This study presents a comparative analysis of large-signal performance for AlGaN/GaN high electron mobility transistor (HEMT) switches on a silicon substrate, isolated using either ion implantation or mesa etching. The investigation focuses on the impact of isolation methods on gate leakage, RF isolation, and harmonic distortion. While DC-IV measurements indicate significantly higher gate leakage in mesa etch isolated devices, small-signal S-parameter measurements indicate that both isolation methods provide effective $\mathbf{R F}$ isolation. Large-signal measurements demonstrate that ion implantation isolated devices exhibit greater non-linearity and harmonic distortion compared to mesa etch isolated devices. For the largest device type, the mesa etch isolated switch had a 37% higher 1 dB compression point. Higher harmonic distortion for ion implantation isolation was attributed to reduced effective substrate resistivity.

This article presents a pioneering study on the pH sensing performance of open-gated gallium nitride (GaN) high-electron mobility transistors (HEMTs) with five different device variants, introducing novel approach to patterning the open gate area. The device variants include normal HEMTs with open gate (O-HEMT) used as a reference, with horizontal lines etched in the open gate area (H-HEMTs), with vertical lines etched in the open gate area (V-HEMTs), fully blank etched (B-HEMTs), and with horizontal and vertical lines or grids etched (G-HEMTs). The fabrication of the GaN HEMTs was achieved through metal-organic chemical vapor deposition (MOCVD), while the source and drain ohmics were fabricated using e-beam evaporation. The sensitivity analysis involved drop casting samples of varying pH onto the open gate area and measuring the device response for acidic (pH = 4), basic (pH = 10), and neutral (pH = 7) solutions. The pH values of 4, 7, and 10 for our analysis have been chosen to showcase the sensor's functionality within a range pertinent to common practical applications, such as biological systems and environmental monitoring-environments, where these pH levels are typically found. The normalized output drain current has been employed as the sensing metric. Our findings reveal the remarkable novelty of this study, as it demonstrates that G-HEMTs exhibit the highest sensitivity among the variants, with an average sensing current of 1.355 mA/pH. The V-HEMTs displayed a sensitivity of 2.15%, followed by H-HEMTs at 1.98%, and normal HEMTs at 1.16%. Notably, the B-HEMT devices initially showcased good sensitivities; however, their performance declined below that of H-HEMTs beyond 15 V. These results underscore the significance of patterning the open gate area in optimizing the pH sensing capabilities of GaN HEMTs.

This paper presents the development of a lateral AlGaN/GaN Schottky barrier diode (SBD) on free-standing GaN epilayers achieved through Si substrate removal for GaN-on-Si technology. Compared to conventional SBDs on GaN-on-Si, the newly developed free-standing AlGaN/GaN SBDs exhibit enhanced RF performance (cut-off frequency (f(C)) and ideality factor (eta(n)) without compromising DC performance. DC results reveal an onset voltage (V-ON) of 1.34 V, on-resistance (R-ON) of 1.6 ohm center dot mm, and eta(n) of 1.7 (14% improvement). Reverse-breakdown voltage (V-BV) exceeds 30 V, with a reverse-bias leakage current (I-R) below 38 mu A/mm. RF performance demonstrates a f(C) of 80.2 GHz at zero-bias (11.14% enhancement). The impact of lossy Si substrate coupling effects on RF performance for both SBD structures was studied using 3D-EM simulations for further performance optimisation. These findings imply integrated GaN-based SBDs on Si technology are suitable for high-power and high-temperature system applications at millimetre-wave frequencies, particularly in conjunction with high-thermal coefficient materials such as diamond and AlN.

