Yen-Hsiang Fang

The amorphous Si/SiC heterojunction color-sensitive phototransistor

An amorphous Si/SiC heterojunction color-sensitive phototransistor was successfully fabricated by plasma-enhanced chemical vapor deposition. The structure is glass/ITO/a-Si(n+-i)/a-SiC(p+-i-n+)/Al. The device is a bulk barrier transistor with wide-bandgap amorphous SiC emitter and base. The phototransistor revealed a very high optical gain of 40 and a response speed of 10 µs at an input light power of 5 µW and a collector current of 0.12 mA at a voltage of 14 V. The peak response occurs at 610 nm under 1-V bias and changes to 420 and 540 nm under 7- and 13-V biases, respectively.

The effect of trimethylgallium flows in the AlInGaN barrier on optoelectronic characteristics of near ultraviolet light-emitting diodes grown by atmospheric pressure metalorganic vapor phase epitaxy

The letter reports a theoretical and experimental study on the device performance of near ultraviolet light-emitting diodes (LEDs) with quaternary AlInGaN quantum barrier (QB). The indium mole fraction of AlInGaN QB could be enhanced as we increased the trimethylgallium flow rate. It was found the AlInGaN/InGaN LEDs can reduce forward voltage and improve light output power, compared with conventional GaN QB. By using advanced device simulation, it should be attributed to a reduction in lattice mismatch induced polarization mismatch in the active layer, which results in the suppression of electron overflow.

Contact resistance in metal-semiconductor systems

Abstract A new method for analyzing the influence of extrinsic parameters, such as contact shape and contact area, on the contact resistance of the metal-semiconductor system is developed. The method incorporates a previously established contact theory in the Transmission Line Model used earlier for rectangular and circular contacts. Theory is also extended to the circular ring contact in this article. Good agreement with measured results substantiates the applicability of the method for the design of contact shapes and areas in semiconductor device fabrication processes. Various contact shapes and areas are investigated and the total contact resistances are compared to one another. It is shown that the contact with long thin stripes possesses the lowest contact resistance.

Reduction of Efficiency Droop in InGaN Light-Emitting Diode Grown on Self-Separated Freestanding GaN Substrates

Using a GaN nanorod template in a hydride vapor phase epitaxy (HVPE) system can manufacture a freestanding GaN (FS-GaN) substrate with threading dislocation densities down to ~ 107 cm-2. In this letter, we report InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown on this FS-GaN substrate. The defect densities in the homoepitaxially grown LEDs were substantially reduced, leading to improved light emission efficiency. Compared with the LED grown on sapphire, we obtained a lower forward voltage, smaller diode ideality factor, and higher light-output power in the same structure grown on FS-GaN. The external quantum efficiency (EQE) of LEDs grown on FS-GaN were improved especially at high injection current, which brought the efficiency droop phenomenon greatly reduced at high current density.

Study of InGaN-Based Light-Emitting Diodes on a Roughened Backside GaN Substrate by a Chemical Wet-Etching Process

The InGaN-based light-emitting diodes (LEDs) with a roughened backside on the N-face surface of GaN substrate were fabricated through a chemical wet-etching process to increase light-extraction efficiency. The stable crystallographic etching planes were formed as the GaN {101̅1̅} planes. When the near-ultraviolet and blue LED were operated as a forward current of 20 mA, the output power of LEDs was improved from 13.2 and 19.9 mW to 25.6 and 24.0 mW, respectively. The different enhanced ratio is attributed to the different transmittance as a function of wavelength is caused from hexagonal pyramid on N-face GaN substrate after wet-etching process.

High efficiency and output power of near-ultraviolet light-emitting diodes grown on GaN substrate with back-side etching

In this paper, we investigate the differences between optical and electrical properties of near-ultraviolet (NUV) InGaN/GaN multi-quantum well light-emitting diodes (LEDs) grown on GaN substrate with a roughened back-side on the N-face surface of GaN substrate through a chemical wet-etching process, and on pattern sapphire substrate (PSS). Back-side etching-treated NUV-LEDs have larger output power than conventional NUV-LEDs, NUV-LEDs with wider wells and NUV-LEDs grown on PSS. When the NUV-LEDs were operated at a forward current of 20?mA, the output power of back-side etching-treated NUV-LEDs was improved by approximately 100, 106 and 8% compared with that of conventional NUV-LEDs, NUV-LEDs with wider wells and NUV-LEDs grown on PSS, respectively. This larger enhancement results from the improved light extraction that was attributed to the different transmittance because a hexagonal pyramid on the N-face GaN that was etched formed at the stable crystallographic etching planes of the GaN {1011} planes.

Comparison of different template structures for high quality and self-separation thick GaN growth

Two different template structures of dot air-bridges and nanorods were used for 300 µm GaN growth by hydride vapor phase epitaxy (HVPE). The selective growth of arrays of dot air-bridges and nanorods whose sidewalls coated with SiO2 are identified and exploited to form a compliant layer to decouple the impact due to the different thermal expansion and lattice mismatch between 300 µm overgrown GaN layer and the host sapphire substrate. As the process temperature cooling down from 1050 °C to room temperature in HVPE system, the 300 μm freestanding GaN substrates were obtained by the self-separation. The dislocation density was estimated by both the etching pit density method and cathodoluminescence (CL). The dislocation densities of the freestanding bulk GaN were lower than 5×106 and 5×107 cm-2 for the template structure of dot air-bridges and nanorods structure, respectively.