A 4410-ppi Resolution Pixel Circuit for High Luminance Uniformity of OLEDoS Microdisplays

This paper proposes a pixel circuit with high resolution and high luminance uniformity for organic light emitting diode-on-silicon (OLEDoS) microdisplays. The proposed pixel circuit employs a simple structure that consists of four n-channel MOSFETs and one capacitor, resulting in high resolution. In addition, this circuit compensates for the threshold voltage ($V_{th}$ ) variation of the driving transistor caused by the body effect, which increases the $V_{th}$ as the source-to-body voltage of the driving transistor increases, thus reducing the emission current deviation, resulting in a high luminance uniformity. Moreover, the proposed pixel circuit extends the data voltage range using the capacitive coupling of the storage capacitance and the parasitic capacitance at the gate node of the driving transistor to precisely control the emission current. To verify the performance of the proposed pixel circuit, a test pattern with an array of the proposed 4T1C pixel circuits was fabricated on a single-crystalline silicon wafer as a backplane using a 110 nm standard CMOS process with 5.5 V high-voltage devices. The proposed pixel circuit occupies a unit sub-pixel area of $5.76 \mu \mathrm {m} \times 1.92 \mu \text{m}$, which corresponds to a resolution of 4410 pixels per inch. The measurement results show that the emission current deviation error of the proposed pixel circuit ranges between 1.16 and 1.14, which is improved from between 45.97 to 45.42 achieved in the conventional current-source type 2T1C pixel circuit, which does not compensate for the $V_{th}$ variation of the driving transistor. Moreover, the measured data voltage range of the proposed pixel circuit is extended to 1.618 V, which is 8.17 times wider than that of the conventional pixel circuit. Therefore, the proposed pixel circuit is very suitable for high resolution and high luminance uniformity of OLEDoS microdisplays.