In this paper, Monolithic Microwave Integrated Circuits (MMICs)-compatible $\mathrm{Si}_{3} \mathrm{~N}_{4}$-based metal-insulator-metal (MIM)-capacitors with elevated-electrode realised on thick SiO 2 has been demonstrated for the first time for GaN-on-Si technology. Comparative analysis of DC and RF characteristics has also been presented with a standard MIM structure and a benchmark sample on SiC substrate. The outcomes show achievement of low-current/RF leakage and high breakdown voltage $\left(\mathrm{V}_{B V}\right)$ capabilities of the proposed capacitor. Further, analysis of RF loss in GaN-on-HR Si substrates was conducted using electromagnetic simulations and small-signal equivalent circuit models that were derived for accurate design of mm-wave integrated circuits. The developed standard and air bridged capacitors showed a $\mathrm{V}_{B V}$ exceeding 167 V and 189 V respectively. The current-density leakage (J) observed is below 1μA /$\mathrm{cm}^{2}$ up to 30 V at 125°C. This 13% increase in $\mathrm{V}_{B V}$ is achieved with tradeoff of low capacitance density of 68.9% attributed to the additional resistance added by the interconnects. In addition, the proposed air-bridged capacitors achieved low RF leakages of 0.4 dB up to 67 GHz which is 50% lower than the standard MIM capacitor.

This work presents performance evaluation of GaN High Electron Mobility Transistor (HEMT) based ultraviolet (UV) detector on Si substrate. In addition to the fabrication and characterization, a systematic study is presented here using simulations extensively to investigate the UV detection mechanism. Output current has been chosen as the sensing metric, the fabricated device exhibits a high UV responsivity of $1.62\times 10^{{7}}$ A/W at $2.5\times 10^{-{10}}$ W, $\text{V}_{{\text {GS}}}={0.5}$ V. Simulations have been done using optical modules available in Silvaco ATLAS TCAD to analyze the energy band bending, Two-Dimensional Electron Gas (2DEG), channel potentials and electric fields in the device. This model can aid in systematic study of HEMT based detectors in terms of dimensional and epi layer design optimizations for sensitivity enhancements. The UV response of the device is found to decrease as the wavelength approaches the visible light wavelength. This makes the photodetectors blind to visible light ensuring selective detection of UV wavelengths. It has been observed that as the area for UV absorption is increased by increasing the W/L ratio, the increases. For a W/L ratio of 100, the detector exhibits a responsivity of $1.86\times 10^{{7}}$ A/W.

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

Presented through this work is a steady state analytical model of the GaN HEMT based gas detector. GaN with high chemical and thermal stability provides promises for detectors in hazardous environments. However, HEMT sensor resolution must be improved to develop high precision gas sensors for automotive and space applications. The proposed model aids in systematical study of the sensor performance and prediction of sensitivities. The linear relation of threshold voltage shift at thermal equilibrium is used in predicting the sensor response. Numerical model for the reaction rates and the electrical dipole at the adsorption sites at the surface and metal/semiconductor interface have been developed and the sensor performance is analyzed for various gas concentrations. The validation of the model has been achieved through surface and interfacial charge adsorption-based gate electrode work function, Schottky barrier, 2DEG and threshold voltage deduction using MATLAB and SILVACO ATLAS TCAD. Further the applicability of $\text{g}_{d}$ (channel conductance) as gas sensing metric is also presented. With high $\text{I}_{D}$ and $\text{g}_{d}$ percentile sensitivities of 118.5% and 92 % for 10 ppm hydrogen concentration. The sensor shows capability for detection in sub-ppm levels by exhibiting a response of 0.043% for 0.01ppm (10 ppb) hydrogen concentration. The detection limit of the sensor (1% sensitivity) presented here is 169 ppb and the device current increases by $34.2~\mu \text{A}$ for 1ppb hydrogen concentration.

This paper presents a novel mm-wave branch line coupler with an enhanced isolation up to 72dB between the input ports, whereas a conventional branch line coupler usually gives less than 40dB of isolation. Similarly, there was also a small increase in the return loss. The two slots are situated a half wavelength apart acting as resonators and bandpass filters. The shape of the slot is rectangular with smaller sides equal to a half of the transmission line (TL) it is placed on (whether the ${Z}_{0}$ or $Z _{0}/\surd {2} ohm TL)$. The two slots must be identical to output symmetrical resonance frequencies. This is a promising coupler design particularly in an integrated microwave circuit such as, mixer or multiplier circuits at high frequencies (mmWave).

