Tai-You Chen

Hybrid Laplace transform/finite element method for one-dimensional transient heat conduction problems

Abstract The powerful method of analysis, involving the combined use of the Laplace transform and the finite element method, is applicable to the problem of time-dependent heat flow systems. The present method removes the time terms using the Laplace transform and then solves the associated equation with the finite element method. The associated temperature is inverted by the method of Honig and Hirdes. The present results are compared in tables with the corresponding exact solutions. It is found that the present method is stable and convergent to the exact solution. There exists no time step, thus the present method is a useful tool in solving long-time problems.

Performance investigation of GaN-based light-emitting diodes with tiny misorientation of sapphire substrates

GaN-based light-emitting diodes (LEDs) grown on c-plane vicinal sapphire substrates are fabricated and characterized. Based on the material quality and electrical properties, the LED with a 0.2 degrees tilt sapphire substrate (device A) exhibits the lowest defect density and high performance, while the LED with a 1.0 degrees tilt sapphire (device D) exhibits the highest one. At 2 mA, the extremely enhanced output power of 23.3% indicates of the reduction of defect-related nonradiative recombination centers in active layers for the device A. At 60 mA, the improved value is up to 45.7%. This is primarily caused by the formation of indium quantum dots in MQW which provides an increased quantum efficiency.

Implementation of an indium-tin-oxide (ITO) direct-Ohmic contact structure on a GaN-based light emitting diode

A GaN-based light-emitting diode (LED) with a direct-Ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and Au thermal-diffused and removed layer, is studied. By depositing an Au metallic film on the Mg-doped GaN layer followed by thermal annealing and removed processes, an ITO direct-Ohmic contact at p-GaN/ITO interface is formed. An enhanced light output power of 18.0% is also found at this condition. This is mainly attributed to the larger and more uniform light-emission area resulted from the improved current spreading capability by the use of an ITO direct-Ohmic contact structure.

Improved Performance of an InGaN-Based Light-Emitting Diode With a p-GaN/n-GaN Barrier Junction

An InGaN-based light-emitting diode with a p-GaN/n-GaN barrier junction is fabricated and investigated. Due to the built-in potential induced by this junction, superior current spreading performance is achieved. In addition, the suppression of the current-crowding phenomenon yields a reduced parasitic effect. Therefore, under an injection current of 20 mA, improved behaviors, including lower turn-on voltage, lower parasitic series resistance, and reduced p-n junction temperature, are achieved. In addition, due to the improved current-spreading ability, longer life-time, driving at medium current injection (60 mA), as well as significantly enhanced electrostatic discharge performance, are obtained.

On a GaN-Based Light-Emitting Diode With an Indium–Tin–Oxide (ITO) Direct-Ohmic Contact Structure

A GaN-based light-emitting diode (LED) with a direct-Ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and Au thermal-diffused and removed layer, is studied. By depositing an Au metallic film on the Mg-doped GaN layer followed by thermal annealing and removed processes, an ITO direct-Ohmic contact at p-GaN/ITO interface is formed. An enhanced light output power of 18.0% is also found at this condition. This is mainly attributed to the larger and more uniform light-emission area resulted from the improved current spreading capability by the use of an ITO direct-Ohmic contact structure.

Improved Current-Spreading Performance of an InGaN-based Light-Emitting Diode with a Clear P-GaN/N-GaN Barrier Junction

The InGaN-based light-emitting diode (LED) with a clear p-GaN/n-GaN barrier junction is fabricated and investigated. Due to the built-in potential induced by this junction, superior current spreading performance is achieved. In addition, the suppression of current crowding phenomenon yields the reduced parasitic effect. Therefore, under an injection current of 20 mA, improved behaviors including lower turn-on voltage, lower parasitic series resistance, and significantly enhanced electrostatic discharge (ESD) performance are presented.

Investigation of the Electrostatic Discharge Performance of GaN-Based Light-Emitting Diodes With Naturally Textured p-GaN Contact Layers Grown on Miscut Sapphire Substrates

The electrostatic discharge (ESD) characteristics of GaN-based light-emitting diodes (LEDs) with naturally textured p-GaN contact layers grown on c-axis miscut sapphire substrates are studied and demonstrated. Based on the machine model, the device grown on a 0.35° miscut sapphire shows the highest ESD tolerance, whereas the device grown on a 0.2° miscut sapphire exhibits the poorest tolerance. It is found that this phenomenon is primarily related to the presence of maximum capacitance Cm values rather than the difference in defect densities between LEDs. The variation in Cm values is caused by the parasitic capacitance effect induced by different p-GaN surface morphologies between the studied devices. This observation gives us a more reliable application in improving the ESD performance based on the device grown on a 0.35° miscut sapphire.

Improved performance of GaN-based light-emitting diodes by using short-period superlattice structures

Abstract An InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) with a ten-period i (undoped) -InGaN/p (Mg doped) -GaN (2.5 nm/5.0 nm) superlattice (SL) structure, was fabricated. This SL structure that can be regarded as a confinement layer of holes to enhance the hole injection efficiency is inserted between MQW and p-GaN layers. The studied LED device exhibits better current spreading performance and an improved quality, compared with a conventional one without SL structure. Due to the reduced contact resistance as well as more uniformity of carriers injection, the operation voltage at 20 mA is decreased from 3.32 to 3.14 V. A remarkably reduced reverse-biased leakage current (10−7–10−9 A) and higher endurance of the reverse current pulse are found. The measured output power and external quantum efficiency (EQE) of the studied LED are 13.6 mW and 24.8%, respectively. In addition, significant enhancement of 25.4% in output power as well as increment of 5% in EQE for the studied devices is observed, as the studied devices show superior current spreading ability and reduction in dislocations offered by the SL structure.

Comprehensive Temperature-Dependent Studies of Metamorphic High Electron Mobility Transistors With Double and Single

The temperature-dependent characteristics of meta morphic high electron mobility transistors (MHEMTs) with double and single δ-doped structure are studied and demonstrated. Due to the use of double δ-doped sheets, the current density in the channel layer and two-dimensional electron gas could effectively be increased. The excellent turn-on voltage of 1.18 (0.80) V, max imum drain saturation current of 544 (524) mA/mm, maximum extrinsic transconductance of 361 (312) mS/mm, unity current gain cutoff frequency of 55.06 GHz, and maximum oscillation frequency of 129.17 GHz are obtained at 300 (510) K for a 0.6 × 100 μm2 gate dimension double δ-doped MHEMT. In addition, using wide-bandgap InAlAs Schottky, spacer, and buffer layers, the carrier confinement could significantly be improved at high temperature. Therefore, excellent thermal stability is achieved for double δ-doped MHEMT. The device with a double δ-doped structure exhibits a considerably low temperature coefficient on threshold voltage (∂Vth/∂T) of 0.06 mV/K when the temperature is increased from 330 to 510 K, which is superior to previous reports of related high electron mobility transistors.

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The electrostatic discharge (ESD) characteristics of GaN-based light emitting diodes (LEDs) with textured p-GaN layers grown on c-axis vicinal sapphire substrates are studied and demonstrated. Based on the machine model, the device grown on a 0.35° tilt sapphire shows the highest ESD tolerance, whereas the one grown on a 0.2° tilt sapphire exhibits the poorest tolerance. This phenomenon is primarily influenced by the presence of maximum capacitance (C m ) values induced by a parasitic capacitance effect at the p-GaN/indium tin oxide interface rather than the difference in dislocation densities between LEDs.