In this work, on-wafer S-parameter characterization of a three-port substrate-integrated-waveguide (SIW)-based D-band power divider is reported, using a two-port VNA. In this configuration, the third port is terminated with a selection of known impedances to enable full extraction of the 3-port S-parameter matrix. A novel methodology for selecting the third-port impedance is proposed for S-parameter reconstruction. To assess the power splitter performance, the reconstructed three-port S-parameters were de-embedded using a thru-reflect-line algorithm, followed by time-domain-gating, and achieved an excess loss of 0.7 ± 0.1 dB and an input return loss of 25 ± 7 dB over the D band (110–170 GHz).

In this work, we present an analytical SPICE-compatible modeling framework for ferroelectric (FE)-gated AlGaN/GaN high-electron-mobility transistors (HEMTs) featuring a metal–ferroelectric–insulator-semiconductor (MFIS) gate-stack. By utilizing Landau–Devonshire (L-D) formalism, a cubic formulation of the 2-D electron gas (2DEG) density is obtained, reflecting multiple physical solutions. To facilitate analytical modeling, a systematic approach is presented in order to arrive at a single generalized solution. By leveraging the Fermi-Dirac statistics, region-wise analytical expressions for the 2DEG charge are formulated, and unified to yield a continuous 2DEG charge model spanning the entire gate bias range. The surface potential formulation is then used to derive closed-form expressions for terminal currents and charges. Owing to its physics-based formulation, the model inherently captures the enhancement in total gate capacitance arising from the FE-induced negative capacitance effect. The model is benchmarked against in-house experimental data as well as data reported in the literature, including DC characteristics and measured S-parameters over the frequency range of 10 MHz–30 GHz, showing excellent agreement and establishing it as a reliable framework for efficient circuit-level simulations.

The design, fabrication, and characterization of novel D-band power dividers in both Y- and T-junction configurations, using substrate integrated waveguide (SIW) technology fabricated on a 100-μm thick 4H-SiC substrates, are reported. The three-port S-parameters of the fabricated dividers were measured and calibrated on-wafer, achieving an excess loss as low as 0.68 ± 0.10 dB and an average return loss of up to 25 dB across full D-band. This is, to the best of our knowledge, the first demonstration of a SiC-based SIW power divider at D-band, offering excellent performance and compatibility with GaN-on-SiC MMIC fabrication processes.

In this study, we present a systematic optimization of 10 nm GaSb thin film grown directly on Si(001) substrate using a 5 nm AlSb buffer layer. By carefully tuning the AlSb buffer layer's growth temperature and thickness, growth conditions of 300 °C and 5 nm are identified that yield a smooth, two-dimensional 10 nm GaSb morphology with a root-mean-square (r.m.s.) surface roughness of 0.90 nm and an X-ray diffraction (XRD) full width at half-maximum (FWHM) of 0.76°. Cross-sectional transmission electron microscopy confirms the high-quality AlSb/GaSb interface and a minimal level of defect propagation through AlSb. In addition, micro-Raman spectroscopy and temperature-dependent Hall measurements reveal a slightly strained GaSb layer with a room-temperature hole mobility of approximately 350 cm2/(V·s). Furthermore, increasing the GaSb thickness to 300 nm improves surface quality and crystallinity, achieving an r.m.s. roughness of 0.40 nm and an FWHM of 0.39°. These results demonstrate the viability of integrating ultrathin, high quality GaSb layers on Si(001), showing their potential for scalable, high performance applications in next-generation CMOS-compatible micro/nanoelectronic devices.

Polarization-graded AlGaN/GaN high-electron-mobility transistors (HEMTs) are experimentally evaluated to assess their potential for improving device breakdown voltage and linearity performance. Incorporation of a 10-nm polarization-graded AlGaN transition layer between the channel and barrier increased the breakdown voltage from 50 V for the conventional abrupt-interface device to 70 V; with a 2-μm source field plate, 90 V was obtained. The graded devices exhibited a 53% reduction in the transconductance second derivative (gm'') and a 54% reduction in the gate-source capacitance second derivative (Cgs'') relative to the abrupt-interface devices, corresponding to an estimated 15.5 dB reduction in intermodulation products. The graded devices exhibited slightly lower fT and fmax (16.6 GHz and 46.3 GHz) compared to abrupt-interface devices (17.5 GHz and 56.5 GHz), attributed to the higher sheet resistance of the graded heterostructures. The increased breakdown voltage nonetheless yields a Johnson’s figure of merit of 1.16 THz·V, 33.2% higher than that of the abrupt-interface device. TCAD simulations calibrated to the measured DC characteristics indicate that the redistribution of polarization charge in the graded structure reduces the peak lateral electric field in the gate-drain region.

