We report on DC and RF measurement results of AlScN/GaN high electron mobility transistors (HEMTs) grown by metal-organic chemical vapor deposition (MOCVD). Comparing the properties with those of a wafer grown with the same MOCVD tool but featuring an AlGaN barrier, the sheet carrier density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {n}_{\text {s}}$ </tex-math></inline-formula> ) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.50\times 10^{{13}}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{2}}$ </tex-math></inline-formula> measured on the AlScN/GaN wafer is around 60 % higher. This translates to a power density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {P}_{\text {out}}$ </tex-math></inline-formula> ) of 8.4 W/mm at a frequency of 30 GHz and a drain bias of 30 V. Also, a high power-added efficiency (PAE) of 48.9% and 46.1% is reached, when biased at 25 V and 30 V, respectively. These early results illustrate the great potential AlScN/GaN devices carry for improving on the achievable output power on device level at millimeter-wave (mmWave) frequencies. 

AlScN/GaN HEMTs Grown by Metal-Organic Chemical Vapor Deposition With 8.4 W/mm Output Power and 48 % Power-Added Efficiency at 30 GHz

Growth of AlScN high‐electron‐mobility transistor (HEMT) structures by metal–organic chemical vapor deposition (MOCVD) is challenging due to the low vapor pressure of the conventionally used precursor tris‐cyclopentadienyl‐scandium (Cp3Sc). It is shown that the electrical and structural characteristics of the AlScN/GaN heterostructure improve significantly by using bis‐methylcyclopentadienyl‐scandiumchloride ((MCp)2ScCl), which has a higher vapor pressure and allows for an increased molar flow and thus higher growth rate (GR). AlScN/GaN HEMT heterostructures with superior electrical characteristics deposited at different barrier growth temperatures are presented. The sheet resistance Rsh of 172 Ω sq−1 obtained at 900 °C barrier growth temperature is among the lowest reported so far for AlScN/GaN HEMT structures. The sheet charge carrier density ns is 3.23×1013 cm−2 and the electron mobility μ is 1124 cm2 Vs−1.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/pssr.202200387

Enhanced AlScN/GaN Heterostructures Grown with a Novel Precursor by Metal–Organic Chemical Vapor Deposition

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 years. 

Advances in GaN Devices and Circuits at higher mm-Wave Frequencies invited paper

GaN HEMTs (high-electron-mobility transistors) on SiC (silicon carbide) wafer with back barrier structures have gained much attention owing to the improved sheet charge density through enhanced carrier confinement, which can accomplish further improvement of RF performance. In this simulation work, the influence of different back barrier materials on RF & DC performance of Fe-doped recess gated AlN/GaN HEMT is explored. This paper also investigates the back barrier thickness effects on carrier confinement of each structure; the results show that better confinement of carriers in the GaN channel is accompanied by an optimized thickness for each back barrier material besides which, the performance is comparatively degraded. As back-barrier based structures have the capability to reduce SCEs by enhancing the carrier confinement, the investigation is done to analyse the impact of gate length scalability on the DC & RF characteristics of the proposed device. L g = 60 nm Fe-doped recess-gated AlN/GaN HEMT with an optimized AlGaN back barrier thickness of 60 nm provided a peak drain current of 4.63 A/mm, the highest transconductance of 2333 mS/mm, and the highest cut-off frequency of 316.6 GHz, and this outstanding DC/RF performance is due to strong carrier confinement and reduced leakage current. This outstanding performance makes them a potential choice for future RF power electronics and MMIC design. • Investigates the influence of back barrier material effects. • Back barrier material thickness effects on RF performance. • Scalability of AlN/GaN HEMTs with various Back Barriers. • Adopted recessed gate technology and Fe-doped buffer. 

