C-Band Resistive SiC-MESFET mixer

In this paper the design and characterization of a linear C-band single ended resistive SiC-MESFET mixer is presented. The mixer has a minimum conversion loss of 7.8 dB and has a third order intermodulation intercept point of 30.3 dBm. The mixer is designed using a harmonic-balance simula-tion load-pull approach. This design method is especially use-ful for high-level mixers, where small-signal approximations cannot be used

Illumination effects on electrical characteristics of GaN/AlGaN/GaN heterostructures and heterostructure field effect transistors and their elimination by proper surface passivation

The effect of ambient illumination is investigated for differently processed GaN/AlGaN/GaN heterostructure materials. For samples of the same material with different passivation, the difference in sheet resistance of illuminated and non-illuminated material can be as large as 130% (for annealed heterostructure without passivation) and as small as 3% (for heterostructure passivated with low pressure chemical vapor deposition (LPCVD) silicon nitride). The time constant for the decay of the persistent photoconductance (PPC) is also very different for the differently processed samples. The majority of the effect on the conductance is from photons with energies between 3.1 and 3.7 eV. The investigation indicates that delayed recombination of electrons emitted from surface states and from deep level states in the AlGaN layer dominates the PPC. A theory is formulated by which the difference in illumination sensitivity for the differently passivated materials can be explained by different distributions of electrons between the channel two dimensional electron gas and an accumulation layer formed in the cap layer. For practical heterostructure field effect transistor (HFET) measurements, the illumination sensitivity is generally lower than that of the Hall measurements. Furthermore, HFETs fabricated with the LPCVD silicon nitride passivation are practically illumination invariant

Mobility and quasi-ballistic charge carrier transport in graphene field-effect transistors

The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physicalmechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gatelengths ranging from 2 μm down to 0.2 μm by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated viathe drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate lengthin the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluationof the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. Themobility term associated with scattering mechanisms is found to be up to 4456 cm2/Vs. Transmission formalism treating the electrons aspurely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that atthe gate length of 2 μm, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 μm, thisnumber increases to above 60%

Investigation of Isolation Approaches and the Stoichiometry of SiNx Passivation Layers in “Buffer-Free” AlGaN/GaN Metal–Insulator–Semiconductor High-Electron-Mobility Transistors

Critical process modules for the fabrication of metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) based on a novel ‘buffer-free’ AlGaN/GaN heterostructure grown with metal–organic chemical vapor deposition (MOCVD) are presented. The methods of isolation and passivation for this type of heterostructure are investigated. Utilizing nitrogen implantation, it is possible to achieve off-state destructive breakdown voltages (BVs) of 2496 V for gate–drain distances up to 25 μm, whereas mesa isolation techniques limit the BV below 1284 V. The stoichiometry of the SiNx passivation layer displays a small impact on the static and dynamic on-resistance. However, MISHEMTs with Si-rich passivation show off-state gate currents in the range of 1–100 μA mm−1 at voltages above 1000 V, which is reduced below 10 nA mm−1 using a stoichiometric SiNx passivation layer. Destructive BVs of 1532 and 1742 V can be achieved using gate-integrated and source-connected field plates for MIHEMTs with stoichiometric and Si–rich passivation layers, respectively. By decreasing the field plate lengths, it is possible to achieve BVs of 2200 V. This demonstrates the implementation of MISHEMTs with high-voltage operation and low leakage currents on a novel “buffer-free” heterostructure by optimizing the SiNx stoichiometry

Low Al-content n-type AlxGa1-xN layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic\ua0chemical vapor deposition\ua0to deliver high-quality\ua0n-AlxGa1−xN (x\ua0= 0\ua0–\ua00.12, [Si] = 1 71017\ua0cm−3)\ua0epitaxial layers\ua0on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13\ua0–\ua00.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5 71015\ua0cm−3\ua0and carbon of 2 71016\ua0cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while\ua0screw dislocations\ua0only raise for\ua0x\ua0above 0.077. The room temperature\ua0electron mobility\ua0of the\ua0n-AlxGa1−xN remain in the range of 400\ua0–\ua0470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties

Novel Low-Loss Millimeter- Wave Transition From Waveguide-to-Microstrip Line Suitable for MMIC Integration and Packaging

This letter presents a unique low-loss transition from microstrip to full height rectangular waveguide at W-band. This microstrip transition can be made as a part of the mm-wave monolithic microwave integrated circuit (MMIC) of arbitrary size, and thus, avoid the use of bond wires at the high-frequency port of the MMIC circuit. As a result, the MMIC can be coupled directly to the waveguide. The working principle of the transition is based on electromagnetic coupling, where the coupling between the microstrip mode and the TE10 waveguide mode is achieved via a resonant cavity. A perfect magnetic conductor (PMC) surface is placed over the cavity to facilitate the smooth coupling of electromagnetic energy from the microstrip line to the cavity and then from the cavity to the waveguide. The PMC surface also suppresses the unwanted waveguide mode coupling to the oversized MMIC substrate. The measured back-to-back transition works over the frequency band of 80-114 GHz (relative BW of 35%) with minimum return loss of 13.5 dB. The total insertion loss of the manufactured prototype is found to be varying from 0.54 to 0.803 dB, which implies a single transition loss of less than 0.27-0.4015 dB in W-band

Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 \ub0C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 \ub0C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3 710-8 m2 K/W, measured at an elevated temperature of 160 \ub0C, in a GaN-on-SiC transistor structure was achieved

Evaluation of Thermal Versus Plasma-Assisted ALD Al2O3 as Passivation for InAlN/AlN/GaN HEMTs

Al2O3 films deposited by thermal and plasma-assisted atomic layer deposition (ALD) were evaluated as passivation layers for InAlN/AlN/GaN HEMTs. As a reference, a comparison was made with the more conventional plasma enhanced chemical vapor deposition deposited SiNx passivation. The difference in sheet charge density, threshold voltage, f(T) and f(max) was moderate for the three samples. The gate leakage current differed by several orders of magnitude, in favor of Al2O3 passivation, regardless of the deposition method. Severe current slump was measured for the HEMT passivated by thermal ALD, whereas near-dispersion free operation was observed for the HEMT passivated by plasma-assisted ALD. This had a direct impact on the microwave output power. Large-signal measurements at 3 GHz revealed that HEMTs with Al2O3 passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schr\uf6dinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 7 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity