Browsing by Author "Wu, Ping-Chen"

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Enhanced Light Extraction of High-Voltage Light Emitting Diodes Using a Sidewall Chamfer Structure
(2017-06-01)
Wu, Ping-Chen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
;
Wuu, Dong-Sing
;
電子工程學系及電子研究所
High-voltage light-emitting diodes (HV-LEDs) were prepared with 4 x 2 microcells. A novel technique for enhancing the light extraction of HV-LEDs by using wet-etched chamfer structures in the sidewalls of each cell is proposed. The thicknesses of the u-GaN layers used were in the range of 3-7 mu m. Simulations revealed that the light extraction of the HV-LEDs with chamfer structures would be enhanced by increasing the u-GaN thickness. The output power (@ 80 mA) of the HV-LEDs without chamfer structures was 538.0-539.1 mW. However, the output powers (@ 80 mA) of the HV-LEDs with chamfer structures and u-GaN thicknesses of 3, 5, and 7 mu m were increased to 555.3, 573.1, and 561.6 mW, respectively. The 7 mu m thick u-GaN layer led to an extremely large thickness of the HV-LED epitaxial structure, which caused difficulties during the wet etching process. Because of the apparent decrease in the wet etching rate of GaN, the damages to the epitaxial structures of the HV-LED with the 7 mu m thick u-GaN was more obvious. Therefore, the device's light extraction was slightly reduced. The results confirm that chamfer structures are highly useful for improving the performance of HV-LEDs, especially for the u-GaN thickness of 5 mu m.
Improved Performance and Heat Dissipation of Flip-Chip White High-Voltage Light Emitting Diodes
(2017-03)
Wu, Ping-Chen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
;
Wuu, Dong-Sing
;
電子工程學系及電子研究所
Three types of white light emitting diodes (LEDs) were constructed with 4 x 1 micro-cells. The first one is the convectional LED series connected with four cells using the wire-bonding process. The other two devices are 4 x 1 lateral-type and 4 x 1 flip-chip high-voltage LEDs (HV-LEDs) using the interconnection technique. The convectional LED, lateral-type HV-LED, and flip-chip HV-LED are denoted as C-LED, L-HV-LED, and FC-HV-LED, respectively. Moreover, the white LEDs were formed by combining the blue LED chips and the phosphor-dispensing method. The thermal resistances (at 20 mA) of C-LED, L-HV-LED, and FC-HV-LED were 14.5, 59.2, and 12.2 K/W, respectively. In addition, the surface temperatures (at 20 mA) of these three devices were 30.74-31.82, 65.93-68.95, and 27.01-27.96 degrees C, respectively. Obviously, the heat dissipation of L-HV-LED was much worse than that of C-LED. However, via the fabrication of the flip-chip structure, the heat dissipation of HV-LED can be enhanced significantly. At an injection current of 100 mA, the luminous efficiencies of C-LED, L-HV-LED, and FC-HV-LED were 80.8, 81.0, and 91.8 lm/W, respectively. Furthermore, at the same forward voltage, a higher current can be driven in the FC-HV-LED, leading to an apparent improvement in the luminous efficiency. According to our calculation, the emission size of HV-LED was only 84% compared with that of C-LED. On the other hand, the fabrication of HV-LED can be performed without the wire-bonding process. This indicates that the FC-HV-LED possesses not only lower production costs but also higher optoelectronic performance than that of the C-LED.
Improved Performance of High-Voltage Vertical GaN LEDs via Modification of Micro-Cell Geometry
(2017-06-01)
Wu, Ping-Chen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
;
Wuu, Dong-Sing
;
電子工程學系及電子研究所
Vertical-type high-voltage light-emitting diodes (HV-LEDs) with 2 x 2 micro-cells were fabricated on Cu substrates, and the micro-cell geometry was modified to enhance the optoelectronic performance. The current spreading in micro-cell is most dominantly affected by the distance between electrode and edge of chip. When square cells were combined in a HV-LED, the device performance was poor due to an obvious current-crowding phenomenon that occurred near the electrodes. This was attributed that the electrodes in these four square micro-cells were all far away from the edges, resulting in the severe current-crowding phenomenon. On the contrary, as the HV-LED was prepared with four rectangle, triangle, or L-shaped micro-cells, the electrodes were close to the edges of micro-cells and the current spreading effect can be easily improved. Although a HV-LED connected with L-shaped cells possessed a better current spreading effect and a lower surface temperature, the light extraction was relatively low because of an electrode-shading loss effect. When triangular cells were used to prepare the HV-LED, the device achieved a superior optoelectronic performance compared with that of other cells because of a lower current-crowding effect and a more uniform light emission. After an epoxy package process, a lower forward voltage of 14.9 V and a higher output power of 353.2 mW were obtained using this HV-LED at an injection current of 80 mA. Additionally, the wall-plug efficiencies of this device at 20 and 80 mA were 41.1% and 29.7%, respectively. The results confirm that the design of triangular cell is beneficial for enhancing the optoelectronic performance of HV-LEDs. Furthermore, the fabrication processes of vertical LEDs have high potential for HV-LED applications.