In this paper, we outline the current state of the art and trends for future development in millimeter-wave (mmw) amplifiers based on gallium nitride (GaN) semiconductors. To that end, we give an overview of recent technological results of currently operational GaN foundries and classify them with respect to their maximum frequency of operation and the current-gain cutoff frequency. Furthermore, focusing on frequencies above 80 GHz, we develop a comprehensive survey of GaN high-power amplifiers (HPAs) and provide a comparison to competing technologies such as indium phosphide (InP) and silicon germanium (SiGe). We also introduce a newly developed 6-stage GaN amplifier that is targeted towards the upper D-band. It provides an output power of up to 22.0 dBm, which sets a new record for GaN HPAs in this band. From our survey, we find that while GaN exhibits an impressive power density, its gain and efficiency characteristics particularly beyond 120 GHz still warrant improvement. Therefore, we describe several areas of future improvement such as the gate module scaling, mitigation of trapping phenomena and recent trends in processing of ohmic contacts and alternative epitaxial stacks. With these further developments, more widespread adoption of GaN-based technologies at even higher frequencies seems feasible in the coming years. 

Advances in GaN Devices and Circuits at higher mm-Wave Frequencies invited paper

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 (<inline-formula> <tex-math notation="LaTeX">$g_{m}$ </tex-math></inline-formula>) of 917 mS/mm at <inline-formula> <tex-math notation="LaTeX">$V_{\mathbf {D}}$ </tex-math></inline-formula> of 3 V, nearly twice as much as prior N-Polar GaN metal-insulator-semiconductor (MIS) HEMTs, with a low gate current of below <inline-formula> <tex-math notation="LaTeX">$40~\mu \text{A}$ </tex-math></inline-formula>/mm up to <inline-formula> <tex-math notation="LaTeX">$+1~V_{\mathbf {G}}$ </tex-math></inline-formula>. For the same device, the peak intrinsic RF<inline-formula> <tex-math notation="LaTeX">$g_{m}$ </tex-math></inline-formula> is 1.17 S/mm. A device with a 50 nm gate length demonstrates the highest N-Polar deep recess HEMT peak <inline-formula> <tex-math notation="LaTeX">$f_{T}$ </tex-math></inline-formula> of 176 GHz and peak <inline-formula> <tex-math notation="LaTeX">$f_{\mathbf {MAX}}$ </tex-math></inline-formula> 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.

Schottky Barrier Gate N-Polar GaN-on-Sapphire Deep Recess HEMT With Record 10.5 dB Linear Gain and 50.2% PAE at 94 GHz

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{g}_{\text{\textit{m},dc}}$</tex-math> </inline-formula> of 843 mS/mm. A HEMT with a gate length ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{\text{\textit{g}}}$</tex-math> </inline-formula> ) of 50 nm demonstrated a peak <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{g}_{\text{\textit{m},RF}}$</tex-math> </inline-formula> of 1028 mS/mm, peak <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{t}$</tex-math> </inline-formula> of 193 GHz, and peak <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\max}$</tex-math> </inline-formula> of 362 GHz. Load–pull was measured at 94 GHz with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\text{DS}}$</tex-math> </inline-formula> of 12 V and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\text{DS}}$</tex-math> </inline-formula> of 250 mA/mm. The TiN HEMT demonstrated a power added efficiency (PAE) of 53.4%—a record for N-polar GaN at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$</tex-math> </inline-formula> -band. The associated output power density was 3.7 W/mm, and the linear power gain (G <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{\textit{p}}}$</tex-math> </inline-formula> ) was an exceptional 10.2 dB. 

N-Polar Deep Recess GaN HEMT With a TiN Schottky Gate Contact Demonstrating 53.4% PAE and 3.7 W/mm Associated P$_{\mathrm{out}}$ at 94 GHz

We report on the development of the super-lattice castellated field effect transistor (SLCFET) technology as a candidate for the next generation of mmW and <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band systems, leveraging the high carrier density and a high degree of charge control offered by this device topology for mmW and <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band power amplification. The SLCFET is built using a superlattice of stacked AlGaN/gallium nitride (GaN) heterostructures that are etched into nanoribbons between epitaxial regrown n+ GaN source and drain contacts and controlled with a 100 nm length T-gate that electrostatically actuates the stacked channels from the sidewalls. The <inline-formula> <tex-math notation="LaTeX">$2\times 20\,\,\mu \text{m}$ </tex-math></inline-formula> amplifier cells of SLCFET devices were measured using load–pull at 94 GHz using a 12-V bias, demonstrating amplifier output power densities of 10.87 W/mm with 43% power added efficiency (PAE) at peak power and a maximum linear gain of 5.4 dB. The SLCFET amplifier process attains this power density due to its extremely high current density, with an IMAX of 4.8 A/mm, along with its minimal dispersion, with current collapse measured using pulsed <inline-formula> <tex-math notation="LaTeX">$I$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> at < 7%. The SLCFET technology, with its previously demonstrated world-class RF switch performance and now record <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band amplifier performance, is ideal for use in next-generation mmW and <inline-formula> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula>-band systems.

GaN SLCFET Technology for Next Generation mmW Systems, Demonstrating Pout of 10.87 W/mm With 43% PAE at 94 GHz

This paper presents an overview of the promising physical properties of the aluminum scandium nitride (AlScN) ternary alloy system and its integration in radiofrequency heterostructure high electron mobility transistors (RF HEMTs). Specifically, heterostructures with AlScN demonstrate enhanced piezoelectric response and enhanced relative dielectric permittivity, as well as ferroelectric behavior. These promising physical phenomena are manifested in HEMTs that show larger on current densities compared to aluminum gallium nitride (AlGaN) HEMTs and display ferroelectric behavior. An overlook on future improvements in thin film deposition and device fabrication is discussed to fully maximize the potential devices and integrated circuits incorporating AlScN.

AlScN High Electron Mobility Transistors: Integrating High Piezoelectric, High K Dielectric, and Ferroelectric Functionality

Development of MBE-grown ScAlN-based heterostructure field effect transistors (HFETs) for millimeter-wave operation at Ka- and W-band frequencies are reported. Fabricated 2-finger transistors with non-alloyed, ohmic regrowth contacts achieved excellent DC performance with maximum current density ($(\mathrm{I}_{\text{MAX}}$) and peak transconductance $(\mathrm{g}_{\mathrm{M}})$ of up to 2.5 A/mm and >0.7 S/mm, respectively. Continuous-wave (cw), large-signal RF (LS-RF) output power density (POUT) and power added efficiency (PAE) of >9 W/mm and >25% were respectively achieved from load pull characterization at 30 GHz when operated up to 40 V. At 94 GHz, POUT and PAE of >3.6 W/mm and 24% respectively were achieved at 10 V operation. This marks not only the best millimeter-wave RF power density performance of ScAlN/GaN transistors at 30 GHz to date but also the first demonstration of RF performance at W-band with $> 2\mathrm{X}$ higher POUT than similar AlGaN-based transistors thereby paving the path for highly compact, millimeter-wave amplifiers (narrow and wideband) with considerably higher power and efficiency.

ScAlN-GaN Transistor Technology for Millimeter-wave Ultra-high Power and Efficient MMICs

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ScAlN-GaN Transistor Technology for Millimeter-wave Ultra-high Power and Efficient MMICs