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See detailSensitivity enhancement of metal-semiconductor-metal photodetectors on low-temperature-grown GaAs using alloyed contacts
Mikulics, M. UL; Marso, Michel UL; Wu, S. et al

in IEEE Photonics Technology Letters (2008), 20(12), 1054-1056

We have fabricated and characterized metal–semiconductor–metal (MSM) photodetectors based on lowtemperature-grown GaAs with alloyed (i.e., ohmic-type) contacts. The annealed contacts optimize the electric ... [more ▼]

We have fabricated and characterized metal–semiconductor–metal (MSM) photodetectors based on lowtemperature-grown GaAs with alloyed (i.e., ohmic-type) contacts. The annealed contacts optimize the electric field distribution inside the photodetector structure which results in an up-to-200% responsivity increase of the devices, compared to conventional MSM detectors with standard nonalloyed (Schottky-type) metallization fabricated on identical material. The improved MSM device with alloyed contacts shows more than three times larger output amplitude at illumination with a 100-fs Ti : sapphire laser, compared to the nonalloyed devices, without degradation of detector speed. [less ▲]

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See detailComparative study on unpassivated and passivated AlGaN/GaN HFETs and MOSHFETs
Heidelberger, G.; Bernát, J.; Gregušová, D. et al

in Physica Status Solidi A. Applications and Materials Science (2006), 203(7), 1876-1881

In this comparative study we investigate AlGaN/GaN-based unpassivated and passivated HFETs and MOSHFETs with regards to DC-, RF-, and power-performance. For optimal comparability, all devices emanate from ... [more ▼]

In this comparative study we investigate AlGaN/GaN-based unpassivated and passivated HFETs and MOSHFETs with regards to DC-, RF-, and power-performance. For optimal comparability, all devices emanate from the same wafer consisting of a SiC-substrate, a 3 μm GaN- and a 30 nm Al0.28Ga0.72N-layer. Devices are processed simultaneously to a large extend. Passivated devices are coated with a 10 nm thick SiO2-layer between the electrodes, MOSHFETs contain a 10 nm thick SiO2-layer serving as gate-insulator underneath the gate and as conventional passivation-layer between the electrodes. Unpassivated devices serve as reference. We present empirical evidence that MOSHFETs outperform both the conventional and the passivated HFETs with respect to DC-, RF-, and power-performance, and we point out the different mechanisms responsible for the behaviour of the devices. [less ▲]

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See detailComparison of AlGaN/GaN MSM Varactor Diodes Based on HFET and MOSHFET Layer Structures
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in IEEE Electron Device Letters (2006), 27

In this letter, the performance of AlGaN/GaN-based metal–semiconductor–metal (MSM) varactor diodes based on HFET and MOSHFET layer systems is investigated. Passivated HFET MSM devices are coated with a 10 ... [more ▼]

In this letter, the performance of AlGaN/GaN-based metal–semiconductor–metal (MSM) varactor diodes based on HFET and MOSHFET layer systems is investigated. Passivated HFET MSM devices are coated with a 10-nm-thick SiO2 layer between the electrodes; in MOSHFET-based diodes, this layer is also used as an insulator underneath the gate. Device fabrication uses standard HFET fabrication technology, allowing easy integration in monolithic microwave integrated circuits. Devices with different electrode geometry are characterized by direct current and by S-parameter measurements up to 50 GHz. The HFET-based varactors show capacitance ratios up to 14 and cutoff frequencies up to 74 GHz. The MOSHFET-based devices, on the other hand, show lower capacitance ratios and poorer stability because of the insulation layer between electrodes and semiconductor. [less ▲]

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See detailComparison of AlGaN/GaN MSM varactor diodes based on HFET and MOSHFET layer structures
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in Proc. 6th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’06 (2006)

In this comparative study we investigate the performance of AlGaN/GaN based MSM varactor diodes based on HFET and MOSHFET layer systems. Device fabrication uses standard HFET fabrication technology ... [more ▼]

In this comparative study we investigate the performance of AlGaN/GaN based MSM varactor diodes based on HFET and MOSHFET layer systems. Device fabrication uses standard HFET fabrication technology, allowing easy integration in MMICs. Devices with different electrode geometries are characterized by DC and by S-parameter measurements up to 50 GHz. The HFET based varactors show capacitance ratios up to 14. The MOSHFET based devices, on the other hand, exhibit lower capacitance ratios and poorer stability because of theinsulation layer between electrodes and semiconductor. [less ▲]

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See detailTechnology related issues regarding fabrication of AlGaN/GaN-based MOSHFETs with GdScO3 as dielectric,
Heidelberger, G.; Roeckerath, M.; Steins, R. et al

in Proc. 6th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’06 (2006)

