A 2822-ppi Resolution Pixel Circuit With High Luminance Uniformity for OLED Microdisplays

Abstract

In this paper, a pixel circuit comprising four p-channel MOSFETs and one capacitor is proposed for a high-resolution organic light emitting diode on silicon microdisplays with high luminance uniformity. The proposed pixel circuit improves the luminance uniformity by compensating for the threshold voltage (V-th) variation of the driving transistors. In addition, it extends the data voltage range to precisely control the emission current (I-em). The test pattern of the proposed pixel circuit is fabricated on a single-crystalline silicon wafer as a backplane using a 90 nm CMOS process and occupies a subpixel area of 3 mu m x 9 mu m, corresponding to a resolution of 2822 pixels per inch. To verify the performance of the proposed pixel circuit, Iem is measured from 24 fabricated test patterns. When the V-th variation is not compensated for, the deviation of the measured Iem ranges from -17.48% to +14.40% with a deviation of V-th from -7.19 mV to +7.19 mV. On the other hand, when the V-th variation is compensated for, the deviation of the measured Iem only ranges from -1.63% to +1.15%. Moreover, the proposed pixel circuit extends the data voltage range up to 1.289 V, which is 1.81 times wider than the source-to-gate voltage of the driving transistor which generates the maximum Iem. In addition, the maximum organic light-emitting diode luminance of the proposed pixel circuit is constantly maintained between 292.5 and 303.8 cd/m(2) in the temperature range of -45 degrees C to 60 degrees C.