In this letter, we report on Schottky barrier (SB)gate N-Polar GaN-on-sapphire deep recess high-electron-mobilitytransistors (HEMTs) with excellent dc, small signal and largesignal performance. A device with a gate length of 77 nm demonstrates a very high extrinsic dc trans conductance (gm)of 917 mS/mm atVDof 3 V, nearly twice as much as priorN-Polar GaN metal-insulator-semiconductor (MIS) HEMTs, witha low gate current of below 40 mu A/mm up to+1VG. For the same device, the peak intrinsic RFgm is 1.17 S/mm. A devicewith a 50 nm gate length demonstrates the highest N-Polar deeprecess HEMT peak f(T) of 176 GHz and peak f (MAX )of 307 GHz.Biased at 10 V and 0.25 A/mm (class-AB), the 77 nm gate lengthdevice exhibits a record 94 GHz large signal performance with alinear 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 signalperformance on low-cost sapphire substrate is very attractive andit paves the way for the next generation high-efficiency wireless communication systems.

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 Al0.24Ga0.76N. 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.

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.

This paper presents a dual-threshold N-polar GaN MISHEMT for linearization of the transconductance in RF applications where low noise and high-linearity are important. A physics-based compact model based on the MIT Virtual-Source GaN (MVSG) HEMT model is presented for the dual-threshold device. This work develops a methodology to extend the single-threshold model to a dual-threshold device structure. IV characteristics, S-parameters, and two-tone load-pull measurements were performed for experimental corroboration. The dual-threshold model has provisions for tuning physical parameters, predicting the device performance, and optimizing the linear gain efficiency(LGE).

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, the first four-finger (4 $\times25\,\,\mu \text{m}$ ) 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 $\times37.5\,\,\mu \text{m}$ ) 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 work, we report on a GaN/AlGaN superlattice based normally-off hole channel FinFET devices. A combination of Schottky gate and 60 nm wide fins led to enhancement mode operation. The device had an on-current of 13 mA/mm and an on-resistance of $300~\Omega $ .mm. simultaneously, a large Ion/Ioff > 107 and a current modulation of more than two orders of magnitude in the enhancement mode regime was also achieved.

This article develops a modified Massachusetts Institute of Technology (MIT) virtual source gallium nitride (GaN) (MVSG)-high-electron-mobility transistor (HEMT) model for N-polar HEMTs and demonstrates the predictive accuracy of the model. This article provides motivation for physics-based modeling approaches and the need to modify these in N-polar devices. The extraction methodology is developed for a millimeter-wave HEMT model from device characterizations. Experimental corroboration of the model demonstrates that the linear gain efficiency (LGE) characterized by the ratio of third-order output intercept point (OIP3) to dc power consumption ( $P_{\text {dc}}$ ) is more than 10 dB with a gain of 7 dB at 30 GHz for a 60-nm N-polar GaN HEMT device. The role of different physical features in the N-polar model is quantitatively described.

Combining excellent dispersion control with large breakdown voltage, deep recess N-polar GaN HEMT technology has demonstrated record high power densities [1] and efficiencies [2]–[3] from 2-finger devices with $50-75\ \mu \mathrm{m}$ gate peripheries at W-band. Employment of this technology in MMICs, however, will benefit from larger periphery multi-finger devices to maximize the power per unit cell and thus reduce or even possibly eliminate power combining for many applications. This will in turn increase the system efficiency by reducing the combining losses and enable more compact phased-arrays at mm-wave frequencies. Recently, a $4\times 25\ \mu \mathrm{m}$ multi-finger N-Polar GaN HEMT with record 712 mW output power was reported [4]. Increasing the gate periphery further ( $> 100\ \mu \mathrm{m}$ ) to maximize the power per cell requires a smaller load line resistance and larger capacitance compensation for optimal matching, thereby pushing the optimum load reflection coefficient magnitude to the values not obtainable by passive load pull, as shown in Fig 1. In this paper, we demonstrate an optimally load line matched $4\times 37.5\ \mu \mathrm{m}$ N-Polar GaN HEMT with 1 W (999.1 mW) output power at 20 V, measured with our new vector active load pull capability [5] at 94 GHz. To the best knowledge of the authors, this power is the highest reported output power from a single transistor to date at W-band.

