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 Al xGa1− 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 Schrödinger 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 [Formula: see text] cm−2) as the non-graded conventional structure, though the mobility drops from [Formula: see text] cm2/V s in the conventional to [Formula: see text] cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller [Formula: see text] and [Formula: see text] as compared to the conventional non-graded channel HEMT implying improved device linearity.

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We report on the successful integration of superconducting Nb gate electrodes to AlGaN/GaN heterostructures and HEMTs for low noise cryogenic applications. First, a specific Nb-gate process was developed and implemented on stand-alone gate test structures. The latter were tested at cryogenic temperatures down to 4 K, using DC end-to-end measurements. The results show a clear transition to a superconducting state at Tc ~ 9.2 K. The superconducting nature of the Nb gates further verified on actual HEMTs, featuring 2 fingers design with gate length of 0.2 µm, through their S-parameters measurements at T

We report on the noise characterization and modeling of AlGaN/GaN HEMTs at a cryogenic temperature of ~10 K within the frequency range of 4.5 to 6.5 GHz. This work is the first model in the literature describing the high frequency noise behaviour of GaN-based HEMTs at cryogenic temperatures using a two-parameter-noise concept. The suggested model, which is based on measured noise figures and scattering parameters, provides the frequency and the bias-dependence of the cryogenic noise properties of AlGaN/GaN HEMTs. The noise contributions from the intrinsic device, the parasitic network and the gate leakage are separately extracted. The contribution of the access network is found of the order of 1 K and increases with the frequency, while the gate leakage has an impact of the order of 0.1 K and increases at low frequency. The model provides a basis for the future design and implementation of GaN-based cryogenic low noise amplifiers.