Chen-Peng Hsu

Performance of Flip-Chip Thin-Film GaN Light-Emitting Diodes With and Without Patterned Sapphires

We report on improved device performance of flip-chip (FC) GaN-based light-emitting diodes (LEDs) by combining patterned sapphire substrate (PSS) and thin-film techniques. It was found that an FC LED grown on a conventional planar sapphire exhibits a power enhancement factor of only 36.3% after the thin-film processes of substrate removal and surface roughening. In contrast, the as-fabricated FC LED grown on a PSS showed a power enhancement factor of up to 62.3% without any postprocess as compared with the light output power of an original conventional FC LED. Further intensity improvement to 74.4% could be achieved for the FC LED/PSS sample with the thin-film processes.

Enhanced light output power of thin film GaN-based high voltage light-emitting diodes

The characteristics of high-voltage light-emitting diodes (HVLEDs) consisting of a 64-cell LED array were investigated by employing various LED structures. Two types of HVLED were examined: a standard HVLED with a single roughened indium tin oxide (ITO) surface grown on a sapphire substrate and a thin-film HVLED (TF-HVLED) with a roughened n-GaN and ITO double side transferred to a mirror/silicon substrate. At an injection current of 24 mA, the output powers of the HVLEDs fabricated using a sapphire substrate and those fabricated using a mirror/silicon substrate were 170 and 216 mW, respectively. Because the TF-HVLED exhibited improved thermal dissipation and light extraction, it produced a greater output power than the HVLED fabricated using the sapphire substrate did.

Novel Electrode Design for Integrated Thin-Film GaN LED Package With Efficiency Improvement

This letter proposes a novel electrode structure for thin-GaN LED applications. The structure enhances light extraction and wall-plug efficiency in thin-GaN LEDs. To enhance light extraction in thin-GaN LEDs and solve current crowding effects caused by electrodes composed of metal, conventional n-GaN electrodes were replaced with ITO conductive films because of their high optical transparency. Simulation results show that the thin-GaN LEDs that use ITO as the nonshielded electrode have a more uniform current density distribution on the n-GaN surface and a higher average internal quantum efficiency than conventional metal electrodes. Furthermore, when a current of 200 mA was applied, the thin-GaN LEDs using the proposed electrode had a 40% increase in light-output power and a significant decrease in chip temperature compared to the use of conventional electrodes. The results indicate that the nonshielded ITO electrode design enhances the light extraction efficiency and avoids the accumulation of heat because of its uniform current density distribution.

A novel integrated structure of thin film GaN LED with ultra-low thermal resistance

This study proposes a novel packaging structure for vertical thin-GaN LED applications by integration of LED chip and silicon-based packaging process. The vertical thin film LED is directly mounted on package submount. The shortest thermal path structure from junction to package submount achieves the lowest thermal resistance of 1.65 K/W for LED package. Experimental results indicate that low thermal resistance significant improved forward current up to 4.6A with 1.125 × 1.125 mm² LED chip size.

Fabrication and Improved Performance of GaN LEDs With Finger-Type Structure

This paper demonstrates that vertical gallium nitride (GaN) light-emitting diodes (LEDs) with a finger-type current spreading structure (referred as F-LEDs), and wing-type vertical LEDs with embedded contact (W-LEDs) exhibit improved performance in output power and current spreading compared with conventional LED (C-LED). Although W-LED and F-LED designs allow improved light shading and current crowding, the extra finger-type structure promotes a better current spread, resulting in performance superior to that of C-LEDs and W-LEDs. Under an injection current of 350 mA, 329.39 mW of output power is obtained in F-LEDs corresponding to a performance enhancement of 39.3% and 20.3% compared with C-LEDs and W-LEDs, respectively. When the driving current was increased to 700 mA, the finger-type structure increased output power and efficiency droop reduction benefits were clearly observed. The F-LEDs exhibited 24% enhanced power and 23% improved droop in comparison with W-LEDs.

