Improvements in reverse breakdown characteristics of THz GaAs Schottky barrier varactor based on metal-brim structure

The excellent reverse breakdown characteristics of Schottky barrier varactor(SBV)are crucially required for the application of high power and high efficiency multipliers.The SBV with a novel Schottky structure named metal-brim is fabricated and systemically evaluated.Compared with normal structure,the reverse breakdown voltage of the new type SBV improves from-7.31 V to-8.75 V.The simulation of the Schottky metal-brim SBV is also proposed.Three factors,namely distribution of leakage current,the electric field,and the area of space charge region are mostly concerned to explain the physical mechanism.Schottky metal-brim structure is a promising approach to improve the reverse breakdown voltage and reduce leakage current by eliminating the accumulation of charge at Schottky electrode edge.

Improvement of breakdown characteristics of an A1GaN/GaN HEMT with a U-type gate foot for millimeter-wave power application

In this study, the physics-based device simulation tool Silvaco ATLAS is used to characterize the electrical properties of an AlGaN/GaN high electron mobility transistor （HEMT） with a U-type gate foot. The U-gate AlGaN/GaN HEMT mainly features a gradually changed sidewall angle, which effectively mitigates the electric field in the channel, thus obtaining enhanced off-state breakdown characteristics. At the same time, only a small additional gate capacitance and decreased gate resistance ensure excellent RF characteristics for the U-gate device. U-gate AlGaN/GaN HEMTs are feasible through adjusting the etching conditions of an inductively coupled plasma system, without introducing any extra process steps. The simulation results are confirmed by experimental measurements. These features indicate that U-gate A1GaN/GaN HEMTs might be promising candidates for use in millimeter-wave power applications.

Experimental study of rotating wind turbine breakdown characteristics in large scale air gaps

When a wind turbine is struck by lightning,its blades are usually rotating.The effect of blade rotation on a turbine＇s ability to trigger a lightning strike is unclear.Therefore,an arching electrode was used in a wind turbine lightning discharge test to investigate the difference in lightning triggering ability when blades are rotating and stationary.A negative polarity switching waveform of 250/2500 μs was applied to the arching electrode and the up-and-down method was used to calculate the 50%discharge voltage.Lightning discharge tests of a 1：30 scale wind turbine model with 2,4,and 6 m air gaps were performed and the discharge process was observed.The experimental results demonstrated that when a 2 m air gap was used,the breakdown voltage increased as the blade speed was increased,but when the gap length was 4 m or longer,the trend was reversed and the breakdown voltage decreased.The analysis revealed that the rotation of the blades changes the charge distribution in the blade-tip region,promotes upward leader development on the blade tip,and decreases the breakdown voltage.Thus,the blade rotation of a wind turbine increases its ability to trigger lightning strikes.

Improved Breakdown Strength in(Ba_(0.6)Sr_(0.4))_(0.85)Bi_(0.1)TiO_3 Ceramics with Addition of CaZrO_3 for Energy Storage Application

（Ba（0.6） Sr（0.4））（0.85） Bi（0.1） TiO3 ceramics doped with x wt%CaZrO3（x= 0-10） were synthesized by solid-state reaction method. The effects of CaZrO3 amount on the dielectric properties and structure of（Ba（0.6）Sr（0.4））（0.85） Bi（0.1） TiO3 ceramics were investigated. X-ray diffraction results indicated a pure cubic perovskite structure for all samples and that the lattice parameter increased till x=5 and then slightly decreased. A homogenous microstructure was observed with the addition of CaZrO3. Dielectric measurements revealed a relaxor-like characteristic for all samples and that the diffusivity γ reached the maximum value of 1.78 at x=5. With the addition of CaZrO3, the dielectric constant dependence on electric field was weakened, insulation resistivity enhanced and dielectric breakdown strength improved obviously and reached 19.9 k V/mm at x=7.5. In virtue of low dielectric loss（tan d〈0.001 5）, moderate dielectric constant（er 〉1 500） and high breakdown strength（Eb 〉17.5 k V/mm）, the CaZrO3 doped（Ba（0.6）Sr（0.4））0.85 Bi（0.1） TiO3 ceramic is a potential candidate material for high power electric applications.

Field plate structural optimization for enhancing the power gain of GaN-based HEMTs

A novel source-connected field plate structure, featuring the same photolithography mask as the gate electrode, is proposed as an improvement over the conventional field plate （FP） techniques to enhance the frequency performance in GaN-based HEMTs. The influences of the field plate on frequency and breakdown performance are investigated simul- taneously by using a two-dimensional physics-based simulation. Compared with the conventional T-gate structures with a field plate length of 1.2 gm, this field plate structure can induce the small signal power gain at 10 GHz to increase by 5-9.5 dB, which depends on the distance between source FP and dramatically shortened gate FE This technique minimizes the parasitic capacitances, especially the gate-to-drain capacitance, showing a substantial potential for millimeter-wave, high power applications.

Influence of channel/back-barrier thickness on the breakdown of AlGaN/GaN MIS-HEMTs

The leakage current and breakdown voltage of AlGaN/GaN/AlGaN high electron mobility transistors on silicon with different GaN channel thicknesses were investigated.The results showed that a thin GaN channel was beneficial for obtaining a high breakdown voltage,based on the leakage current path and the acceptor traps in the AlGaN back-barrier.The breakdown voltage of the device with an 800 nm-thick GaN channel was 926 V@1 m A/mm,and the leakage current increased slowly between 300 and 800 V.Besides,the raising conduction band edge of the GaN channel by the AlGaN back-barrier lead to little degradation for sheet 2-D electron gas density,especially,in the thin GaN channel.The transfer and output characteristics were not obviously deteriorated for the samples with different GaN channel thickness.Through optimizing the GaN channel thickness and designing the Al GaN back-barrier,the lower leakage current and higher breakdown voltage would be possible.

Experimental Study on Insulation Properties of Epoxy Casting Resins Using High-frequency Square Waveforms

Although the transmission efficiency of high-voltage high-frequency transformers improve as the rated frequency increases,their insulation performance is severely affected.Through a high-frequency experimental platform,insulation tests on epoxy casting resins are carried out using high-frequency square wave signals.Subsequently,the variations of discharge inception and flashover voltages with frequency were analyzed based on the Secondary Electron Emission Avalanche(SEEA)model.The breakdown characteristics of epoxy casting resins are studied based on the Weibull distribution analysis and inverse power function model.Finally,an insulation test standard is proposed based on the breakdown characteristics of epoxy casting resins for dry-type HV-HF transformers.The results show that,the flashover voltages are noticeably reduced with the increase of frequency while the discharge inception is hardly affected by frequency.Moreover,the drop in breakdown voltages and insulation lifetime shortening are caused by high-frequency thermal effects.Also,the applied voltages of epoxy resin cast dry-type HV-HF transformers are recommended to be 3.27 to 3.49 times their maximum operating voltages in 1min applied withstand tests.The applied voltages of the induced withstand tests should be 2.20 to 2.52 times their maximum operating voltages.