We report on the ferroelectric properties of single-phase wurtzite ScGaN grown on GaN by plasma-assisted molecular beam epitaxy. Distinct ferroelectric switching behavior was confirmed by detailed electrical characterization. Coercive fields in the range of 2.0–3.0 MV/cm and large, retainable remnant polarization in the range of 60–120 μC/cm² are unambiguously demonstrated for ScGaN epilayers with Sc contents of 0.31–0.41. Taking advantage of the widely tunable energy bandgap of III-nitride semiconductors, the demonstration of ferroelectricity in ScGaN, together with the recently reported ferroelectric ScAlN, will enable a broad range of emerging applications with combined functionality in ferroelectric, electronic, optoelectronic, photovoltaic, and/or photonic devices and systems.

Molecular beam epitaxy of single-phase wurtzite N-polar Sc_xAl_1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc_0.18Al_0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc_0.11Al_0.89N and Sc_0.30Al_0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc_0.18Al_0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm²/V s is measured for a 15-nm-thick Sc_0.18Al_0.82N barrier sample with a sheet electron concentration of 4.1 × 10¹³ cm⁻², and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of Sc_xAl_1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.

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 $D$ -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.

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.

AlGaN/GaN high electron mobility transistor (HEMT) with 1.2-μm-thick AlN super back barrier (SBB) for high-voltage applications was illustrated. Different from the conventional back barriers, AlN SBB in this paper tremendously enhances the carrier confinement in the view of energy band. The average breakdown field of AlN SBB HEMTs is 2.07×10 ⁵ V/cm, without field plate and other junction terminal technologies, almost 1.5 times larger than of the control GaN buffer HEMTs. The buffer leakage current in device with AlN SBB is in the ~ 10 ^-5 A/mm level, lower than in device with GaN buffer by two orders of magnitude. Moreover, the buffer leakage of AlN SBB device increases negligibly with the drain voltage, while in GaN buffer device the current increases until breakdown. Benefiting from the AlN SBB, The current collapse is suppressed and the RF characteristics are promoted.