N-polar GaN deep recess MISHEMTs devices have shown record mm-wave power density and efficiency under CW conditions, and high linearity receive performance. Digital communication systems require high linearity from both transmitters and receivers, and low noise figure and high transmit efficiency. This manuscript shows several advances to enable this, including a very high 20 dB OIP3/P DC with 1.9 dB NF at 30 GHz, which uses a peaked OIP3 at a g m3 zero-crossing. Two approaches for reducing bias sensitivity of distortion are shown. A transmit device is shown at a higher current density with a high power 30 GHz OIP3 to P DC of 6.7 dB, with low bias sensitivity of linearity, and associated high power and PAE. Initial results of an approach to broaden OIP3 peaks versus gate bias is shown with a multi-threshold device structure. To reduce power consumption, a fabrication process for deeper gate length scaling was used to obtain a 94 GHz CW efficiency of 34% at 6.25 W/mm power density, a 5% increase, with the shorter gate length increasing linear gain to above 8 dB.

We present results on 1200V GaN switches made with HEMTs on sapphire substrates. These are fast-switching, low loss devices extending the high performance of GaN switches to higher voltage levels. The insulating nature of sapphire substrates can help to extend the rated voltage of GaN HEMTs to 1200V and beyond, while simultaneously using a much thinner buffer layer compared to GaN-on-Si for similar voltages. Using a 70 mΩ GaN-on-sapphire 2-chip normally-off GaN FET in TO-247 package, we obtained >99% efficiency for a 900:450V buck converter operating at 50kHz. The GaN die has R ON,sp of 6.1mΩ.cm 2 and the device shows excellent switching FOMs with R ON .Q G = 0.9 Ω.nC, and R ON .Q RR = 11 Ω.nC. The sapphire substrate is thinned to below 200μm to give a thermal resistance comparable to that of packaged GaN-on-Si switches. These results indicate that a correctly engineered GaN-on-sapphire technology can be a very competitive platform for the 1200V power device market.

In this paper, we report on N-polar GaN-on-sapphire deep recess MIS-HEMTs with breakthrough performance at W-band. The devices show a high 8.8 dB linear gain at 94GHz at a 260mA/mm current density bias point, enabling excellent output power ($\text{P}_{\text{O}}$) density of 5.83 W/mm with record 38.5% power-added efficiency (PAE) at 14 V. Furthermore, at 12 V the device demonstrated even higher PAE of 40.2% with an associated 4.85 W/mm of output power density, a record. This power performance shows significant efficiency improvement over previous N-polar GaN-on-SiC, as well as Ga-polar devices and circuits, demonstrating the great potential of N-polar GaN-on-sapphire technology for mm-wave application with simultaneous high efficiency and power density.

Transphorm Inc. is supplying N-polar GaN epitaxial wafers for ultra-high-performance RF and mm-wave electronics on sapphire and silicon carbide (SiC) substrates. In this work, we report on the manufacturing facility, epitaxial growth on both 100-mm sapphire and 100-mm SiC, and device electrical performance. Results show good material quality, a low 2DEG sheet resistance with good non-uniformity, and good dynamic behavior with no current dispersion at 60 V at both room temperature and high temperature (150°C).

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.

The advantage of GaN is the capability of producing amplifiers with high output power and efficiency. At microwave frequencies, this performance has been achieved; however, when transitioning device design into mm-wave frequencies, the output power and efficiency of GaN HEMTs decrease. Traditionally, the approach taken to develop Ka-band (30–40 GHz) GaN devices has been to modify a device designed for a lower frequency. By contrast, this work modified a N-polar GaN deep recess HEMT developed for W-band power performance (94 GHz), for improved performance in the Ka-band. In this Letter, we first report on improvement in the 30 GHz continuous-wave (CW) power density through modification of the W-band device with the demonstration of 10.3 W/mm at 47.4% power-added efficiency (PAE). We then report on the two-tone linearity performance of the device when measured under the same bias and matching conditions. While the evaluation of GaN HEMTs has traditionally focused on the use of one-tone CW power measurements, with the increasing adoption of GaN transistors into communication systems, such as mm-wave 5 G cellular communication, simply demonstrating high power density and efficiency does not provide a sufficient understanding of the device as high linearity is required to transmit data using complex modulation schemes. Under two-tone stimulus, the device demonstrates an OIP3 to PDC ratio greater than 6.7 dB and a C/IM3 ratio of greater than 37 dBc under backoff conditions greater than 10 dB from the peak one-tone PAE.

