Recent advances in technology have made active matrix technology more attractive to employ in Organic Light Emitting Diode (OLED) video displays. Specifically, Thin Film Transistor (TFT) based active matrix displays provide the size and manufacturing cost efficiencies that are attractive to OLED video displays. In the conventional active matrix display, such as those employing Liquid Crystal Display elements (AMLCD), TFTs are used as driving elements to vary the orientation of the liquid crystals and thereby switch corresponding pixels off and on. In the case of OLEDs, active matrix technology allows efficient DC operation of each pixel, rather than row-by-row high frequency pulsed operation, as is required in passive matrix displays. For this reason, active matrix technology is expected to be widely used in OLED displays.
A complication associated with employing TFTs to drive OLEDs stems from the fact that TFTs become unstable when operating within the driving requirements of OLEDs. TFTs that are biased on for a long time suffer from a phenomena called “threshold drift.” Threshold drift takes effect because when the TFT is on for a long period, electrons gain enough energy to allow them to leave the silicon and tunnel into the gate oxide. These trapped electrons in the oxide increase the threshold of NMOS devices. Corresponding hole processes can increase the negative threshold of PMOS devices. Eventually, such threshold variation makes the TFT unsuitable as a switching element. In the conventional AMLCD, the TFTs are biased on for a short time period, usually at the row rate of the display, because LCD elements do not need to continually receive current to properly operate. On the other hand, in the OLED display, the TFTs should be biased for a full duty cycle because the OLEDs require continuous current to properly operate. Accordingly, TFT threshold drift will be greater in the AMOLED devices. Therefore, there is a need for a method and system for stabilizing TFTs in AMOLED devices, reducing the effects of threshold drift.
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