White thin-film flip-chip LEDs with uniform color temperature using laser lift-off and conformal phosphor coating technologies

We fabricated a phosphor-conversion white light emitting diode (PC-WLED) using a thin-film flip-chip GaN LED with a roughened u-GaN surface (TFFC-SR-LED) that emits blue light at 450 nm wavelength with a conformal phosphor coating that converts the blue light into yellow light. It was found that the TFFC-SR-LED with the thin-film substrate removal process and surface roughening exhibits a power enhancement of 16.1% when compared with the TFFC-LED without a sapphire substrate. When a TFFC-SR-LED with phosphors on a Cu-metal packaging-base (TFFC-SR-Cu-WLED) was operated at a forward-bias current of 350 mA, luminous flux and luminous efficacy were increased by 17.8 and 11.9%, compared to a TFFC-SR-LED on a Cup-shaped packaging-base (TFFC-SR-Cup-WLED). The angular correlated color temperature (CCT) deviation of a TFFC-SR-Cu-WLED reaches 77 K in the range of -70° to + 70° when the average CCT of white LEDs is around 4300 K. Consequently, the TFFC-SR-LED in a conformal coating phosphor structure on a Cu packaging-base could not only increase the luminous flux output, but also improve the angular-dependent CCT uniformity, thereby reducing the yellow ring effect.

The Efficiency and Reliability Improvement by Utilizing Quartz Airtight Packaging of UVC LEDs

This paper investigates a novel packaging that can effectively improve the efficiency and reliability of 280-nm deep ultraviolet (DUV) light-emitting diodes (LEDs). The experimental results showed that use of a quartz airtight package dramatically improves light extraction efficiency compared with conventional silicone encapsulant under a rated current of 20 mA. The transmittance of silicone decreases significantly as the wavelength below 300 nm. By enabling high transparency over a wide range of wavelengths in the DUV region, the quartz airtight package improved the light extraction efficiency of ultraviolet C LEDs. Thermal resistances of LEDs measured for two package types showed that junction temperatures were almost the same under the same driving current. In addition, the maintenance of radiative power when using the quartz airtight package was 13% higher than that when using the silicone package after 500 h aging test under 20 mA and 25°C. The silicone package LEDs exhibited significant intensity decay with aging time. The experimental results showed that the quartz airtight package is a reliable design that can effectively improve both efficiency and long-term reliability of DUV LEDs.

Enhancement of Light Extraction for InGaN LEDs by Means of Beveled Sapphire and Cup-Shaped Copper Sheeting

In this letter, InGaN light-emitting diode (LED) structures fabricated with beveled sapphire substrates and embedded in cup-shaped copper sheets are presented. Using Trace-Pro simulation, the shaped sapphire with an optimum beveled angle of 75° was determined to enhance the light extraction of LEDs. The thermal dissipation for high power LEDs can be improved by embedding them in cup-shaped copper sheets. At an injection current of 350 mA, the output powers of LED with conventional structure and LEDs with original and beveled sapphires, both embedded in cup-shaped copper sheets, are 325.4, 372.6, and 395.6 mW, respectively, while the power efficiencies are 27.6%, 31.5%, and 33.3%, respectively. It indicates that the light extraction and output power of LED devices can be enhanced with the aid of beveled sapphires and cup-shaped copper sheets.

Effect of the Flat and Pattern Surface Texturing on Light Extraction of GaN Flip-Chip Light-Emitting Diodes

Device performances of GaN-based flip-chip light-emitting diodes (FC LEDs) with planar and patterned sapphire substrates (PSS) were compared in this study. It was found that for the FC LED with planar sapphire, enhancement factor of luminous intensity can be raised to 107.5% after the processes of substrate removal and surface roughening. By contrast, for the FC LED with PSS, the intensity enhancement factor is already up to 169.5% without any post-processes as compared with the intensity of an as-fabricated conventional FC LED. Further intensity improvement to 205.1% can be achieved for the FC LED with PSS by employing subsequent processes such as substrate removal and surface roughening. These results indicate that the PSS approach is useful in improving light extraction of a nitride-based FC LED.