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.

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.

A stack of TiN/Ru was employed as the Schottky gate metal for a nitrogen-polar deep-recess GaN high electron mobility transistor (HEMT) on a sapphire substrate. TiN/Ru Schottky diodes fabricated on the same HEMT epitaxy demonstrated a 6 $\times$ reduction in reverse-bias leakage current compared to Ru-only counterparts. The addition of TiN also resulted in a clear improvement in breakdown voltage for scaled N-polar Schottky gate HEMTs, as measured by the drain-current injection method (DCI). TiN HEMTs exhibited a peak $\textit{g}_{\text{\textit{m},dc}}$ of 843 mS/mm. A HEMT with a gate length ( $L_{\text{\textit{g}}}$ ) of 50 nm demonstrated a peak $\textit{g}_{\text{\textit{m},RF}}$ of 1028 mS/mm, peak $f_{t}$ of 193 GHz, and peak $f_{\max}$ of 362 GHz. Load–pull was measured at 94 GHz with a $V_{\text{DS}}$ of 12 V and $I_{\text{DS}}$ of 250 mA/mm. The TiN HEMT demonstrated a power added efficiency (PAE) of 53.4%—a record for N-polar GaN at $W$ -band. The associated output power density was 3.7 W/mm, and the linear power gain (G $_{\text{\textit{p}}}$ ) was an exceptional 10.2 dB.

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.

In this letter, we report the first $D$ -band operation of N-Polar GaN technology with record large signal performance using co-planar waveguide (CPW) pre-matching networks. The pre-matched devices were designed without utilizing a large signal model. Load line resistance and output capacitance were extracted from the 94 GHz passive load–pull measurements of equivalent un-matched devices for output pre-matching network design at $D$ -band. These parameters were then further verified with the pulsed-IV and S-parameter measurements. For the input pre-matching network, conjugate matching at the small signal model was achieved. An N-Polar GaN pre-matched device biased at 12 V and 0.5 A/mm showed 2 W/mm (100 mW) output power with 10.6% power-added efficiency (PAE) at 132 GHz, without de-embedding the matching network losses. With matching network losses de-embedded, the power density and PAE at the device level are 2.7 W/mm and 21.8% at 1.6 dB compression, respectively. Device level OP1dB of 2.2 W/mm at 132 GHz closely agrees with OP1dB of 2.3 W/mm measured by 94-GHz load pull, showcasing the excellent output match as well as the saturated power being limited by drive power at 132 GHz. To the best knowledge of the authors, the reported output density of 2 W/mm is the highest reported GaN output power density at $D$ -band.

In this letter, we report the first N-polar InAlGaN quaternary back barrier high-electron-mobility transistor (HEMT). The epitaxial device heterostructure was grown by metal organic chemical vapor deposition (MOCVD). Hall measurements of the heterostructure showed a two-dimensional electron gas (2DEG) density of $2.85\times 10^{{13}}$ cm $^{-{2}}$ and a mobility of 1048 cm $^{{2}}\,\,\text{s}^{-{1}}\,\,\text{V}^{-{1}}$ . Transfer length method measurements showed a remarkably low sheet resistance of $179~\Omega /\Box $ in the source-drain direction. A HEMT with a gate length of $0.6~\mu \text{m}$ and source-drain spacing of $3.1~\mu \text{m}$ showed a peak transconductance of 212 mS/mm and a high peak DC drain current of 1.92 A/mm. Small signal measurements of an equivalent HEMT yielded a current-gain cut-off frequency ( $\text{f}_{\text {T}}{)}$ and power-gain cut-off frequency ( $\text{f}_{\text {max}}{)}$ of 18 GHz and 28 GHz, respectively, at peak $\text{f}_{\text {max}}$ bias conditions (VGS = −9 V and VDS = 5 V).