An Ultra-High 6318-PPI Pixel Circuit for Micro-OLED Displays With V_th Compensated up to 10-bit Gray Levels

An eight FETs and two capacitors (8T2Cs) pixel circuit with dual data voltages (V_data 's) is proposed for micro-organic light-emitting diode (OLED) displays, resulting in and validated successfully with an ultra-high pixel-per-inch (PPI) of 6318. This designed pixel circuit can also realize 10-bit threshold voltage (V_th)-compensated gray levels. It is known that conventional displays are 1024 gray levels by inputting 10-bit V _data 's to the pixel circuit for generating pixel currents (I _pixel's), resulting in the V _data range of at least 5.12 V with the DAC's 1-LSB of 5 mV assumed. However, for the CMOS processes of dozens of nanometers, only high-voltage FETs (HVFETs) in large sizes can drive with such large V _data range, undermining the chance of high PPI. The new pixel circuit proposes herein low-voltage FETs (LVFETs) in place of most HVFETs successfully by synthesizing two V _data 's to generate the required I _pixel 's for 10-bit gray levels with the dynamic range of V _data reduced to 0.64 V, while the V _th 's can still compensate. This circuit featuring 6318 PPI is successfully fabricated in the chips of the 55-nm CMOS process for validations. The simulation result show that, with V_th's compensated, the Δ I_pixel's with respect to the currents at corner TT are improved from the averaged +285% of FF to +11.1% and the averaged -69.1% of SS to -9.1%. Favorable voltage linearities were also seen between gray levels. Both DNLs and INLs in the range of ±1 LSB were measured on a testkey-form circuit, appearing consistency with the simulation results and verifying the expected performance of the designed dual-data-driven pixel circuit.