| dc.description.abstract | High voltage light-emitting diode (HV LED) is a novel chip array formed as a series connection structure by joining multi-microchips. HV LED can be operated not only by using the DC voltage directly but also by the full-wave voltage converted from the AC one via the full-wave rectification. HV LED technology can be used to drive the LED at high operated voltage so that the current in each micro-chip can be decreased. Since the HV LED is based on designs of multi-microchips and small working current, the current spreading of LED chip will be more uniform.
In this study, we propose and establish a numerical model using finite element method. This numerical model can be used to calculate and analyze the distributions of current density and temperature in HV LED under two operating situations, DC or full-wave. Moreover, the simulation results of our proposed model were verified by two measuring experiments, temperature and optical measurements. The simulation results of the current density distribution in the active layer are agreed with the measured ones obtained by the optical experiment. We found that the phenomenon of current crowding will appear more obviously around the electrode edge when the input power is increased.This phenomenon also causes that the temperature near the crowded area is raised. When the input power is at 280mW, the light output powers of the HV LED by operating at DC and full-wave voltages are 45.63mW and 54.48mW, respectively. Because the HV LED driven by full wave voltage can decrease the heat accumulation, they will have lower junction temperature. Thus, the full-wave driving condition is better than the DC one under the same input power.
Finally, our proposed model can be further used to analyze the design of the electrode pattern. For the p electrode design, the current crowding effect will appear at the n electrode edge when the lateral length of p electrode is increased. Then the hot spot phenomenon at the n electrode edge is generated, the junction temperature in LED chip will be also increased due to the more serious current crowding at the n electrode edge. When the p-electrode is enlarged to the length of 40μm, the junction temperature can be reduced about 0.9K. When the length of p-electrode is reduced as 40μm, the junction temperature will be arisen about 2K. When we change the n-electrode pattern like the long narrow strip one, the current density distribution is more uniform on each micro-chip. Moreover, the p-electrode and modified n-electrode are both extended as 80μm; the junction temperature in LED chip at quasi-steady state is 340.5K (4.6K lower than the conventional one). The more extended length, the better thermal and electrical behaviors.
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