The Physics of Low-dimensional Semiconductors: An Introduction; J.H. Davies, Cambridge University Press, UK, ISBN 0-521-48491-X, $44.95

GaN technology is not only gaining traction in power and RF electronics but is also rapidly expanding into other application areas including digital and quantum computing electronics. This paper provides a glimpse of future GaN device technologies and advanced modeling approaches that can push the boundaries of these applications in terms of performance and reliability. While GaN power devices have recently been commercialized in the 15–900 V classes, new GaN devices are greatly desirable to explore both higher-voltage and ultra-low-voltage power applications. Moving into the RF domain, ultra-high frequency GaN devices are being used to implement digitized power amplifier circuits, and further advances using the hardware–software co-design approach can be expected. On the horizon is the GaN CMOS technology, a key missing piece to realize the full-GaN platform with integrated digital, power, and RF electronics technologies. Although currently a challenge, high-performance p-type GaN technology will be crucial to realize high-performance GaN CMOS circuits. Due to its excellent transport characteristics and ability to generate free carriers via polarization doping, GaN is expected to be an important technology for ultra-low temperature and quantum computing electronics. Finally, given the increasing cost of hardware prototyping of new devices and circuits, the use of high-fidelity device models and data-driven modeling approaches for technology-circuit co-design are projected to be the trends of the future. In this regard, physically inspired, mathematically robust, less computationally taxing, and predictive modeling approaches are indispensable. With all these and future efforts, we envision GaN to become the next Si for electronics.

Emerging GaN technologies for power, RF, digital, and quantum computing applications: Recent advances and prospects

We report 60-nm gate-length graded-channel AlGaN/GaN HEMTs fabricated with a mini-field-plate T-gate. The devices exhibit an extrinsic fT and $\text{f}_{\text {MAX}}$ of 156 GHz and 308 GHz, respectively. At 60-nm gate length, the fT*Lg of 9.4 GHz $\ast \mu \text{m}$ is comparable to that of conventional scaled short gate-length AlGaN/GaN HEMTs. Due to minimal current collapse, the graded-channel GaN HEMTs demonstrated linear RF power scaling up to 5.5 W/mm at 30 GHz. This results in a record $\text{f}_{\text {T}}~\ast $ power density product of 858 GHz $\ast \text{W}$ /mm for Ga-polar GaN devices, indicating the promise of this device architecture for mmW amplifiers.

Power Scaling of Graded-Channel GaN HEMTs With Mini-Field-Plate T-gate and 156 GHz f T

AlScN/GaN epitaxial heterostructures have raised much interest in recent years, because of the high potential of such structures for high-frequency and high-power electronic applications. Compared to conventional AlGaN/GaN heterostructures, the high spontaneous and piezoelectric polarization of AlScN can yield to a five-time increase in sheet carrier density of the two-dimensional electron gas formed at the AlScN/GaN heterointerface. Very promising radio-frequency device performance has been shown on samples deposited by molecular beam epitaxy. Recently, AlScN/GaN heterostructures have been demonstrated, which were processed by the more industrial compatible growth method metal-organic chemical vapor deposition (MOCVD). In this work, SiNx passivated MOCVD-grown AlScN/GaN heterostructures with improved structural quality have been developed. Analytical transmission electron microscopy, secondary ion mass spectrometry and high-resolution x-ray diffraction analysis indicate the presence of undefined interfaces between the epitaxial layers and an uneven distribution of Al and Sc in the AlScN layer. However, AlScN-based high-electron-mobility transistors (HEMT) have been fabricated and compared with AlN/GaN HEMTs. The device characteristics of the AlScN-based HEMT are promising, showing a transconductance close to 500 mS mm-1 and a drain current above 1700 mA mm-1.

https://iopscience.iop.org/article/10.1088/1361-6641/abd924/pdf

Improved AlScN/GaN heterostructures grown by metal-organic chemical vapor deposition

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 prescriptions 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.

Polarization induced interface and electron sheet charges of pseudomorphic ScAlN/GaN, GaAlN/GaN, InAlN/GaN, and InAlN/InN heterostructures

We report the RF power results of Sc(Al,Ga)N/GaN high electron mobility transistors (HEMTs). We show dc, small-signal RF and load-pull performance at 30 GHz with two barrier alloys—a ternary of ScAlN and a quaternary of ScAlGaN. The active layers are grown by molecular beam epitaxy on a GaN-on-SiC template. The Sc(Al,Ga)N HEMTs with 120 nm gate length achieve transconductance >700 mS/mm and >70 GHz cutoff frequency. The quaternary ScAlGaN sample shows reduced current collapse during pulsed I-V and load-pull characterization. The ScAlGaN HEMT delivers 5.77 W/mm output power ( ${V}_{D}= {20}$ V) and 47% power-added efficiency ( ${V}_{D}= {15}$ V) when tuned for maximum power and efficiency, respectively.

Michael Elliot

Papers

RF Power Performance of Sc(Al,Ga)N/GaN HEMTs at Ka-Band