We demonstrate plasma-assisted MBE selective-area growth (SAG) of GaN/AlGaN core-shell structures on Si(111) as a path to vertical GaN devices. The SAG enables the full vertical and uniform core-shell fin structures to be realized in a continuous growth without breaking vacuum. TEM images and energy dispersive spectroscopy mapping of the core-shell structures show well-aligned crystal structures and sharp heterointerfaces. Dislocation filtering was observed in STEM imaging of the fin structure. p-n junction diodes fabricated with GaN/AlGaN core-shell structures reveal ideality factors as low as 1.5, and the reverse-bias leakage is consistent with trap-assisted space-charge-limited conduction. While practical challenges exist, including material-related leakage, growth-related challenges, and optimization, this demonstration of the p-n junction by this method may provide a path to vertical superjunction device concepts in GaN and related materials.

Tunable and reconfigurable devices for efficiently manipulating THz waves in advanced sensing, imaging and communication are challenging to realize. An especially promising approach for developing this class of THz devices–based on spatially-resolved photoconductivity modulation (SRPM) in semiconductors–will be introduced, followed by prototype demonstrations. The SRPM concept can be further enhanced by using mesa-array structures to achieve higher modulation depth and improved spatial resolution for more advanced tunable and reconfigurable THz devices. High-performance THz optically-controlled switches (OCSs) enabling next-generation adaptive integrated THz circuits/systems will then be discussed, and their use for achieving reconfigurable THz filters will also be presented. Finally, in addition to amplitude modulation, a unique approach for achieving high-performance THz phase shifting will be described. In total, the SRPM concept provides a critical solution that enables complete, systematic, and versatile technology for manipulating THz waves across a wide range of applications.

We report the first experimental demonstration of an alternative approach for achieving integrated broadband terahertz (THz) switching functionality, based on photoconductivity modulation in thin film Si on low-loss sapphire substrates. By employing a silicon thin film mesa (14 x 14 mu m(2)) on a silicon-on-sapphire (SoS) substrate (with 1.5-mu m-thick silicon and 600-mu m-thick sapphire), an optically-controlled THz switch integrated with low-loss coplanar waveguide transmission lines operating from 110 to 220 GHz has been designed, fabricated, and characterized. To control the switch in this prototype demonstration, a 915 nm laser diode was used to illuminate the silicon mesa through an optical fiber. The switch performance was measured on-wafer in both D- and G-band; an average on-state insertion loss of similar to 3.5 dB (with minimum insertion loss of 2.5 and 1.8 dB at 160 and 200 GHz), and an off-state isolation greater than 20 dB across the entire frequency range have been achieved. This performance can be further greatly improved, and is comparable to or better than competing approaches, with the advantage that this approach allows seamless integration of the switch with passive components on low-loss sapphire substrates. This makes the reported approach promising for developing high-performance and compact THz switches for next-generation adaptive circuits and systems.

High-gain GaN impact-ionization avalanche transit-time (IMPATT) diodes have been fabricated and experimentally characterized. The devices feature a "Hi-Lo" doping profile design; temperature-dependent reverse I - V measurement verified impact ionization avalanche as the dominant breakdown mechanism. IMPATT operation was confirmed, with reflection gain observed between 19.3 and 50.1 GHz using pulsed bias conditions of -175 V and current density of 7.6 kA/cm(2). Apeak reflection gain of 1.97 dB was observed around 30.8 GHz. On-wafer S-parameter measurements revealed a large negative differential resistance (NDR) of -992 Omega , comparable to state-of-the-art Si IMPATT diodes. The RF output power density is projected to be 40.3 kW/cm(2) at 35 GHz. The short-pulse on-wafer S-parameter-based measurement technique used here facilitates direct extraction of device impedance, providing insights into the optimal impedance matching of the peripheral resonant circuit and the appropriate operating frequency. Temperature-dependent characterization shows that the avalanche frequency has a much stronger dependence on current density rather than bias voltage. Therefore, biasing with a current source is preferred compared to a voltage source for stable operation in applications. The maximum operating frequency is found to increase at higher temperatures, possibly due to a decrease in the series resistance associated with the anode's p-contact resistance. These devices are promising for high-power, high-efficiency K-band to V-band signal generation across a broad temperature range.

