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

Abstract

In this paper, a pixel circuit comprising four p-channel mosfet s 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 $\rm{(V}_{\rm{th}}\rm{)}$ variation of the driving transistors. In addition, it extends the data voltage range to precisely control the emission current $\rm{(I}_{\rm{em}}\rm{)}$ . 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 μ m × 9 μ m, corresponding to a resolution of 2822 pixels per inch. To verify the performance of the proposed pixel circuit, $\rm{I}_{\rm{em}}$ is measured from 24 fabricated test patterns. When the $\rm{V}_{\rm{th}}$ variation is not compensated for, the deviation of the measured $\rm{I}_{\rm{em}}$ ranges from –17.48% to +14.40% with a deviation of $\rm{V}_{\rm{th}}$ from –7.19 mV to +7.19 mV. On the other hand, when the $\rm{V}_{\rm{th}}$ 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 $\rm{I}_{\rm{em}}$ . 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 °C to 60 °C.