investigated by the backgating current deep level transient spectroscopy (DLTS) method. Hereby we detected majority and minority carrier traps in the GaN buffer at the energies EV + 0.41 eV and EC – 0.55 eV, respectively. The latter energy can be attributed to the well known “E2” level in GaN. [less ▲]

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See detailMaterial and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Microelectronics Journal (2003), 34

Selected material and device issues related to the performance of AlGaN/GaN HEMTs on (111) Si substrates are reported. It is shown that these devices can sustain significantly higher dc power (16 W/mm ... [more ▼]

Selected material and device issues related to the performance of AlGaN/GaN HEMTs on (111) Si substrates are reported. It is shown that these devices can sustain significantly higher dc power (16 W/mm) than those grown on sapphire. Consequently smaller degradation in the device performance at higher temperatures (up to 400 8C) is demonstrated. Photoionisation spectroscopy reveals trap level of 1.85 eV, additional to two another levels found before in GaN-based HEMTs prepared on sapphire. Thus, AlGaN/GaN HEMTs on Si substrates demonstrate the viability of this technology for commercial application of high power rf devices. [less ▲]

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See detailInfluence of doping concentration on DC and RF performance of AlGaN/GaN HEMTs on silicon substrate
Marso, Michel UL; Javorka, P.; Dikme, Y. et al

in Physica Status Solidi A. Applications and Materials Science (2003), (1), 179-182

AlGaN/GaN HEMTs with different carrier supply layers are fabricated on Si substrate, using MOVPE growth technique. The influence of the carrier supply doping concentration on the HEMT properties is ... [more ▼]

AlGaN/GaN HEMTs with different carrier supply layers are fabricated on Si substrate, using MOVPE growth technique. The influence of the carrier supply doping concentration on the HEMT properties is investigated by Hall measurements and by electrical DC and RF characterisation of transistor devices. Hall mobility is found to decrease with increasing sheet concentration, while the gate leakage current increases. The device with the highest carrier supply doping concentration of 1019 cm–3 is provided with a GaN cap layer to reduce gate leakage. The transistors based on modulation doped layer structures show much higher DC performance than the undoped device. Best RF properties are obtained for a doping level of 5 × 1018 cm–3. High frequency measurements exhibit fT and fMAX values of 35 GHz and 37 GHz, respectively, for devices with 300 nm gate length. [less ▲]

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See detailFabrication and performance of AlGaN/GaN HEMTs on (111) Si substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Physica Status Solidi A. Applications and Materials Science (2002), 194(2), 472-475

In the current work the performance of AlGaN/GaN HEMTs fabricated on silicon substrates is presented. The AlGaN/GaN material structures were grown on (111) Si by MOVPE. Static I–V characteristics with a ... [more ▼]

In the current work the performance of AlGaN/GaN HEMTs fabricated on silicon substrates is presented. The AlGaN/GaN material structures were grown on (111) Si by MOVPE. Static I–V characteristics with a saturation current of 0.91 A/mm and a peak extrinsic transconductance of 122 mS/mm were measured and show minimal thermal effects. For devices with a gate length of 0.7 um and 0.5 um, a unity gain frequency of 20 GHz and 32 GHz and a maximum frequency of oscillation of 22 GHz and 27 GHz, respectively were obtained. The unity gain frequencies are the highest values reported so far on AlGaN/GaN/Si HEMTs and fully comparable to those known for devices using sapphire and SiC substrates. However, the fmax to fT ratio is only about 1, which indicates on parasitic conduction through the Si substrate under small signal conditions. It is shown that the saturation current and the transconductance decrease much less with increased temperature than known for similar devices grown on sapphire. [less ▲]

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See detailAlGaN/GaN HEMTs on (111) Silicon Substrates
Javorka, P.; Alam, A.; Wolter, M. et al

in IEEE Electron Device Letters (2002), 23(2002), 4-6

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE ... [more ▼]

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE. Devices exhibit a saturation current of 0.91 A/mm, a good pinchoff and a peak extrinsic transconductance of 122 mS/mm. A unity current gain frequency of 12.5 GHz and fmax/fT=0.83 were obtained. The highest saturation current reported so far, static output characteristics of up to 20 V and breakdown voltage at pinchoff higher than 40 V demonstrate that the devices are capable of handling 16 W/mm static heat dissipation. [less ▲]

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See detailInvestigation of current collapse in doped and undoped AlGaN/GaN HEMTs
Wolter, M.; Javorka, P.; Marso, Michel UL et al

in Proc. 4th Intern. Conf. Advanced Semicon. Dev. & Microsystems (2002)

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See detailMaterial and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Javorka, P.; Alam, A.; Marso, Michel UL et al

in Mat. Res. Soc. Symp. Proc. Vol. 743 (2002), 743, L9.1.1

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed ... [more ▼]

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm-2 and μ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3−0.7 μm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ~16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 μm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices. [less ▲]

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See detailMSM Diodes Based on an AlGaN/GaN HEMT Layer Structure for Varactor and Photodiode Application
Marso, Michel UL; Bernát, J.; Wolter, M. et al

in , Proc. 4th Intern. Conf. Advanced Semicon. Dev. & Microsystems (2002)

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See detailInvestigation of AlGaN/GaN HEMTs on Si substrate using backgating
Marso, Michel UL; Wolter, M.; Javorka, P. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2002), (1), 65-68

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN HEMT on silicon (111) substrate is investigated. This effect, known as backgating, is used to study traps that are located ... [more ▼]

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN HEMT on silicon (111) substrate is investigated. This effect, known as backgating, is used to study traps that are located between substrate and 2DEG channel. The transient of the drain current after applying a negative substrate voltage is evaluated for measurements with and without illumination. Several trap contributions are resolved by measurements at different photon energies. A photocurrent is observed up to 600 nm wavelength. Up to this wavelength the backgating effect can be compensated and the drain current restored by a short light pulse. The experiments are performed on completed HEMTs, allowing investigation of the influence of device fabrication technology. [less ▲]

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See detailPhotoionization spectroscopy of traps in doped and undoped AlGaN/GaN HEMTs
Wolter, M.; Javorka, P.; Marso, Michel UL et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2002), (1), 82-85

Deep-level defects and surface states are supposed to be responsible for the limitation of AlGaN/GaN high electron mobility transistor (HEMT) performance. In order to investigate the influence of these ... [more ▼]

Deep-level defects and surface states are supposed to be responsible for the limitation of AlGaN/GaN high electron mobility transistor (HEMT) performance. In order to investigate the influence of these traps, photoionization spectroscopy was used to study doped and undoped HEMTs grown on sapphire in different metalorganic vapour-phase epitaxy reactors. This measurement technique is based on the optical reversion of the current collapse and it allows one to determine photoionization cross-sections of the participating traps. For doped and undoped HEMTs nearly the same two defect levels with excitation energies of 3.2 eV and 2.9 eV were determined. By varying the source–gate voltage it was found that the photoionization cross-section is reduced for positive gate bias, i.e. the virtual gate on the gate–drain access region is partially neutralized due to the removal of trapped electrons from surface states. [less ▲]

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See detailPhotoionization spectroscopy of traps in AlGaN/GaN HEMTs
Wolter, M.; Javorka, P.; Marso, Michel UL et al

in Journal of Electronic Materials (2002), 31(12), 1321-1324

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See detailInvestigations on the influence of traps in AlGaN/GaN HEMTs
Wolter, M.; Javorka, P.; Marso, Michel UL et al

in EDMO (2001)

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See detailOptimization of AlGaN/GaN HEMT performance
Javorka, P.; Wolter, M.; Alam, A. et al

in Proc. EDMO (2001)

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