In this letter we report the morphological, electrical and thermal transport properties of a high electron mobility transistor (HEMT) style epitaxial wafer, where an approximately 2000 nm thick GaN layer has been directly deposited on a bulk single crystal AlN (BCS AlN) substrate with no buffer layer in between, and also the experimental results of DC and RF properties of a HEMT device based on such a wafer. The buffer-free HEMT style sample achieved very smooth surface morphology and ultra-low microscopic roughness down to Ra = 0.172 nm over an area of 1 μm × 1 μm in AFM measurements. Contact electrical transport measurements with Van der Pauw geometry showed sheet carrier concentration of 7.3 × 10¹² cm⁻², Hall mobility of 2220 cm²/(V·s) and sheet resistance of 386 Ω/sq, resulting from the charge carriers of the two dimensional electron gas at the AlGaN/GaN heterojunction. The measured maximum trans-conductance G_m of the fabricated HEMT device was 250 mS/mm at a gate bias voltage of −1.8 V. With a relatively long gate length of 500 nm and a gate-to-drain distance of 4.7 μm, the f_T and f_max, derived from S-parameters measurements, are 25.9 and 54 GHz, respectively. Large-signal RF measurement exhibited a high linear power gain (G_p) of 25.2 dB and a peak output power (P_out) density of 7.2 W/mm@1.5 GHz, associated with a power-added efficiency (PAE) of 40.9%. Comparing with the structure with a 500 nm thick AlGaN buffer, the total thermal resistance of the structure in our device decreased by 44%. This work confirms the technical feasibility of fabricating GaN HEMT devices on BCS AlN substrates without any additional buffer layer, and the excellent electric and thermal transport properties of the simplified wafer structure indicate a bright future of BCS AlN-based GaN HEMT devices in ultra-high-frequency and high-power-density nitride electronics.

Bulk single-crystal aluminum nitride (BSC AlN) substrates are known to be ideal platforms for constructing high-power and DUV optoelectronic nitride devices. However, high-quality epitaxial growth of nitride films on BSC AlN and related characterization is still far from being well studied. The challenges and uncertainties in doing accurate thermal characterization on such heterostructures are not fully recognized. In this study, we successfully fabricated a buffer-free thin GaN/AlN heterostructure on a BSC AlN substrate via metal-organic chemical vapor deposition (MOCVD) technology. This heterostructure consists of a 140 nm-thick AlN homoepitaxial layer and a 480 nm-thick GaN epitaxial layer. Characterization results indicate that the prepared heterojunction has excellent crystal quality and smooth surface morphology. To accurately obtain the thermophysical parameters of the heterostructure, this study employed broadband frequency domain thermoreflectance (BB-FDTR) technology, and careful measurements with detailed data analysis were demonstrated. In addition to showing the feasibility of epitaxial growth of high-quality thin film GaN directly on BSC AlN substrates, this study also provides key experimental data for evaluating the heat dissipation advantages of GaN/AlN heterostructures.

In this paper, a compact and low-power sub-THz direct-conversion receiver with a second-harmonic-remixed LO chain is proposed. Based on a common-mode second-harmonic-enhanced network, the common-mode second-harmonic voltage at the drains of the common-source differential pair in the tripler is enhanced and mixed with the fundamental voltage at the gate to generate additional differential third-harmonic voltage. Hence, the saturation output power and efficiency of the triplers used in the LO chain have been significantly improved. The power consumption of the LO chain employed in the receiver is as low as 65 mW. Measurement results demonstrate that the receiver achieves a conversion gain of 30.5 dB and a 3-dB RF bandwidth of 34 GHz, while the in-band minimum noise figure is 9.9 dB.

In recent years, position-sensitive detectors (PSDs) have found widespread application in displacement measurement, optical measurement, imaging, and laser communication, owing to their high spatial resolution and rapid response capabilities. However, the performance and operating mechanisms of perovskite-based PSDs remain insufficiently elucidated. In this work, we fabricated a high-sensitivity self-powered PSD based on a ZnO/P(VDF-TrFE)-CH₃NH₃PbI₃(MAPbI₃) heterojunction. Systematic optimization revealed an optimal P(VDF-TrFE) doping concentration of 5 mg/mL, enabling the device to achieve a remarkable positional sensitivity (PS) of 307.03 mV/mm with a minimum nonlinearity of 1.02%. Furthermore, the intrinsic pyroelectric property of P(VDF-TrFE) induces a significant pyroelectrically enhanced lateral photovoltaic effect (LPE), boosting the PS to 511.33 mV/mm—an enhancement of 166.5%. The heterojunction PSD maintains effective operational performance over an electrode spacing range of 0.5−2.2 mm. While the LPE response declines with increasing spacing, a considerable pyroelectric effect (PE)-enhanced PS of 70.67 mV/mm is retained even at 2.2 mm. Importantly, we demonstrate multi-wavelength imaging by exploiting both the inherent LPE response and its pyroelectrically enhanced counterpart, with imaging intensity tunable via electrode spacing control. This study provides crucial insights into the LPE behavior of the heterojunction and systematically clarifies the mechanism by which the PE modulates device performance and imaging capabilities.