Starting out from our well established process for AlGaN/GaN HFETs we discuss ways to enrich the process in order to fabricate Metal-Oxide-Semiconductor HFETs (MOSHFETs) with a Gadolinium Scandate (GdScO3 ... [more ▼]

Starting out from our well established process for AlGaN/GaN HFETs we discuss ways to enrich the process in order to fabricate Metal-Oxide-Semiconductor HFETs (MOSHFETs) with a Gadolinium Scandate (GdScO3) insulation layer. In particular, adequate processing orders, various etching procedures and possible drawbacks of the GdScO3 deposition process on ohmic contacts are discussed. Making use of the gained knowledge we fabricated GdScO3-MOSHFETs for the first time. Compared to a conventional HFET the new device shows a higher saturation drain current and a lower gate leakage current. Nevertheless, the potential insulating properties of GdScO3 are not fully exploited yet and further optimization of the deposition process is needed. [less ▲]

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See detailRF characterization and modeling of AlGaN/GaN based HFETs and MOSHFETs
Fox, A.; Marso, Michel UL; Heidelberger, G. et al

in Proc. 6th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’06 (2006) (2006)

An increased RF-Performance of heterojunction field-effect transistor was found to be due to passivation and in addition a SiO2 insulation underneath the gate metallization. This leads to an increase of ... [more ▼]

An increased RF-Performance of heterojunction field-effect transistor was found to be due to passivation and in addition a SiO2 insulation underneath the gate metallization. This leads to an increase of cutoff frequency from 17 GHz up to 24 GHz for devices with 500 nm gate length. The RF output power increased from 4.1 to 6.7 W/mm at 7 GHz. RF simulation based on measured S-parameter showed a decrease of gate-source-capacitance and transconductance for devices with a dielectric layer underneath the gate metallization. The increase of the ratio gm/Cgs of about 25% is in agreement with the measured cutoff frequency ft. [less ▲]

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See detailAn AlGaN/GaN two-color photodetector based on AlGaN/GaN/SiC HEMT layer structure
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2006), 3(6), 2261-2264

The optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two-dimensional electron gas acts as a ... [more ▼]

The optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two-dimensional electron gas acts as a barrier for the depletion layer. Therefore, only the upper AlGaN barrier layer contributes to the photocurrent. In the high voltage regime the depletion region penetrates the GaN buffer that adds the spectral responsivity of GaN to the pure AlGaN behaviour in the low voltage regime. The ratio of the responsivities at 350 nm and 300 nm wavelength can be switched from 0.01 at 2.5 V to 0.8 at 4 V bias. This property makes the MSM-2DEG a candidate for use as two-color photodetector. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. [less ▲]

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See detailSiO2 /AlGaN/GaN MOSHFET with 0.7 µm gate-length and fmax / fT of 40/24 GHz
Bernát, J.; Gregusová, D.; Heidelberger, G. et al

in Electronics Letters (2005), 41(11), 667-668

The performance of SiO2/AlGaN/GaN MOSHFETs is described. The C–V measurements showed slight increase in sheet carrier density after applying 12 nm-thick PECVD SiO2. The devices exhibited gate leakage ... [more ▼]

The performance of SiO2/AlGaN/GaN MOSHFETs is described. The C–V measurements showed slight increase in sheet carrier density after applying 12 nm-thick PECVD SiO2. The devices exhibited gate leakage current of 5 .10 ^-10 A/mm. Small-signal RF characterisation of 0.7 mm gate length devices yielded an fT of 24 GHz and an fmax of 40 GHz, which are comparable to those typical for state-of-the-art AlGaN/GaN HFETs. [less ▲]

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See detailHigh-power SiO2/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors
Kordoš, P.; Heidelberger, G.; Bernát, J. et al

in Applied Physics Letters (2005), 87(14), 143501-143504

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure ... [more ▼]

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure field-effect transistors HFETs were also investigated for comparison. Deposited 12 nm thick SiO2 yielded an increase of the sheet carrier density from 7.6 .10^12 to 9.2 .10^12 cm−2 and a subsequent increase of the static drain saturation current from 0.75 to 1.09 A/mm. The small-signal rf characterization of the MOSHFETs showed an extrinsic current gain cutoff frequency fT of 24 GHz and a maximum frequency of oscillation fmax of 40 GHz. The output power of 6.7 W/mm of the MOSHFETs measured at 7 GHz is about two times larger than that of HFETs. The results obtained demonstrate the suitability of GaN-based MOSHFETs for high-power electronics. [less ▲]

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See detailInfluence of carrier supply doping on the RF properties of AlGaN/GaN/SiC high-electron-mobility transistors
Marso, Michel UL; Bernát, J.; Javorka, P. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2005), 2(7), 2611-2614

We have fabricated undoped as well as modulation-doped AlGaN/GaN HEMTs on SiC substrate. The influence of the carrier supply layer on device performance is investigated by Hall, channel conductivity ... [more ▼]