There is a strong need for a large band gap pFET device with good performance for an efficient high voltage CMOS platform for power conversion applications. In this work, we report on GaN/ AlGaN superlattice based MES-FinFET devices with MOCVD regrown p+ contacts around the fins. 75 nm wide FinFETs showed a normally-off operation with an on-current of 65 mA/mm, highest ever reported for any GaN based E-mode pFETs. Simultaneously, a large I on /I off > 10 7 was also achieved.

We investigate the static and dynamic (trapping) performance of N-polar Gallium Nitride MIS-HEMT devices as a function of the aluminum concentration in the top cap layer (22%, 34% and 46%). The analysis is based on combined dc characterization, double pulse measurements, and threshold voltage transient investigation. The de results demonstrate that the use of high aluminum concentrations in the cap layer results in a lower gate leakage current (around 9 µA/mm for % Al=46, compared to 70 µA/ mm for %Al=22, measured at V Gs =-7 V and V DS =15 V). In addition, pulsed and transient investigation showed that the use of high Al concentration in the cap layer can substantially suppress the current collapse (slump ratio = 15 % for %Al=46, compared to 26 % for %AI=22). Trapping is ascribed to the presence of a defect state located at E C -0.5 eV, which is responsible for a threshold voltage shift. The results point out the key role of the AlGaN cap layer on the performance of AlGaN-based HEMTs, and give indication on how to optimize the performance of the devices.

We investigated the influence of aluminum concentration in the AlGaN cap layer on the stability of threshold voltage under gate and drain stress voltages. The devices examined are N-polar GaN MIS-HEMTs with a 2.6nm layer of AlxGa1-xN cap under the gate contact. The devices have three different Al concentration (x=22%, x=34%, x=46%) in the AlGaN cap layer. DC measurements show that the devices with a higher Al concentration have a lower gate leakage current. In this work we found out: 1) the amount of threshold voltage shift during stress in these devices increases linearly with the value of gate leakage current at the different bias conditions 2) a higher Al percentage in AlGaN cap layer can suppress gate leakage current and thus reduce the instability of threshold voltage under high gate and drain filling voltages.

Source and drain contacts regrown by metal-organic chemical vapor deposition (MOCVD) were successfully used in nitrogen-polar GaN metal-insulator-semiconductor (MIS) high-electron-mobility transistors (HEMTs) targeting high voltage switching applications. Previous work on N-polar GaN devices utilized either alloyed ohmic contacts, or ohmic contacts regrown by molecular beam epitaxy (MBE). Using MBE regrowth, ultra-low contact resistances (R-C) were demonstrated. In this study, MOCVD was used for the contact regrowth, eliminating the need for an MBE growth step. A metal-to-2DEG (two-dimensional electron gas) contact resistance (R-C) of 0.16 omega mm was demonstrated, comparable to the ultra-low contact resistances that were previously reported for contacts regrown by MBE. N-Polar MISHEMTs fabricated using this technology achieved breakdown voltages over 2000 V, with a specific on-resistance (R-ON) of 3.5 m omega cm(2)( 8.8 omega mm), and a dynamic R-ON increase of 12% at 400 V. The drain current density was 620 mA mm(-1) at V-GS = 1 V for device dimensions of L-G = 1 mu m, L-GS = 1 mu m, L-GD = 28 mu m, and W-G = 50 mu m. The successful contact regrowth by MOCVD eases the adoption of N-polar transistors.

This paper reports on the hot-carrier effects and semi-on-state behavior of nitrogen-polar GaN MIS-HEMTs at cryogenic temperatures (from 300 K down to 100 K). In the semi-on-state (V-G approximate to -2 V), holes are generated by impact ionization in the high field region at the drain-side of the gate-edge. At room temperature, holes overcome the SiN/AlGaN stack and are collected at the gate-terminal, resulting in measurable hole gate-current (similar to 11 nA/mm). Conversely, at cryogenic temperatures, the top SiN/AlGaN stack confines the holes within the GaN channel, thus inducing a negative threshold voltage shift (-0.4 V) and a sharp increase in drain current (0.18 A/mm). This behavior, referred to as "kink," is readily observable on the ID-VD characteristics. We demonstrated that the kink is related to impact ionization and follows a non-monotonic behavior maximized in the semi-on-state. Our interpretation is supported by a quantitative analysis based on the latest experimental impact-ionization coefficients available in the literature.