Trap Characterization Techniques for GaN-Based HEMTs: A Critical Review.

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have been considered promising candidates for power devices due to their superior advantages of high current density, high breakdown voltage, high power density, and high-frequency operations. However, the development of GaN HEMTs has been constrained by stability and reliability issues related to traps. In this article, the locations and energy levels of traps in GaN HEMTs are summarized. Moreover, the characterization techniques for bulk traps and interface traps, whose characteristics and scopes are included as well, are reviewed and highlighted. Finally, the challenges in trap characterization techniques for GaN-based HEMTs are discussed to provide insights into the reliability assessment of GaN-based HEMTs.

Molecular subtyping and prognostic risk characterization of head and neck squamous cell carcinoma based on lysosome-related genes.

Lysosomes possess a multitude of biological functions and are known to play a crucial role in the proliferation and metastasis of head and neck squamous cell carcinoma (HNSCC). This study aims to systematically investigate the potential role of lysosomes-related genes (LRGs) in the development of heterogeneity and prognosis in HNSCC. Publicly available transcriptome and clinical data of HNSCC were obtained and analyzed using consensus clustering to identify molecular subtypes. A risk model based on LRGs was developed and evaluated, including its correlation with clinical features, immune infiltration, drug sensitivity, and response to immune therapy. Gene set enrichment analysis was conducted to explore relevant pathways, and a prognostic nomogram model for HNSCC was constructed and evaluated. In this study, we identified 542 LRGs that exhibited differential expression in HNSCC, with 116 of these being significantly associated with overall survival. Two LRGs-derived molecular subtypes were identified, which displayed significant differences in prognosis and immune cell infiltration. Additionally, a prognostic risk model was developed, which included 13 LRGs. This model successfully divided HNSCC into low-risk and high-risk groups with different prognoses and immune cell infiltrations. The LRGs-derived risk signature was associated with immune infiltration, clinical features, drug sensitivity and immunotherapy response. The good prognosis of the low-risk group was linked to the activation of immune response-related processes and the inhibition of pathways such as necroptosis and neutrophil extracellular trap formation. Patients in the low-risk group had better immune therapy response, while those in the high-risk group had higher drug sensitivity. Finally, our nomogram, which combines clinical N staging and LRG-derived model, demonstrated excellent prognostic evaluation performance as shown by decision curve analysis and calibration curve. The study provides a comprehensive analysis of the expression and prognostic significance of LRGs in HNSCC, leading to the identification of 2 distinct molecular subtypes and the development of a risk model based on LRGs.

High-speed Ge-on-GaAs photodetector.

In this work, a germanium (Ge) on gallium arsenide (GaAs) photodetector is demonstrated with the optical response from 850 nm to 1600 nm, which has potential for monolithic integration with VCSELs on GaAs platform as transceiver working beyond 900 nm. The device exhibits a responsivity of 0.35A/W, 0.39 A/W and 0.11 A/W at 1000 nm, 1310 nm and 1550 nm, respectively and dark current of 8 nA at -1 V. The 10 µm diameter back-illuminated device achieves a 3-dB bandwidth of 9.3 GHz under -2 V bias. A donor-like trap at the interface between the Ge and GaAs collection layers is verified by capacitance-voltage curve and deep-level transient spectroscopy (DLTS) measurement, which impedes the depletion in GaAs collection layers.

Analysis of AM-to-PM conversion in MUTC photodiodes based on an equivalent circuit model.

High-speed, high power-handling photodiodes with sufficiently low amplitude-to-phase (AM-to-PM) conversion coefficients are critical components in the systems that generate ultra-stable microwave signals. This paper reports the AM-to-PM conversion in modified uni-traveling carrier photodiodes (MUTC-PDs) with 20 µm and 40 µm diameters. The contributions of AM-to-PM conversions from the carrier transit-time and impedance were quantified systematically based on a photocurrent-dependent nonlinear equivalent circuit model. It is found that the AM-to-PM conversion in 40 µm PD is dominated by the nonlinear impedance, while for 20 µm PD, the transit-time impacts the AM-to-PM conversion more significantly. These results imply that, for large PDs, the nonlinearity of the PDs' photocurrent-dependent impedance is the critical reason causing AM-to-PM conversion.

Timing Performance Simulation for 3D 4H-SiC Detector.

To meet the high radiation challenge for detectors in future high-energy physics, a novel 3D 4H-SiC detector was investigated. Three-dimensional 4H-SiC detectors could potentially operate in a harsh radiation and room-temperature environment because of its high thermal conductivity and high atomic displacement threshold energy. Its 3D structure, which decouples the thickness and the distance between electrodes, further improves the timing performance and the radiation hardness of the detector. We developed a simulation software-RASER (RAdiation SEmiconductoR)-to simulate the time resolution of planar and 3D 4H-SiC detectors with different parameters and structures, and the reliability of the software was verified by comparing the simulated and measured time-resolution results of the same detector. The rough time resolution of the 3D 4H-SiC detector was estimated, and the simulation parameters could be used as guideline to 3D 4H-SiC detector design and optimization.

Performance improvement of GaN-based light-emitting diodes grown on patterned Si substrate transferred to copper.

LEDs on Si offer excellent potential of low cost manufacturing for solid state lighting and display, taking advantage of the well-developed IC technologies of silicon. In this paper, we report how the performance of LEDs grown on Si can be improved. Multiple quantum well InGaN LED structure was grown on patterned silicon substrates and circular LEDs 160 µm in radius were processed. Fabricated LEDs were then transferred to an electroplated copper substrate with a reflective mirror inserted by a double-flip transfer process, to improve the light extraction efficiency and heat dissipation. The light output power of LEDs on copper increased by ~80% after the transfer. The operating current before the onset of light output power saturation also increased by 25% because of the good thermal conductivity of copper. The light output power of packaged LEDs on copper was 6.5 mW under 20 mA current injection and as high as 14 mW driven at 55 mA.