We have fabricated undoped as well as modulation-doped AlGaN/GaN HEMTs on SiC substrate. The influence of the carrier supply layer on device performance is investigated by Hall, channel conductivity, small signal RF and delay time evaluation. While the doping layer improves the DC performance it degrades the RF behaviour of the device with the highest carrier supply doping of 5x1018 cm–3. The channel conductivity measurements show identical dependence of the mobility on carrier concentration for all samples. The saturation velocity, extracted by evaluation of the total delay time as function of the inverse drain current, decreases from 0.86x107 cm/s for the undoped device to 0.7x107 cm/s for the highest doped HEMT. This result shows that the degradation of RF performance is due to the reduction of the effective saturation velocity caused by the carrier supply layer. [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 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 detailInfluence of layer structure on performance of AlGaN/GaN High Electron Mobility Transistors before and after passivation
Bernát, J.; Javorka, P.; Fox, A. et al

in Journal of Electronic Materials (2004), 33((2004)), 436-439

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See detailDC and Pulsed Behaviour of Undoped and Doped AlGaN/GaN/SiC HEMTs before and after Si3N4 passivation
Bernát, J.; Marso, Michel UL; Fox, A. et al

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

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See detailGeneration of 1 THz radiation by photomixing in low-temperature-grown MBE GaAs
Mikulics, M.; Camara, I.; Marso, Michel UL et al

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

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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 detailInfluence of doping density on small and large signal characteristics of AlGaN/GaN/SiC HEMTs
Fox, A.; Marso, Michel UL; Bernát, J. et al

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

The small signal S-parameter and load pull measurements on AlGaN/GaN HEMTs with different doping densities are reported. The cut-off frequencies ft and fmax of 38 and 67 GHz respectively, and the maximal ... [more ▼]

The small signal S-parameter and load pull measurements on AlGaN/GaN HEMTs with different doping densities are reported. The cut-off frequencies ft and fmax of 38 and 67 GHz respectively, and the maximal output powers of 4.9 and 3.8 W/mm at 2 and 7 GHz respectively, were achieved. The parameter extraction from small signal S-parameters shows an increase of gate-source capacitance and transconductance with the doping concentration and hence its influence on small signal (ft and fmax) and power performance. [less ▲]

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See detailEffect of surface passivation on performance of AlGaN/GaN/Si HEMTs
Bernát, J.; Javorka, P.; Fox, A. et al

in Solid-State Electronics (2003), 47((2003)), 2097-2103

Performance of intentionally undoped and doped AlGaN/GaN/Si high electron mobility transistors (HEMTs) before and after passivation with SiO2 and Si3N4 is investigated. Hall effect measurements show ... [more ▼]

Performance of intentionally undoped and doped AlGaN/GaN/Si high electron mobility transistors (HEMTs) before and after passivation with SiO2 and Si3N4 is investigated. Hall effect measurements show higher impact of Si3N4 than SiO2 passivation on the carrier concentration increase in the channel. Improvements in DC performance of HEMTs after passivation with SiO2 and Si3N4 correspond to the changes in sheet carrier concentration. Small signal microwave characterisation shows a decrease (from 18.6 to 9 GHz) and an increase (from 18.4 to 28.8 GHz) of the current gain cut off frequency after SiO2 and Si3N4 passivation, respectively. Similar effect of passivation is found in microwave power changes––only about a half of the power is obtained after SiO2 passivation but more than doubled power results from Si3N4 passivation, measured at 2 GHz. Higher density of interface states for SiO2 than Si3N4 passivation is supposed to be responsible for these effects. However, for an optimal design of GaN-based power devices additional studies related to the interface between a passivation layer and GaN are needed. [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 detailInfluence of SiO/sub 2/ and Si/sub 3/N/sub 4/ passivation on AlGaN/GaN/Si HEMT performance,
Javorka, P.; Bernát, J.; Fox, A. et al

in Electronics Letters (2003), 39((2003)), 1155-1157

Different influence of SiO2 and Si3N4 passivation on performance of AlGaN=GaN=Si HEMTs is reported. DC characteristics are less enhanced by using SiO2 than Si3N4. This is in agreement with carrier ... [more ▼]

Different influence of SiO2 and Si3N4 passivation on performance of AlGaN=GaN=Si HEMTs is reported. DC characteristics are less enhanced by using SiO2 than Si3N4. This is in agreement with carrier concentration changes after passivation, as follows from Hall data. Small signal RF performance is degraded after applying SiO2 and enhanced after Si3N4 passivation, e.g. for unpassivated devices fTffi17 GHz which decreased to 9 GHz and increased to 28 GHz for SiO2 and Si3N4 respectively. The fmax=fT ratio has not changed after passivation. [less ▲]

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