Papers by Author: W. Chang

Abstract: Electron-hole recombination-induced stacking faults have been shown to degrade the electrical characteristics of SiC power pin and MPS diodes and DMOSFETs with thick drift epitaxial layers. In this paper, we investigate the effects of bipolar injection induced stacking faults on the electrical characteristics of p+ ion-implanted high-voltage vertical-channel JFETs with 100-μm drift epilayers. The JFETs were stressed at a fixed gate-drain bipolar current density of 100 A/cm² for five hours, which led to degradation of the forward gate-drain p-n junction and on-state conduction. The degradation was fully reversed by annealing at 350 °C for 96 hours. Forward and reverse gate-source, transfer, reverse gate-drain, and blocking voltage JFET characteristics exhibit no degradation with bipolar stress. Non-degraded characteristics remain unaffected by annealing events. Consequently, should minority carrier injection occur in JFETs operating at elevated temperatures no stacking fault induced degradations are expected. This eliminates the need for specialty substrates with suppressed densities of basal plane dislocations in the fabrication of high-voltage SiC JFETs for high temperature applications.

Abstract: Myliobatidae is a family of large pelagic rays including cownose, eagle and manta rays. They are extremely efficient swimmers, can cruise at high speeds and can perform turn-on-a-dime maneuvering, making these fishes excellent inspiration for an autonomous underwater vehicle. Myliobatoids have been studied extensively from a biological perspective; however the fluid mechanisms that produce thrust for their large-amplitude oscillatory-style pectoral fin flapping are unknown. An experimental robotic flapping wing has been developed that closely matches the camber and planform shapes of myliobatoids. The wing can produce significant spanwise curvature, phase delays down the span, and oscillating frequencies of up to 1 Hz, capturing the dominant kinematic modes of flapping for myliobatoids. This paper uses dye flow visualization to qualitatively characterize the fluid mechanisms at work during steady-state oscillation. It is shown that oscillatory swimming uses fundamentally different fluid mechanisms than undulatory swimming by the generation of leading-edge vortices. Lessons are distilled from studying the fluid dynamics of myliobatoids that can be applied to the design of biomimetic underwater vehicles using morphing wing technology.