Influence of carrier supply doping on the RF properties of AlGaN/GaN/SiC high-electron-mobility transistors

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 ▲]

MSM varactor diodes based on AlGaN/GaN/SiC HEMT layer structures

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 ▲]

Influence of a carrier supply layer on carrier density and drift mobility of AlGaN/GaN/SiC high-electron-mobility transistors

in Applied Physics Letters (2004), 84 (2004)

Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel ... [more ▼]

Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel conductivity measurements. Sheet carrier concentration and average mobility at 0 V gate bias correspond to standard Hall results. Sheet carrier density increases from 6.831012 cm22 for the undoped sample up to 131013 cm22 for the device with the highest doping concentration, while the mobility decreases from 1800 to 1620 cm2/V s. The local mobility, on the other hand, depends only on the actual sheet carrier density and is not influenced by the doping concentration of the carrier supply layer. It reaches a maximum value of 2100 cm2/Vs at a carrier density of 331012 cm22. [less ▲]

Influence of doping density on small and large signal characteristics of AlGaN/GaN/SiC HEMTs

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 ▲]

Influence of SiO/sub 2/ and Si/sub 3/N/sub 4/ passivation on AlGaN/GaN/Si HEMT performance,

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 ▲]

Material and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates

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 ▲]

Peculiarities of low frequency noise in GaN-based high electron mobility transistors

in Proceedings of 17th Internat. Conf. Noise and Fluctuations ICNF (2003)

Effect of surface passivation on performance of AlGaN/GaN/Si HEMTs

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 ▲]

Investigation of AlGaN/GaN HEMTs on Si substrate using backgating

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 ▲]

Determination of channel temperature in AlGaN/GaN HEMTs grown on sapphire and silicon substrates using DC characterization method

in IEEE Transactions on Electron Devices (2002), 49(8), 1496-1498

Self-heating effects and temperature rise in AlGaN/GaN HEMTs grown on silicon and sapphire substrates are studied, exploiting transistor dc characterization methods. A negative differential output ... [more ▼]

Self-heating effects and temperature rise in AlGaN/GaN HEMTs grown on silicon and sapphire substrates are studied, exploiting transistor dc characterization methods. A negative differential output resistance is observed for high dissipated power levels. An analytical formula for a source-drain current drop as a function of parasitic source resistance and threshold voltage changes is proposed to explain this behavior. The transistor source resistance and threshold voltage is determined experimentally at different elevated temperatures to construct channel temperature versus dissipated power transfer characteristic. It is found that the HEMT channel temperature increases rapidly with dissipated power and at 6 W/mm reaches values of 320 C for sapphire and 95 C for silicon substrate, respectively. [less ▲]