In this letter, the design and demonstration of a frequency doubler is presented utilizing the in-house developed technology of AlGaN/GaN multichannel Schottky barrier diode (SBD). The results show a promising GaN frequency multiplier that can be used in 5G transceivers. The conversion loss is about 15dB across the entire band (5-25 GHz). Further, the input return loss is below -10dB and the second harmonic output power is sitting at 5dBm across the whole band. Additionally, the output power of the fundamental harmonic is 40dBm or more less than the second harmonic output power level. Moreover, the conversion efficiency is about 21% for the optimum input power (20dBm). To our knowledge, this is the first multi-channel GaN-base Schottky diode frequency multiplier to be demonstrated.

In this article, we report the optimization of ohmic contact formation on AlGaN/GaN on low-resistivity silicon. For achieving this, a strategy of uneven AlGaN/GaN was introduced through patterned etching of the substrate under the contact. Various pattern designs (holes, horizontal lines, vertical lines, grid) and varied etch depth (above and below the 2-D electron gas) were investigated. Furthermore, a study of planar and nonplanar ohmic metallization was investigated. Compared to a traditional fabrication strategy, we observed a reduced contact resistance from 0.35 to 0.27 Omega. mm by employing a grid etching approach with a "below channel" etch depth and nonplanar ohmic metallization. In general, measurements of "below channel" test structures exhibited improved contact resistance compared to "above channel" in both planar and nonplanar ohmic metallization.

Intrinsic limits to temperature-dependentsub-strate loss for GaN-on-Si technology, due to the change in resistivity of the substrate with temperature, are evaluated using an experimentally validated device simulation framework. Effect of room temperature substrate resistivity on temperature-dependent coplanar waveguide (CPW) line loss at various operating frequency bands is then presented. CPW lines for GaN-on-high-resistivity Si are shown to have a pronounced temperature dependence for temperatures above 150 degrees C and have lower substrate losses for frequencies above the X-band. On the other hand, GaN-on-low-resistivity Si is shown to be more temperature insensitive and has lower substrate losses than even highly resistive Si for lower operating frequencies. The effect of various CPW geometries on substrate loss is also presented to generalize the discussion. These results are expected to act as a benchmark for temperature-dependent substrate loss in GaN-on-Si RF technology.

High performance coplanar waveguides (CPWs) on GaN membrane technology for AlGaN/GaN high electron mobility transistors (HEMTs) grown on low-resistivity (LR) Si substrates have been demonstrated in this work. The developed CPW technology shows a remarkable improvement in RF losses when the lossy Si beneath CPW structures is removed, resulting in comparable RF performance to that of CPW realized on high-resistivity (HR) Si and semi-insulating (SI) SiC substrates, with similar AlGaN/GaN top epitaxial layers. Experimental results, first to be reported, demonstrate transmission losses ( S21) of 0.47 dB and Q-factor of 20.77 for the CPW on GaN membrane technology, compared to S21 of 2.95 dB and Q-factor of 4.51 for the CPW on GaN-on-LR Si, at 40 GHz. Furthermore, the influence of substrate parasitics on RF performance of CPW on GaN-based HEMTs grown on various substrates was studied and analyzed by the extraction of transmission line parameters for frequencies up to 40 GHz. These findings offer viable integrated GaN-based HEMTs on LR Si technology suitable for high-power and high-temperature system applications at RF and millimeter-wave frequencies, when used in conjunction with high thermal coefficient materials such as diamond and AIN.

We demonstrate a quadrature branch-line coupler operating at 90 GHz on GaN-on-low resistivity silicon substrates (ρ <; 40 Ω.cm). To reduce the losses offered by the low-resistivity silicon at 90 GHz, a shielding technique is used where the silicon substrate is covered by a ground plane (Al metal). SiO 2 dielectric of thickness 10 μm is used as a spacer between the top metal and ground plane to further improve the performance of the coupler. Measured results showed return loss and isolation as low as -25 dB and -16 dB respectively, and coupling loss of -4 ± 0.5 dB from 81 GHz to 101 GHz. The output amplitude imbalance achieved was less than 1 dB. The coupler validates the shielding MMIC technology on GaN-on-low resistivity silicon substrate.

