Optimization of photomixers and antennas for continuous-wave terahertz emission

We have studied terahertz emission from interdigitated finger photomixers coupled to planar antenna structures. Using both pulsed and continuous-wave excitation, polarization measurements reveal that the antenna design dominates the properties of the radiated output at frequencies below 0.6 THz, while the efficiency at higher frequencies is additionally dependent on the design of the photomixer fingers. We have produced terahertz maps of the device, characterizing the photomixer by measuring the generated power as a function of the excitation position. Together, these measurements have allowed us to understand better the distinct roles of the photomixer and antenna in emission at different fre

Low-Cost Compact Integrated Rectenna for Implantable Medical Receivers

This work describes a novel fully integrated rectenna circuit using tunnelling-based devices for implanted medical devices. An ASPAT (Asymmetric Spacer Layer Tunnel Diode) was used as the active rectifier due to its high non-linearity and temperature insensitivity features. A miniaturized geometry rectenna ( 1×5 mm 2 ) with improved matching characteristics was demonstrated, by integrating a Cockcroft-Walton rectifier with an L-shaped planar folded antenna structure operating at ISM frequency bands. The circuit performance was experimentally explored at various separation distance between transmitter and receiver units. For a 5cm transmission set-up, the rectenna with a single-stage rectifier delivered 0.8V output at 20dBm transmit power. An extended doubler configuration exhibited enhanced performance when multiple stages are used, is predicted to reach 0.24mW output power at 23dBm transmit power and yielding ~1.6V output voltage with an efficiency of 0.12%. These findings can assist in compensating for the degraded antenna gain attributed to the extremely small effective-radiating area of 0.04λ . Furthermore, the ability of controlling the antenna input impedance helps in circumventing the requirement for a matching circuitry thereby offering further reduction in chip size.UWIPOM2 project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 857654

Miniaturized Folded Antenna with Improved Matching Characteristic for mm-wave Detections

UCMMT 2021 - UK-Europe-China Work. Millimetre-Waves Terahertz Technol., Sep. 2021UWIPOM2 project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 857654

The synthesis and characterization of Cu2ZnSnS4 thin films from melt reactions using xanthate precursors

Kesterite, Cu2ZnSnS4 (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu2ZnSnS4 (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10+15 vs 4.55 × 10+17 cm−3) and mobility (87.1 vs 11.1 cm2 V−1 s−1). The Hall coefficients were 2.36 × 103 versus 13.7 cm3 C−1

Optimization of Gate Recess Step and Elimination of the Dome Effect for Highly Reliable and Reproducible Novel pHEMT Device

We report a comprehensive etching study on the gate recess-processing step in novel pseudomorphic High Electron Mobility Transistor (pHEMT) fabrication. The experiments focused on the etchant composition and elimination of the cap layer residue (Dome Effect) at the etching trenches. The optimised processing flow using highly selective Succinic Acid is aimed for moderate cap layer etching, with an etching rate of 240 Å/min and InGaAs-InAlAs selectivity of 140. The percentage of the Dome height to etching depth is 30%, consistent throughout the etching surface, however, can be further improved by controlling the etching time to the etch-stop layer. The optimised processing steps will enhance the device’s robustness, especially in the complete processing steps for Monolithic Microwave Integrated Circuit (MMIC) fabrication, tailored for high-gain and low noise in satellite communication applications

An X-band MMIC Low Noise Amplifier Design with In0.7Ga0.3As/In0.52Al0.48As pHEMT

A low noise amplifier (LNA) design operating at Xband frequency range of 8 – 12 GHz using 0.25 µm In0.7Ga0.3As/In0.52Al0.48As pHEMT is presented. The target specifications of the MMIC LNA design are then addressed, the performance constraints and compromises that arise in the design of circuit topologies, biasing networks and matching configurations are also discussed. The design and analysis of the single input single-ended output, single and double stage LNAs are presented using all of the criteria. The simulations setup successfully showed an X-band LNAs are designed to match a 50 Ω input and output impedance. The proposed design is an MMIC LNA that combines high performance with low cost and avoids expensive external components

Critical state alignment and charge accumulation in triple barrier resonant tunnelling structures

We report observations of resonant tunnelling features in the current-voltage (I(V)) characteristics of a series of triple barrier resonant tunnelling structures (TBRTS) due to the critical alignment of the n=1 confined states of the two quantum wells within the active region. Charge accumulation in the first QW of these structures has a significant effect on the I(V) characteristics of the resonances. A nominally symmetric TBRTS and asymmetric TBRTS, with decreasing second well widths, have been studied, with observations of charge accumulation affecting the critical alignment in both symmetric and asymmetric designs. We demonstrate that in highly asymmetric structures the critical alignment can occur coincident to the Fermi level in the emitter, and remains on resonance at higher bias than is expected due to charge accumulation in the structure. With great renewed interest in tunnelling structures for high frequency (THz) operation, the understanding of device transport and charge accumulation is critical