High-gain GaN impact-ionization avalanche transit-time (IMPATT) diodes have been fabricated and experimentally characterized. The devices feature a "Hi-Lo" design; temperature dependent reverse I-V measurement verified impact ionization avalanche as the dominant breakdown mechanism. IMPATT operation was confirmed, with reflection gain observed between 20 GHz and 45 GHz using pulsed bias conditions of -177 V and current density of 7.5 kA/cm(2). A peak reflection gain of 1.4 dB was observed around 30 GHz. On-wafer S-parameter measurements revealed large negative differential resistance of -803 Omega, comparable to state of the art Si IMPATT diodes. The RF output power density is projected to be 42.1 kW/cm(2) at 35 GHz. The short-pulse on-wafer S-parameter-based measurement technique used here facilitates direct extraction of device impedance, providing insights into the optimal impedance matching of the peripheral resonant circuit and the appropriate operating frequency. These devices are promising for high-power, high-efficiency K-band to V-band signal generation.

Conventional power combiners based on microstrip or coplanar transmission lines are hindered by high loss above 110 GHz. By contrast, power combiners based on substrate-integrated waveguides (SIWs) can have low loss, high-power capacity, and minimum crosstalk at the same frequencies. Recently, we demonstrated a D-band (110-170 GHz) distributed amplifier by embedding transistors in the middle of an SIW as voltage probes. The voltage probes are realized through hot through-substrate vias (TSVs), which are not supported by all semiconductor foundries. As an alternative, this work embeds transistors along the sidewalls of an SIW as current probes through standard TSVs grounded to the backside metal layer. The new approach combines twice as many transistors per unit length of the SIW without requiring either hot TSVs or interconnects from the sidewalls to the middle of the SIW. This results in order-of-magnitude improvement in the power amplifier. For example, the measured output power of the new distributed amplifier at 145 GHz is 24 dBm with an associated power added efficiency of 9%, making its performance approaching state-of-the-art InP and SiGe D-band power amplifiers. Since the innovative power-combing approach is material agnostic and device agnostic, it can also be used to improve the performance of InP and SiGe power amplifiers above 110 GHz.

Advancing electromechanical resonators towards terahertz frequencies opens vast bandwidths for phononic signal processing. In quantum phononics, mechanical resonators at these frequencies can remain in their quantum ground state even at kelvin temperatures, obviating the need for millikelvin cooling typically required for GHz resonators. However, electrical actuation and detection at such high frequencies are challenging, primarily due to device miniaturization needed to support acoustic waves with nanometer-scale wavelengths. This requires thinning piezoelectric films to a thickness that matches the acoustic wavelength. In this work, we reduce the thickness of lithium niobate from 300 nm to 67 nm through several stages, and fabricate suspended Lamb-wave resonators at each thickness level. These resonators achieve resonant frequencies of nearly 220 GHz, doubling the previous record and approaching the terahertz threshold. While ultrathin films exhibit a clear advantage in frequency gains, they also experience increased acoustic losses. Our results suggest that future advances in terahertz nanomechanics will critically rely on mitigating surface defects in sub-100 nm thin films.

Electron and hole impact ionization coefficients are obtained from measurements of high‐Al content Al x Ga 1‐ x N p–n diodes grown on AlN substrates. The photomultiplication method using a 193 nm pulsed laser is applied to measure the multiplication factor. The impact ionization coefficients are modeled using Chynoweth's formulation, based on electric field profiles determined from the solution of Poisson's equation. A least‐squares fit of the theoretical multiplication factor to the measured multiplication factor yields the impact ionization coefficients and the extracted electron impact ionization coefficients are consistent with previous numerical predictions. Furthermore, the theoretical breakdown voltage and critical electric field are computed based on the extracted impact ionization coefficients. These results provide much‐needed data to further optimize the design of optoelectronic, power switching, and high‐power RF devices based on ultrawide bandgap AlGaN.

