Generally, a gate driver circuit of a display panel uses a N-type thin film transistor (TFT). In order to ensure that a thin film transistor of a pixel on a scan line (a gate line) is continuously turned off, a gate driver circuit needs to maintain a voltage of the scan lines (a gate voltage of the thin film transistor) at a low level. In a gate driver circuit, a pull-down circuit for providing the low level needs to be continuously turned on, which results in a continuing bias voltage for the thin film transistor. Under the operating condition of the continuing bias voltage in high voltage, the thin film transistor may generate a characteristic variation (e.g., a characteristic degradation and/or a threshold voltage variation). The characteristic degradation of the thin film transistor may reduce a working life thereof, whereas a threshold voltage Vth variation of the gate driver circuit may affect a reliability of the gate driver circuit.
Take a push-pull output circuit for example, in the circumstance where a push-pull output signal of the driver circuit are to be maintained at a low logic level, a bias voltage of a pull-down transistor (e.g., a N-type transistor) of a pull-push output circuit in the driver circuit are maintained at a high logic level. Because the pull-down transistor for providing a low output level are continuously applied by high voltage, a characteristic variation (a characteristic degradation and/or a threshold voltage variation) may occur on a transistor characteristic thereof.
In order to solve aforesaid problem, a plurality of pull-down circuits/devices are utilized in some embodiments, so as to reduce a duty cycle of the pull-down circuits/devices. However, with extension of operating time, the threshold voltage variation may still occur on the pull-down circuits/devices. Furthermore, a process variation may occur during a manufacturing process of the driver circuit. The process variation may change a characteristic of the transistor (e.g., variation may occur on the threshold voltage of the transistor). It is an issue to be solved as how to improve the reliability of the driver circuit while effectively reducing influences caused by the device variation of the threshold voltage.