Ying Wang

A compact differential microwave sensor, based on an “eye-like” complementary split-ring resonator (CSRR), for complex permittivity detection to address the dual challenges of mitigating interresonator coupling and achieving a high sensitivity, is proposed in this work. The “eye-like” CSRR is split into two separate differential half-CSRRs by a slot gap with a “ocular pupil” (OP) at the center; thus, a differential sensing mode against environmental common mode interference is realized. By optimizing the slot gap space and the radii of the “OP,” the interresonator coupling is minimized. Moreover, by integrating two symmetric interdigital structures on both upper and lower “eyelids,” the electrical field distribution is intensified substantially, thereby enhancing sensitivity. The geometry and dimensions of the sensor are optimized based on a mathematical analysis of an equivalent lumped RCL circuit model of CSRR, which also provides the design principle of the sensor. The developed sensor is fabricated and tested with a compact geometric size of 19×14×0.76 mm. The device prototype is measured and characterized using water–ethanol solutions. Compared with the previous similar works, the sensor is small in size and has a high frequency detection resolution (FDR) of 20.1 MHz/ $\Delta \varepsilon ^{\prime }$ , with a sample volume as small as $0.41~\mu $ L.

The etching process is pivotal in the fabrication of GaN-based devices, directly influencing their performance. This study presents an enhanced low-damage hybrid cyclic etching technology for AlGaN/GaN heteroepitaxy structures, combining oxygen plasma oxidation with HCl etching to achieve atomic-scale precision. By carefully controlling the oxidation layer thickness, this method preserves a low surface roughness of 0.228 nm and achieves an etching rate of 0.45 nm/cycle with excellent consistency. Compared to conventional ICP dry etching and KOH wet etching evaluated under identical experimental conditions, the proposed technique significantly reduces etching-induced-damage. When applied to the ohmic contact formation of AlGaN/GaN HEMTs using the same metallization scheme and annealing process, devices fabricated with the new hybrid etching method demonstrated a significantly lower contact resistance of 0.76 Ω7mm compared to those using conventional ICP etching (1.21 Ω7mm) or no additional etching after SiN removal (1.37 Ω7mm). Furthermore, these devices exhibited a threshold voltage of -6.5 V, a saturation drain current density of 880.55 mA/mm, and a knee voltage of approximately 4 V. This method also demonstrates potential for recessed gate etching and post-dry-etching damage repair, offering a promising solution for low-roughness surfaces processing with broad application prospects.

Gallium nitride high-electron mobility transistors (GaN-on-Si HEMTs) on silicon substrates exhibit significant potential in the field of radio frequency devices due to its high frequency, high-speed, and low-cost advantages. Nevertheless, the thermal stress caused by the mismatch in lattice parameters and the coefficient of thermal expansion between the Si substrate and GaN epitaxial layer constrains the device performance. Due to different temperature distributions in different scenarios, there is an urgent need for a model that can balance the contradiction between epitaxial growth and device operation for the joint design of GaN-on-Si HEMTs’ epitaxial structures. This work presents thermal stress formulas specifically designed for GaN epitaxial growth and device operation, based on Townsend’s theory of multilayer film curvature and stress. Furthermore, our work innovatively proposes a multiscenario coupled GaN-on-Si HEMTs’ epitaxial structure design model based on multiobjective optimization. The model represents a combined objective function incorporating thermal stress experienced during device operation and epitaxial growth, as well as the total thermal resistance and bulk thermal conductivity of the epitaxial layer. Through simulation validation, the model has been proved to effectively calculate the optimized epitaxial structure parameters across multiple scenarios to meet diverse requirements. And it can effectively reduce the operating temperature. This model can accelerate the process of GaN epitaxial design and provide an effective method for enhancing the performance of GaN-on-Si HEMTs.

In this paper, we propose a novel design of GaN-based planar Gunn oscillator as terahertz signal source. The oscillator has multiple gates and each gate can be individually biased. By controlling gate bias voltage and distance to the barrier layer, the output oscillating current can be formed in such a way that the higher harmonics are more powerful than the fundamental one. This is because multiple Gunn domains created under the gates in the channel are synchronized. Compared to the conventional single-domain diode, this multi-gate configuration not only increases the frequency and output power but also provides superior harmonic control, leading to higher efficiency and more stable operation at terahertz frequencies. We will review the design details and analysis on domain forming conditions using a physics-based numerical model. The optimal parameters for high power, high DC-RF conversion efficiency and high frequency will be given. Specifically, in dual-gate device, when it works in the dual-domain mode, the second harmonic is enhanced, reaching a frequency of 310.5 GHz with 7.6 mW of power and 11.2% efficiency. The tri-gate device operating in tri-domain mode further enhances the third harmonic to 417.0 GHz, with 9.57 mW of power and 9.23% efficiency. The multi-gate structure allows for more efficient harmonic generation, greater frequency tunability, and better power management, all of which are crucial for advanced terahertz application such as mixing, frequency multiplexing, and signal amplification.

To improve the breakdown voltage of Ga2O3 vertical Schottky Barrier diode (SBD), this article innovatively proposes a novel field plate-trench composite structure. Different from the traditional field plate structures and trench structures, the proposed composite structure innovatively adds a shallow trench under the field plate structure, greatly reducing the depth of the trench and relieving the etching difficulty. At the same time, simulation results show that the field plate-shallow trench composite structure can increase the breakdown voltage of the device from 538 V to 945 V, which is nearly 1.76 times higher than that of traditional devices. The new field plate-trench composite structure greatly improves the breakdown voltage of Ga2O3 vertical SBD, while also increasing the forward characteristics and the reliability of the device, having superiority in size, and alleviating the difficulty of the process. Our research work also has guiding significance for the research of other wide bandgap power devices.

In this paper, we propose a novel gallium nitride-based multi-two-dimensional-electron-gas (2DEG)-channel self-parallel Gunn diode (SPD) for the first time. In the SPD, a trench anode is etched through at least the bottommost 2DEG channels, which splits all 2DEG channels into two shorter channels with lengths of L₁ and L₂. Therefore, one SPD is just equal to several shorter diodes in parallel; as a result, we call it a self-parallel Gunn diode. In the symmetrical SPD, the component of fundamental frequency is nearly multiplied as compared with the regular Gunn diode. In the asymmetrical SPD (L₂ = nL₁, n is a positive integer), the harmonic components are greatly enhanced, specially the n^th harmonic. Our work demonstrates that the GaN-based terahertz SPD not only offers an easy transfer between two different frequencies, but also realizes the simultaneous enhancement of oscillation power and frequency.

GaN-based planar Gunn diodes are promising terahertz sources for monolithic microwave and terahertz integrated circuits (MMICs and MTICs, respectively) due to high output power and easiness of fabrication and circuit integration. However, high lateral current in the 2DEG channel may lead to failures such as early breakdown and suppression of oscillations. In this paper, we will, for the first time, systematically investigate the thermal effect on DC IV and output RF characteristics of AlGaN/GaN hetero-structural planar Gunn diodes on different substrates including diamond, SiC, Si and sapphire. Our simulation results show that the best RF output performance comes with the devices on diamond substrate and no oscillating current is observed for devices on sapphire substrate. The suppress of Gunn oscillation in the device on sapphire is mainly due to the excessive heat generated in the channel that leads to increase of the dead zone and attenuation of electronic domains. These results will lay theoretical and experimental foundation for realizing not only milliwatt GaN-based terahertz semiconductor oscillators but also other power devices.