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, 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.

Achieving high-quality, relaxed InGaN substrates for longer-wavelength light emitting diodes (LEDs) is of great interest for the development of micro-LED based display technology. This work demonstrates molecular beam epitaxy (MBE)-grown In_0.2Ga_0.8N with a strain relaxation of 60% corresponding to an equivalently fully relaxed In composition of 12%. This was done by growing on a GaN-on-porous GaN pseudo-substrate (PS). The surface morphology of this film was found to be free of V-defects on the surface and with a threading dislocation density comparable to that of the GaN layers beneath. While InGaN grown on planar GaN-on-sapphire substrates remained nearly strained to the GaN underlayer, InGaN grown under identical conditions on PS displayed elastic-like relaxation. Furthermore, an increase in indium (In) composition was observed for the InGaN grown on PS. Where past work of InGaN grown on porous GaN PS by metalorganic chemical vapor deposition also resulted in relaxed InGaN templates suitable for device application, the surfaces of these relaxed films exhibited V-defects for thicker layers. Employing MBE, thicker films with higher In composition can be achieved with smooth surface morphology, thus enabling pseudo-substrates with a wide range of lattice constants. These pseudo-substrates of varying in-plane lattice constant are attractive for III-nitride based optoelectronics, particularly for green, amber, and red micro-LEDs.

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 wet-etching technique based on a mixture of hydrochloric (HCl) and nitric (HNO₃) acids is introduced, demonstrating exceptional 42:1 selectivity for etching N-polar GaN over Al_0.24Ga_0.76N. In the absence of an AlGaN etch stop layer, the etchant primarily targets N-polar unintentionally doped (UID) GaN, indicating its potential as a suitable replacement for selective dry etches in the fabrication of GaN high-electron-mobility transistors (HEMTs). The efficacy and selectivity of this etchant were confirmed through its application to a gate recess module of a deep-recess HEMT, where, despite a 228% over-etch, the 2.6 nm AlGaN etch stop layer remained intact. We also evaluated the proposed method for the selective etching of the GaN cap in the n+ regrowth process, achieving a contact resistance matching that of a BCl₃/SF₆ ICP process. These findings underscore the applicability and versatility of the etchant in both the electronic and photonic domains and are particularly applicable to the development of N-polar deep-recess HEMTs.