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, 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).

N-polar GaN HEMT technology has emerged as a disruptive technology that outperforms Ga-polar GaN HEMTs in terms of high-frequency power amplification capability. In this paper, the authors present a comprehensive review of the evolution of N-polar GaN HEMT technology from the perspective of crystal growth, dielectrics, and metals on N-polar GaN, transistor design, and performance. Specifically, the authors discuss the progress of the N-polar GaN HEMTs toward high-frequency, high-power, and high-efficiency applications with recent record-level performances, demonstrated by the authors, at mmWave frequencies.

A varying channel thickness was induced on nitrogen-polar GaN through undulations associated with step-bunching on a miscut vicinal substrate. Subsequently, the fabricated deep-recess metal insulator semiconductor high electron mobility transistors (MISHEMT) exhibited a varying threshold voltage along the gate width. A low third-order transconductance ( ${g}_{{m}3}$ ) was observed, indicating a third-order output-referred intercept point over dc power (OIP3/PDC) which was bias-insensitive. Two-tone 30-GHz load–pull demonstrated a peak OIP3/PDC of 11.95 dB, which remained above 8 dB for two $\Delta V_{\text{GS}}$ windows of 0.5 and 0.3 V at $V_{\mathbf{DS}}$ of 7 V. Single-tone 94-GHz load–pull measured at $V_{\text{DS}}$ of 12 V showed a peak output power density of 4.9 W/mm and an associated PAE of 33.9%.

Atomic layer deposition (ALD) is an excellent growth technique to achieve high-quality, high-uniformity, and highly conformal films with precise growth control at low (<400 °C) substrate temperatures. In this work, ALD was used to deposit low-resistance GaN layers on nitrogen-polar (N-polar) semi-insulating (S.I.) GaN substrates at 300 °C; film conductivity was significantly increased by adding a Si-precursor dose step immediately after the Ga-precursor step in the group III half-cycle and before the nitrogen-based plasma step in the group V half-cycle. Hall measurements revealed remarkably lower resistivities and five orders of magnitude increases in charge density in the Si-doped GaN films (∼3.5 × 1019 cm−3) relative to unintentionally doped films (∼2 × 1014 cm−3), with a Hall mobility of ∼30 cm2/V-s. Moreover, the charge density was further increased to 6.0 × 1019 cm−3 by utilizing a dual plasma process and a sub-saturation dosing of the Ga-precursor prior to the Si dose in the group III half-cycle. The sample surface remained smooth in most experiments.

This study investigates Mg-doped p-GaN gates integrated into N-polar GaN HEMT heterostructures grown by metal–organic chemical vapor deposition. Three epitaxial designs were compared to identify p-GaN growth temperatures and Al(Ga)N spacer designs (between the p-GaN and GaN channel) that suppress Mg back-diffusion into the GaN channel, as quantified by secondary ion mass spectrometry, while preserving the underlying two-dimensional electron gas (2DEG). A 2.7 nm Al0.27Ga0.73N spacer combined with 1020 °C p-GaN growth resulted in Mg pileup in the channel and a loss of measurable 2DEG conduction. In contrast, reduced Mg back-diffusion into the channel with preserved 2DEG transport was achieved through a 2 nm AlN spacer with 1020 °C p-GaN growth or a 2.7 nm Al0.27Ga0.73N spacer with 920 °C p-GaN growth. After selective removal of the p-GaN gate layer, Hall measurements yielded a 2DEG sheet density ns = 1.05 × 1013 cm−2 and 2DEG mobility μ = 730 cm2/V s for the AlN spacer with 1020 °C p-GaN growth sample, and ns = 1.58 × 1013 cm−2 and μ = 1344 cm2/V s for the Al0.27Ga0.73N spacer with 920 °C p-GaN growth sample. C–V measurements at 1 MHz further confirm functional p-GaN gating, with threshold voltages VTH ≈ −1.92 V for the AlN spacer with 1020 °C p-GaN growth sample and VTH ≈ −3.69 V for the Al0.27Ga0.73N spacer with 920 °C p-GaN growth sample. These results establish an epitaxial growth space for p-GaN-gated N-polar GaN HEMTs through spacer engineering and p-GaN growth-temperature control.