Electroluminescence (EL) measurement technology for micro-LEDs is one of the most important issues to be addressed to accelerate the commercialization of micro-LED displays. Since it has a submicron dimension in the electrode of a micro-LED, EL inspection that requires contact with the electrode need too much time and cost in the process. Only photoluminescence (PL) measurement with low accuracy or measurement after the final transfer to the display panel has been performing at least in the mass production area. 

 To address this issue, we introduced and developed the concept of a probing wafer. This is a technology that can measure the electrical characteristics of all individual micro-LED chips in an on-wafer state with a single voltage application by contacting the probing wafer to the LED wafer in the form of a flip chip. The possibility of our proposed method was demonstrated through the luminescence inspection and measurement of electrical characteristics of a micro-LEDs cell unit by using the probing wafer. 

A pixel circuit is one of the display technologies, comprising FETs (Field effect transistors) and LEDs (Light emitting diodes). In order to integrate micro-LEDs in a pixel circuit, a more complex driving and complicated circuit are required. This problem causes not only the low-resolution of the micro-LED display, but also reduction in the transmissive area of the transparent display with a decrease in transparency in it. Instead, the application of our proposed device was designed by the monolithic integration of LEDs and FETs on the single epi wafer can offer a simple driving and circuit.

In order to fabricate a monolithically integrated device of an LED with two transistors, a p-type depletion metal oxide semiconductor field effect transistors (MOSFETs) structure were proposed and fabricated on p-GaN of a GaN LED epitaxial wafer. It was confirmed that this device operates as a MOSFET through on/off operation by controlling the current variable with the gate voltage, which means that the p-GaN layer of GaN LED epi wafer can be used as a MOSFET channel.