Real-Time External Compensation of Threshold Voltage Shift Using Double-Gate Oxide TFTs in a Gate Driving System

In order to suppress the malfunction caused by the shift of the threshold voltage (VTH) of oxide thin film transistors (TFTs) to a negative value, double-gate TFTs are used in the shift register of the gate driving system to control VTH by adjusting the top gate bias. The proposed circuit detects the current consumption of the shift register and adjusts VTH so that the current consumption of the shift register is regulated within the desired value. The system includes a compensation algorithm, which can search for an optimized top gate bias in various circumstances such as process fluctuations and ambient temperature change. The proposed system provides a stable operation compared with a conventional structure especially at high temperature. Experimental results show that, in the conventional system without compensation, the output voltage of the shift register deteriorates at 80°C and above, and the power consumption increases from 1.15 to 2.14 mW after 21600 s of continuous operation at 60°C. On the other hand, the proposed system provides a stable gate output up to 100°C and keeps the power consumption below 1.10 mW by adjusting the top gate bias responding to environment changes.