We report the experimental characterization and theoretical analysis of high-frequency GaN IMPATT diodes featuring a Hi-Lo doping profile. On-wafer small-signal measurements of reflection gain over frequency and temperature confirm IMPATT operation covering Ka through V bands. Temperature-dependent reverse I–V measurements also confirmed avalanche breakdown as the dominant breakdown mechanism. Under a pulse bias of −175 V and current density of 7.6 kA/cm2, the fabricated IMPATT diode exhibited gain between 19.3 and 50.1 GHz, with peak gain of 1.97 dB at ∼30.8 GHz. Temperature dependent characterization revealed that while the bias voltage required for operation at a given frequency changes significantly with temperature, consistent and stable performance can be achieved using a current source for biasing. For a fixed current density, both the avalanche frequency and device impedance remained relatively stable over a broad ambient temperature range. For use as a free-running oscillator, the minimum required Q for the external resonator was found to be less than 3.6, which is well within the range of what can be achieved in practice at these frequencies. Theoretical predictions of avalanche frequency were conducted and showed good agreement with the measured avalanche frequency, facilitating precise design of oscillators for applications. The fabricated IMPATT diodes demonstrate significant potential for high-power and high-efficiency signal generation from K-band to V-band across a wide temperature range.

This article presents an approach for modeling polarization-graded gallium nitride (GaN) high-electron-mobility transistors (HEMTs). Unlike conventional GaN HEMTs, where a 2-D electron gas (2DEG) forms at the barrier-channel interface, graded structures feature a 3-D electron distribution. TCAD simulations are used to extract carrier density and energy band diagrams, which form the basis for model development. The derivation uses refined approximations for the Fermi-Dirac integral solution, ensuring differentiability while accurately correlating carrier density with the applied gate bias through the use of potential balance. A surface-potential-based approach is subsequently used to model terminal currents and charges. Validation of the model is done through comparison with on-wafer measurements and published data, including dc transfer and output characteristics and measured S-parameters over the frequency range of 10 MHz-110 GHz. Furthermore, model accuracy in representing linearity is verified by comparing to large signal and intermodulation measurements at 10 GHz.

Compact and efficient millimeter-wave sources are critical components for wireless communications, radar, and a range of remote sensing applications. In the mm-wave frequency regime, impact ionization avalanche transit-time (IMPATT) diodes offer the potential for both very high power density and DCRF conversion efficiency. While IMPATT diodes have been widely reported in GaAs and Si, the higher critical electric field and high saturation velocity in GaN offers a significant performance benefit in terms of power density and operational frequency. We report experimental demonstration of a GaN-based “Hi-Lo” IMPATT diode exhibiting high reflection gain exceeding 1.48 dB and an operation range in the mm-wave range from $20-45 \text{GHz}$. The significant gain achieved enables the use of compact resonators with modest $Q$ for mm-wave oscillator applications in Ka to V band systems.

Gallium nitride (GaN)‐based high‐electron‐mobility transistors (HEMTs) are advantageous for power RF applications due to their dense 2DEG and high mobility. However, at mm‐wave frequencies, the lateral scaling required to achieve high‐frequency operation reduces breakdown voltage, limiting output power. Polarization grading of the barrier offers a solution by enhancing both RF performance and breakdown voltage. This study evaluates nonlinear grading profiles, which improve power handling without the need for additional field plates. Simulations using Synopsys Sentaurus technology computer‐aided design (TCAD) reveal that polarization‐graded HEMTs, featuring a graded AlGaN barrier layer (6–18 nm), exhibit more negative threshold voltage, increased drain current, and flatter transconductance compared to conventional abrupt‐interface HEMTs. The graded layer reduces peak lateral electric fields by as much as 39%, resulting in breakdown voltages that are 2.2 times higher than reference HEMT structures without grading. Polarization‐graded HEMTs with an 18 nm graded layer achieve a maximum output power of 8.8 W mm −1 , nearly three times that of conventional HEMTs. These results demonstrate the potential of polarization‐graded structures for mm‐wave applications, enhancing intrinsic field and power‐handling capabilities.

The high-power performance of a D-band (110-170 GHz) traveling wave amplifier (TWA) is reported. The amplifier was designed and fabricated using a GaN-on-SiC high-electron mobility transistor (HEMT) technology integrated with a substrate integrated waveguide (SIW) structure for low-loss on-chip power combining. Active injection load-pull measurements of both discrete HEMTs as well as the completed MMIC TWA were performed. The discrete HEMT measurements at D-band supplement the available design data for these scaled GaN HEMTs. The TWA achieved a peak power-added efficiency (PAE) of 9.1% at 145 GHz. The available output power exceeded 23.5 dBm from 135–14 5GHz, with a maximum output power of 24.7 dBm(295 mW) at 140 GHz.