Investigation of back barrier material effects on the scalability of Fe-doped recess-gated AlN/GaN HEMTs for next generation RF power electronics

In this letter, we report on Schottky barrier (SB) gate N-Polar GaN-on-sapphire deep recess high-electron-mobility transistors (HEMTs) with excellent dc, small signal and large signal performance. A device with a gate length of 77 nm demonstrates a very high extrinsic dc transconductance (<inline-formula> <tex-math notation="LaTeX">$g_{m}$ </tex-math></inline-formula>) of 917 mS/mm at <inline-formula> <tex-math notation="LaTeX">$V_{\mathbf {D}}$ </tex-math></inline-formula> of 3 V, nearly twice as much as prior N-Polar GaN metal-insulator-semiconductor (MIS) HEMTs, with a low gate current of below <inline-formula> <tex-math notation="LaTeX">$40~\mu \text{A}$ </tex-math></inline-formula>/mm up to <inline-formula> <tex-math notation="LaTeX">$+1~V_{\mathbf {G}}$ </tex-math></inline-formula>. For the same device, the peak intrinsic RF<inline-formula> <tex-math notation="LaTeX">$g_{m}$ </tex-math></inline-formula> is 1.17 S/mm. A device with a 50 nm gate length demonstrates the highest N-Polar deep recess HEMT peak <inline-formula> <tex-math notation="LaTeX">$f_{T}$ </tex-math></inline-formula> of 176 GHz and peak <inline-formula> <tex-math notation="LaTeX">$f_{\mathbf {MAX}}$ </tex-math></inline-formula> of 307 GHz. Biased at 10 V and 0.25 A/mm (class-AB), the 77 nm gate length device exhibits a record 94 GHz large signal performance with a linear gain of 10.5 dB, 50.2% peak power-added efficiency (PAE) with 2.8 W/mm power and 3.2 W/mm peak power with 46.3% PAE and 6.1 dB compressed gain. The outstanding large signal performance on low-cost sapphire substrate is very attractive and it paves the way for the next generation high-efficiency wireless communication systems.

Schottky Barrier Gate N-Polar GaN-on-Sapphire Deep Recess HEMT With Record 10.5 dB Linear Gain and 50.2% PAE at 94 GHz

This article presents improvements of large-signal RF power performance at Ka-band of gallium nitride high electron mobility transistors (HEMTs) utilizing a scandium aluminum nitride barrier. Three samples were fabricated and measured having different passivation schemes, including gallium nitride, silicon nitride, and aluminum gallium nitride. The pinning of the Fermi level against the valence band reduced the gain and output power density of the gallium nitride passivated device, resulting in 4.5 W/mm at a drain bias of 32 V. With silicon nitride alone as the passivation, dc/RF dispersion was reduced, and gain increased significantly, boosting the output power for this device to 8.0 W/mm at 28 V, though at the cost of a reduced breakdown voltage. Adding an epitaxial aluminum gallium nitride passivating layer increased the maximum bias voltage that the devices can operate at compared to devices with the silicon nitride passivation. When biased at 40 V, the output power of the devices reached 10.8 W/mm at 30 GHz. These devices show the importance of the passivation design to large-signal RF performance in scandium aluminum nitride-based HEMTs and will help guide further improvements.

Passivation Schemes for ScAlN-Barrier mm-Wave High Electron Mobility Transistors

Development of MBE-grown ScAlN-based heterostructure field effect transistors (HFETs) for millimeter-wave operation at Ka- and W-band frequencies are reported. Fabricated 2-finger transistors with non-alloyed, ohmic regrowth contacts achieved excellent DC performance with maximum current density ($(\mathrm{I}_{\text{MAX}}$) and peak transconductance $(\mathrm{g}_{\mathrm{M}})$ of up to 2.5 A/mm and >0.7 S/mm, respectively. Continuous-wave (cw), large-signal RF (LS-RF) output power density (POUT) and power added efficiency (PAE) of >9 W/mm and >25% were respectively achieved from load pull characterization at 30 GHz when operated up to 40 V. At 94 GHz, POUT and PAE of >3.6 W/mm and 24% respectively were achieved at 10 V operation. This marks not only the best millimeter-wave RF power density performance of ScAlN/GaN transistors at 30 GHz to date but also the first demonstration of RF performance at W-band with $> 2\mathrm{X}$ higher POUT than similar AlGaN-based transistors thereby paving the path for highly compact, millimeter-wave amplifiers (narrow and wideband) with considerably higher power and efficiency.

ScAlN-GaN Transistor Technology for Millimeter-wave Ultra-high Power and Efficient MMICs

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Passivation Schemes for ScAlN-Barrier mm-Wave High Electron Mobility Transistors

ScAlN-GaN Transistor Technology for Millimeter-wave Ultra-high Power and Efficient MMICs