We demonstrate GaN-based double-layer electrode flip-chip light-emitting diodes (DLE-FCLED) with highly reflective indium-tin oxide (ITO)/distributed bragg reflector (DBR) p-type contact and via hole-based n-type contacts. Transparent thin ITO in combination with TiO 2 /SiO 2 DBR is used for reflective p-type ohmic contact, resulting in a significant reduction in absorption of light by opaque metal electrodes. The finely distributed via hole-based n-type contacts are formed on the n-GaN layer by etching via holes through p-GaN and multiple quantum well (MQW) active layer, leading to reduced lateral current spreading length, and hence alleviated current crowding effect. The forward voltage of the DLE-FCLED is 0.31 V lower than that of the top-emitting LED at 90 mA. The light output power of DLE-FCLED is 15.7% and 80.8% higher than that of top-emitting LED at 90 mA and 300 mA, respectively. Compared to top-emitting LED, the external quantum efficiency (EQE) of DLE-FCLED is enhanced by 15.4% and 132% at 90 mA and 300 mA, respectively. The maximum light output power of the DLE-FCLED obtained at 195.6 A/cm 2 is 1.33 times larger than that of the top-emitting LED obtained at 93 A/cm 2 .

We investigated the reverse leakage current characteristics of InGaN/GaN multiple quantum well (MQW) near-ultraviolet (NUV)/blue/green light-emitting diodes (LEDs). Experimental results showed that the NUV LED has the smallest reverse leakage current whereas the green LED has the largest. The reason is that the number of defects increases with increasing nominal indium content in InGaN/GaN MQWs. The mechanism of the reverse leakage current was analyzed by temperature-dependent current–voltage measurement and capacitance–voltage measurement. The reverse leakage currents of NUV/blue/green LEDs show similar conduction mechanisms: at low temperatures, the reverse leakage current of these LEDs is attributed to variable-range hopping (VRH) conduction; at high temperatures, the reverse leakage current of these LEDs is attributed to nearest-neighbor hopping (NNH) conduction, which is enhanced by the Poole–Frenkel effect.

High power flip‐chip light‐emitting diodes with distributed n‐type via‐hole‐based two‐level metallization electrodes (TLM‐FCLED) were fabricated and investigated. Comparison tests altering Ni metal thickness and annealing temperature were performed to optimize the reflectivity of Ni/Ag reflective layer, which enhanced the light extraction efficiency. On the other hand, via‐hole‐based n‐contact electrodes structure increased the utilization ratio of active region area, and the introduction of first metallization layer allowed n‐contact to be arranged uniformly on the entire n‐GaN surface, which exhibited a more favorable current spreading uniformity. As a result, the light output power (LOP) of TLM‐FCLED was 9.23 and 26.55% higher than that of conventional high power LED (CHP‐LED) at 350 and 1050 mA. The CHP‐LED exhibited 13.39% external quantum efficiency (EQE) degradation from 350 to 1050 mA, whereas the TLM‐FCLED exhibited only 6.88% EQE degradation. It is noted that the maximum LOP was about 1264 mW at 1830 mA, thereby, suggesting the potential of its application in ultra‐high power applications.

Nitride‐based high power LEDs with finger‐like SiO2 current blocking layer (CBL), three‐dimensional (3D) patterned step‐like ITO double layers and wavy sidewalls were fabricated. The finger‐like SiO2 CBL beneath finger‐like p‐electrode was designed to prevent current crowding effect, thereby facilitating uniform current spreading over the entire chip. In addition, 3D patterned step‐like ITO double layers, including alternating 230 nm thick patterned upper step ITO layer and 100 nm thick lower step ITO layer, were formed by combining photolithography and aqua regia etchant. We showed that the top light extraction efficiency of high power LEDs can be significantly enhanced by taking 3D patterned step‐like ITO. The light output power of high power LEDs with 3D patterned step‐like ITO double layers is 13.9% higher than that of LEDs with smooth ITO layer. High‐power LEDs with wavy sidewalls was fabricated by an optimized mask design in conjunction with dry etching process based on Cl2/BCl3 to improve light extraction efficiency at the horizontal direction. We demonstrated that light output power of high power LEDs with wavy sidewalls can be improved by 11% as compared to LEDs with flat sidewalls.

In this work, one type of direct current high voltage light‐emitting diode (DC‐HV LED) and six types of alternating current high voltage LEDs (AC‐HV LEDs) are demonstrated. Comparative current‐voltage (I–V) and light output power (LOP)‐current (L‐I) characteristics are performed between 24 V DC‐HV LED and 24 V AC‐HV LEDs with eight working cells. The AC‐HV LED has relatively larger wall‐plug efficiency (WPE) than DC‐HV LED over a current density range from 0 to 152 A cm−2. The effect of the layouts on the optical and electrical properties of AC‐HV LEDs is further investigated. Owing to larger heat dissipation area and fewer number of chips, our combined numerical and experimental results demonstrate that the AC‐HV LED I has a more favorable current spreading uniformity compared to other AC‐HV LEDs. In addition, it is found that the LOP of AC‐HV LEDs is dependent on both the number of working cells and the ratio of radiation area to total chip area. Larger ratio of light emission area to total chip area can be obtained by decreasing the area of rectifier cells. Therefore, in order to achieve a much higher LOP, AC‐HV LEDs have to be designed with smaller area of rectifier cells and with more working cells simultaneously.

Through the analysis of the temperature-dependent current–voltage (I–V) characteristics of the fabricated InGaN/GaN high-power blue-light-emitting diodes (LEDs), the low-bias region was confirmed to be dominated by tunneling current, while the medium-bias region was dominated by diffusion-recombination current. Electrons and heavy holes appeared to play similar roles in the tunneling current of the fabricated LEDs, with no apparent dominant tunneling entity determined by characteristic energy as previous works suggested. After 1000 h of high-temperature/current stress, the medium-bias regions of the I–V curves of LEDs remained almost unchanged, while the current in the low-bias region was greatly enhanced by the stress, which confirmed the different carrier transport mechanism behaviors in the low- and medium-bias regions. Further comparison between the I–V characteristics of the unstressed and stressed LEDs suggested that the change in I–V curve was associated with the increase in defect density and the apparent doping concentration in the InGaN/GaN multiple-quantum-well (MQW) active region.

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