Effects of TFT mobility variation in the threshold voltage compensation circuit of the OLED display

A pixel circuit that compensates for the non-uniform thin-film transistor (TFT) characteristics across a display panel is inevitably used in the organic light-emitting diode (OLED) display to exhibit uniform brightness. The existing pixel circuits aim to compensate only for the non-uniformity of the threshold voltage (Vth). They do not compensate for the deviation of the other parameters, such as the mobility and the subthreshold swing. In this study, how the OLED current changes when the mobility of a TFT in the Vth compensation circuit increases was examined. If the mobility increases, the enhanced drain current drives the TFT into a deeper subthreshold mode during the Vth compensation phase, and the |VGS| of the TFT decreases compared with the normal mobility case. The resultant OLED current, however, can be higher or lower than the normal mobility case depending on the gray level because the effects of the current increase due to the higher mobility and of the current decrease due to the reduced |VGS| appear simultaneously but differently depending on the VGS value of the TFT. The analysis results show that the OLED current increases for the high gray level but decreases for the low gray level. It remains almost unaffected by the mobility increase for the mid-gray level. © 2016 The Korean Information Display Society.