Simulation Analysis Of Pixel Circuits Based On A-Si TFT Organic Light Emitting Displays

The rapid development of organic light emitting device (OLED) has made possible their application to high-resolution large-area flat panel displays.Such high-resolution displays require thin-film-transistor (TFT) based active-matrix (AM) driving schemes to reduce their power dissipation. In all pixel electrode circuts, the output current characteristics dependend on the TFT W/L ratio, storage capacitance and driven voltage. To verify the circuit's ability and to decrease the risk in parameter design, circuit simulation was performed by using AIM-SICE.We designed a 2.4 inch QVGA organic light-emitting display based on hydrogenated amorphous silicon thin-film transistor (a-Si: H TFT) pixel electrode circuit. The best set of circuit parameters such as geometrical size of TFT and storage capacitance have been proposed with the help of AIM-SPICE. The pixel structure of the circuit was designed and at the same time, the RC delay in scan and data line were calculated and analyzed. The simulation result proved that the pixel electrode circuit performs perfectly.Current-source four-TFT pixel electrode circuit and voltage-source four-TFT pixel electrode circuit were investigated in the paper. We thought that the drive TFT operated in the identical region in ON and OFF states should be emphasized on compensating threshold voltage shift and eliminating flicker at the two kinds of four-TFT pixel electrode circuit. The simulation results showed that the operating region of the drive TFT can be adjusted by supplying different drive voltage value such as Vselectl, Vselect2 and Vdd. We maintained the drive TFT operated in the identical satuation regions in ON and OFF states by opitimizing the set of voltage. The simulation results indicate that the output current decrease is less than 0.1uA when the pixel circuit is swiched from ON to OFF state.Based on the pixel electrode circuit analyses, the best sets of pixel electrode circuit parameters are listed. The pixel electrode circuits showed excellent electrical reliability even when a large TFT threshold voltage shift was assumed. When the threshold voltage of drive TFT was shifted by 2.0V from its initial value, the simulation result illustrates that the output current changed by only 6.0% at high input current (1.5uA) and by 29.8% at low input current (0.2uA) for the current-source four-TFT pixel electrode circuit while the output current changed by only 12.4% at high input voltage (6V) and by 16.2% at low input voltage (0.5V) for the voltage-source four-TFT pixel electrodecircuit.