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In this paper, we outline the current state of the art and trends for future development in millimeter-wave (mmw) amplifiers based on gallium nitride (GaN) semiconductors. To that end, we give an overview of recent technological results of currently operational GaN foundries and classify them with respect to their maximum frequency of operation and the current-gain cutoff frequency. Furthermore, focusing on frequencies above 80 GHz, we develop a comprehensive survey of GaN high-power amplifiers (HPAs) and provide a comparison to competing technologies such as indium phosphide (InP) and silicon germanium (SiGe). We also introduce a newly developed 6-stage GaN amplifier that is targeted towards the upper D-band. It provides an output power of up to 22.0 dBm, which sets a new record for GaN HPAs in this band. From our survey, we find that while GaN exhibits an impressive power density, its gain and efficiency characteristics particularly beyond 120 GHz still warrant improvement. Therefore, we describe several areas of future improvement such as the gate module scaling, mitigation of trapping phenomena and recent trends in processing of ohmic contacts and alternative epitaxial stacks. With these further developments, more widespread adoption of GaN-based technologies at even higher frequencies seems feasible in the coming.

In this letter, we report on the realization of a two-stage 16-way solid-state power amplifier (SSPA) in the Ka-band. To this end, we describe the design of a high-power amplifier (HPA) in a 100-nm gallium nitride (GaN) process and its integration into a split-block waveguide module. The PA module achieves an output power of more than 7.6 W between 28 and 39 GHz. In conjunction with 16 of these PA modules, we then employed a custom low-loss radial splitter and combiner to create a compact SSPA system. The two-stage SSPA configuration exhibits a small-signal gain of up to 44 dB and a peak output power of 127 W at 31 GHz in 5 dB of gain compression. Furthermore, we measured output power of close to 100 W and state-of-the-art efficiency values of more than 19% between 28 and 38 GHz. To our knowledge, this is the most broadband high-power SSPA demonstrated so far in this frequency range.

The piezoelectric and spontaneous polarization of wurtzite ScxAl1−xN, GaxAl1−xN, and InxAl1−xN ternary compounds dramatically affects the electrical properties of pseudomorphic MexAl1−xN/GaN, MexAl1−xN/AlN, and MexAl1−xN/InN heterostructures and devices (Me: = Sc, Ga, In), due to bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. We have calculated the piezoelectric and spontaneous polarization of undoped, metal polar ScxAl1−xN barrier layers (0 ≤ x≤ 0.5) pseudomorphically grown on InN, GaN, and AlN buffer layers, in order to compare the polarization induced surface and interface charges determined to the ones predicted and measured in heterostructures with GaxAl1−xN and InxAl1−xN barriers (0 ≤ x ≤ 1.0). To facilitate the inclusion of the predicted polarization in future simulations, we give explicit pre scriptions to calculate polarization induced bound interface charges for arbitrary x and barrier thicknesses up to 50 nm in each of the ternary III-N alloy heterostructures. In addition, we predict the electron sheet charges confined in heterostructures with positive polarization induced interface charges taking limitations for the epitaxial growth by strain and critical barrier thicknesses into account. Based on these results, we provide a detailed comparison of the sheet resistances and current-carrying capabilities of the heterostructures investigated, pointing to a superior potential of ScAlN/GaN based heterostructures for processing improved high electron mobility transistors for high-frequency and power electronic applications.

GaN devices for high-frequency and high-power applications often need n-doped GaN layers on top of their structures. Such layers can be either grown in an epitaxial reactor or formed by implantation or annealing of Si-containing layers (e.g., a SiO2 mask). These processes are typically performed at high temperatures, which generate the undesired effect of atom diffusion between the different epitaxial layers; consequently, the electrical performance of the final device will be hampered. Herein, an optimized epitaxial growth process of n-GaN layers is developed with the focus on minimizing the atom diffusion process, while preserving a high material quality and excellent electrical characteristics, such as very low contact resistance for n-GaN ohmic contacts or high electron mobility in GaN npin structures. A low growth temperature process combined with improved growth conditions to minimize the incorporation of impurities is successfully optimized and demonstrated on different epitaxial reactors.

Alloys of scandium with AlN exhibit an enhanced piezoelectric coefficient that can boost the performance of nitride-based electronic and optoelectronic devices such as high electron mobility transistors (HEMTs). Consequently, there is increasing interest in the epitaxial growth of high-quality AlScN/GaN heterostructures. So far, only very recent reports on AlScN HEMT structures grown by molecular beam epitaxy (MBE) have been published. Herein, the motivation for depositing AlScN epitaxial layers by metal-organic chemical vapor deposition (MOCVD) as well as the challenges associated with this approach are explained. For the first time, the successful deposition of epitaxial layers with a Sc content up to 30% (Al0.7Sc0.3N)is reported. It is shown that the deposited films consist of wurtzite-type AlScN with high crystalline quality, demonstrating that MOCVD is suitable for the growth of HEMT structures with Sc-based ternary nitrides.