This letter reports on the W-band power performance of N-polar GaN deep recess MIS-high electron mobility transistors (HEMTs) using a new atomic layer deposition (ALD) ruthenium (Ru) gate metallization process. The deep recess structure is utilized to control the DC-RF dispersion and increase the conductivity in the access regions. The ALD Ru effectively fills the narrow T-gate stems aiding realization of shorter gate lengths with lower gate resistance than in prior work. In this work, the gate length was scaled down to 48 nm, resulting in the demonstration of a record high 8.1-dB linear transducer gain measured at 94 GHz by load pull. This increased gain has enabled a record 33.8% power-added efficiency (PAE) with an associated output power density ( P _O ) of 6.2 W/mm.

In this article, N-polar GaN-on-sapphire deep-recess metal–insulator–semiconductor (MIS)-high-electron-mobility transistors (HEMTs) with a breakthrough performance at ${W}$ -band are presented. Compared with prior N-polar GaN MIS-HEMTs, a thin GaN cap layer and atomic layer deposition (ALD) ruthenium (Ru) gate metallization were used along with high-quality GaN-on-sapphire epitaxy from Transphorm Inc. Before SiN passivation, 94 GHz large signal load–pull shows that the transistor obtains a record-high 9.65 dB linear transducer gain and demonstrated 42% power-added efficiency (PAE) with associated 4.4 W/mm of output power density at 12 V drain bias. By biasing the drain at 8 V, the device shows an even higher PAE of 44% with an associated 2.6 W/mm of output power density. After SiN passivation, the fabricated N-polar GaN-on-sapphire HEMTs show a high PAE of 40.2% with an associated 4.85 W/mm of output power density. Furthermore, a very high output power density of 5.83 W/mm with 38.5% PAE is demonstrated at a 14 V drain bias. This power performance shows significant efficiency improvement over previous N-polar GaN-on-SiC and demonstrates a combined efficiency and power density beyond what has been reported for Ga-polar devices, in spite of the low-thermal-conductivity sapphire substrate. This shows that N-polar GaN-on-sapphire technology is an attractive candidate for millimeter-wave power amplifier applications with simultaneous high efficiency and power density.

This work presents recent progress in the W-band (94 GHz) power performance of N-polar GaN deep recess HEMTs grown on sapphire substrates. While SiC has been the substrate of choice to achieve the highest level of performance, sapphire substrates are a lower cost alternative. In this work we show that N-polar GaN deep recess HEMTs grown on sapphire match the power performance of a device on SiC up to 14 V with 5.1 W/mm of output power density. At 16 V the device on sapphire starts to suffer from thermal effects but still demonstrated 5.5 W/mm with an associated 20.6% power-added efficiency. This work also examines the impact of encapsulating the device in a low dielectric constant film often used for the implementation of a RF wiring environment.

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 ( $g_{{m}}$ ) of 917 mS/mm at $V_{\mathbf{D}}$ of 3 V, nearly twice as much as prior N-Polar GaN metal-insulator-semiconductor (MIS) HEMTs, with a low gate current of below 40 $\mu$ A/mm up to $+$ 1 $V_{\mathbf{G}}$ . For the same device, the peak intrinsic RF $g_{{m}}$ is 1.17 S/mm. A device with a 50 nm gate length demonstrates the highest N-Polar deep recess HEMT peak $f_{{T}}$ of 176 GHz and peak $f_{\mathbf{MAX}}$ 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.

In this letter, the first four-finger (4 $\times$ 25 $\mu $ m) N-polar GaN high-electron-mobility transistor (HEMT) with an outstanding large signal performance of 712-mW (7.1 W/mm) with 31.7% power-added efficiency (PAE) is demonstrated at 94 GHz. To the best of our knowledge, this is a record output power from a single device cell in any semiconductor technology at $W$ -band. An equivalent two-finger device (2 $\times$ 37.5 $\mu $ m) exhibits 6.9 W/mm with 30.6% PAE, demonstrating no degradation of either power density or efficiency with increased number of fingers and gate width. Additionally, the design considerations for multifinger devices are presented with the simulations of airbridge parasitics using COMSOL Multiphysics and Ansys high-frequency structure simulator (HFSS), along with the exploration of the design space in number and width of fingers for the best gain performance for $>$ 1 W. Simultaneous high-output power with high efficiency ( $>$ 30%) shows the great potential of multifinger N-polar GaN HEMT technology for the state-of-the-art $W$ -band power amplifiers.

Crack-free 1.3 μm thick elastically relaxed Al_xGa_1−xN layers were demonstrated on compliant high fill-factor 10 × 10 μm² tile patterned GaN-on-porous-GaN pseudo-substrates (PSs). Porous GaN was utilized as a semi-flexible underlayer. The Al_xGa_1−xN layer was grown in steps of 200, 300, 400, and 400 nm. While the Al_xGa_1−xN layer regrown on a co-loaded GaN-on-sapphire template cracked after deposition of only 200 nm AlGaN, on the GaN-on-porous GaN patterned substrates, no cracks were observed, and smooth films were achieved. In addition, an enhanced aluminum uptake was observed as the AlGaN growth progressed on the GaN-on-porous-GaN PS, owing to the composition pulling effect, until the point when the tiles started to coalesce. Upon further regrowth on these coalesced tiles, the aluminum uptake saturated, while the surface remained crack-free and smooth.