Very bright high-res microdisplays are a necessity for certain augmented reality applications, such as aircraft pilot helmet display systems. In research conducted for the Hilico project, CEA-Leti is developing a gallium nitride (GaN) microdisplay with these characteristics.
GaN-based materials have been used to make high-yield light-emitting diodes (LEDs). However, only LEDs that emit blue or green light efficiently can currently be fabricated. The kind of color microdisplay being developed here requires efficient blue, green, and red LEDs. CEA-Leti researchers came up with an array in which each pixel is made up of four blue LEDs. One of the four LEDs has a coating that converts blue to green; two others have a coating that converts blue to red. An application-specific integrated circuit (ASIC) powers each LED independently to deliver the desired color to each pixel.
The array of LEDs (8 x 8 µm2 pixels at a 9.5 µm pitch) combined with the the color converting coatings and the circuit should lead to the first 1.5 cm (side) color microdisplay demonstrator with luminance of at least 50,000 candelas per sq. m, around twice that of today's top-performing LCD and OLED microdisplays.