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 $W$ -band systems, leveraging the high carrier density and a high degree of charge control offered by this device topology for mmW and $W$ -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 $2\times 20\,\,\mu \text{m}$ 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 $I$ – $V$ at < 7%. The SLCFET technology, with its previously demonstrated world-class RF switch performance and now record $W$ -band amplifier performance, is ideal for use in next-generation mmW and $W$ -band systems.

The super-lattice castellated field-effect transistor (SLCFET) is a multi-channel AlGaN/GaN HEMT device emerging as a technology platform for RF front ends, integrating world-class RF switches with high-performance RF amplifiers on the same wafer. This paper reports the performance results of a SLCFET amplifier device using an ALD TiN T-gate, which are improved with respect to previously reported data, with measured f t and f max up to 99 and 152 GHz respectively. The improvement is attributed to a reduced gate capacitance due to a higher T-gate hat without significant penalty of loss in gate control or increase of gate resistance. We also report W-band (94 GHz) large signal load-pull performance of the device with 4.33 W/active mm output power and 19.2% PAE. The process also demonstrates excellent DC/RF dispersion gate lag <1% and drain lag <2% and high breakdown voltage of 55 V.

Low resistance n+GaN contact materials were experimentally studied for GaN HEMT applications by selective area epitaxy regrowth on a patterned SiC substrate. Epitaxy was performed by metal organic chemical vapor deposition using 100% H2 or 100% N2 as the carrier gas. Thin film characterization demonstrated that n+GaN grown in N2 carrier gas has a superior morphology with improved crystalline quality to that grown in H2 carrier gas. The results also indicated that the surface morphology of n+GaN grown in N2 carrier gas is less sensitive to mask pattern density and micro-loading effects with Si doping concentrations up to 1 × 1020/cm3. Secondary ion mass spectrometry analysis shows that C and O impurity levels in n+GaN are one order of magnitude lower with N2 carrier gas than with H2. The electrical measurement of transmission line model structures shows an n+GaN sheet resistance of 15 Ω/sq and an Ohmic metal to n+GaN contact resistance of 0.02 Ω-mm for structures grown in N2 carrier gas. These values represent 7.1× and 2.5× improvements compared to H2 carrier gas.

This paper reports frequency performance improvements in Superlattice Castellated Field Effect Transistor (SLCFET) amplifier through device scaling. Device scaling incorporates the variations in castellation ridge width, castellation trench width, castellation length, gate stem length, gate hat length, gate offset, gate dielectric thickness, and passivation thickness. Highest ${f_{T}}$ and Fmax (70GHz/150GHz) values were achieved on devices with shortest castellation length. Shortening castellated access region reduces series resistance resulting in improved frequency performance. Thinner gate dielectric improves transconductance resulting enhancement in frequency performance as well.

Accurate compact model for simulating radio-frequency (RF) switch characteristics of a Super Lattice Castellated Field Effect Transistor (SLCFET) is presented. The new model is developed by modifying the industry standard ASM-HEMT compact model formulations. It is found that multiple channels of SLCFET cause unique I-V and S-parameters behavior of this technology. An accurate, and computationally efficient compact model is obtained after modifying ASM-HEMT formulations for multi-channel device electrostatics in SLCFET. The model is shown to be in good agreement with measurements for DC and RF device characteristics.

This report describes the second generation (Gen2) of the Superlattice Castellated Field Effect Transistor (SLCFET) amplifier. The SLCFET amplifier is a new device that uses 3-dimensional device geometry to modulate a superlattice of multiple AlGaN/GaN channels. The superlattice enables extremely low source and drain resistances and high output current and power density, while the 3-dimensional, low-resistance T-gate provides good electrostatic control and high gain. The wide bandgap material system also provides a high breakdown voltage. These properties lead to a robust, high performance device for low-noise and power amplifier applications. Process improvements allow the Gen2 device to reach FT/FMAX of 76/130 GHz. At 10 GHz, noise characterization achieves a minimum NFmin of 0.699 dB and load pull measurements achieve over 70% peak Power Added Efficiency (PAE).