This paper presents novel multi-channel RF lateral Schottky-barrier diodes (SBDs) based on AlGaN/GaN on low resistivity (LR) (sigma = 0.02 Omega.cm) silicon substrates. The developed technology offers a reduction of 37 % in onset voltage, V-ON (from 1.34 to 0.84 V), and 36 % in ON-resistance, R-ON (1.52 to 0.97 to Omega.mm) as a result of lowering the Schottky barrier height, Phi(n), when compared to conventional lateral SBDs. No compromise in reverse-breakdown voltage and reverse-bias leakage current performance was observed as both multi-channel and conventional technologies exhibited V-BV of (V-BV > 30 V) and I-R of (I-R < 38 mu A/mm), respectively. Furthermore, a precise small-signal equivalent circuit model was developed and verified for frequencies up to 110 GHz. The fabricated devices exhibited cut-off frequencies of up to 0.6 THz, demonstrating the potential use of lateral AlGaN/GaN SBDs on LR silicon for high-efficiency, high-frequency integrated circuits applications.

In this paper we present for the first time a Gallium Nitride-on-Silicon (GaN-on-Si) anenometric flow sensor based on a gold (Au) thermoresistive hot-wire. The device was fabricated in a custom GaN process, with an etching process to release a membrane made of the GaN stack. This membrane thermally isolates the hot-wire, increasing its thermal efficiency, which was measured to be 1.04°C/mW. Testing was performed at mass flow rates from 0-4 SLPM using a custom gas rig. The sensitivity of the device, driven in a constant current mode at 3 different zero-flow temperatures, was compared showing an increase in peak sensitivity of 67% at 250°C compared to 150°C.

Novel MMIC spiral inductors on GaN-on-low-resistivity silicon (LR-Si) substrates (sigma < 40 Omega . cm) are demonstrated with enhanced self-resonance frequency (f(SRF))and Q-factor. The developed technology improves inductor performance by suppressing substrate coupling effects using air-bridge technology above benzocyclobutene dielectric as an interface layer on the lossy substrate. A 0.83-nH spiral inductor with peak Q-factor enhancement of 57% (Q = 22 at 24 GHz) and maximum f(SRF) of 59 GHz was achieved because of the extra 5-mu m elevation in air. An accurate broad-band model for the fabricated inductors has been developed and verified for further performance analysis up to 40 GHz. The proposed inductors utilize cost-effective, reliable, and MMIC-compatible technology for the realization of high-performance RF GaN-on-LR Si MMIC circuits for millimeter-wave applications.

We demonstrate that the highly resistive Si substrate in GaN-on-Si RF HEMTs does not act as an insulator, but instead behaves as a conductive ground plane for static operation and can cause significant back-gate-induced current collapse. Substrate ramp characterization of the buffer shows good agreement with device simulations and indicates that the current collapse is caused by charge-redistribution within the GaN layer. Potential solutions, which alter charge storage and leakage in the epitaxy to counter this effect, are then presented.

In this work, a viable passive components and transmission media technology is presented for THz-Monolithic Integrated Circuits (THz-MIC). The developed technology is based on shielded microstrip (S-MS) employing a standard monolithic microwave integrated circuit compatible process. The S-MS transmission media uses a 5-µm layer of benzocyclobutene (BCB) on shielded metalized ground plates avoiding any substrate coupling effects. An insertion loss of less than 3 dB/mm was achieved for frequencies up to 750 GHz. To prove the effectiveness of the technology, a variety of test structures, passive components and antennas have been design, fabricated and characterized. High Q performance was demonstrated making such technology a strong candidate for future THz-MIC technology for many applications such as radar, communications, imaging and sensing.