Analysis of a-Si TFT active driver array protection circuit for OLED
OLED(organic electrical emitting device) active drive array is integrated with two or more thin film transistors in each unit pixel [1, 2] to drive the light-emitting lattice, so as to provide continuous working signals for the pixel within the whole frame cycle. The active drive overcomes the problems caused by the pulse signal drive with small duty cycle in the passive drive mode [3], and is conducive to the realization of large-area and high-resolution OLED display. In the design process of OLED A-SI TFT active driver circuit, we adopted a driver circuit that contains two TFT(thin film transistor) in each unit pixel [4 ~ 6], as shown in Figure 1. Due to abnormal changes in external signals, the scanning line, data line and source line often bring burst high voltage, and in the process of TFT array and OLED production, due to friction in some operations, it will also bring up to tens of thousands of volts of static voltage. These occasional abnormal voltage signals will directly act on the gate of the T1 tube or T2 tube, breakdown the gate insulation medium, resulting in complete damage to the device. Therefore, some protection circuits must be added on the outside of the T1 and T2 gate, and in order to prevent the high-voltage signal from the power line from breaking down the OLED, it is also necessary to add protection circuits on the power line. Common gate protection circuits include diode gate protection circuit, source-drain breakdown protection, field-induced inversion protection, gate modulated breakdown protection, etc. [7]. Because the source breakdown is recoverable, and considering the consistency of the peripheral protection circuit and the drive array process, we choose the form of source breakdown protection, using the breakdown characteristics of a-Si (amorphous silicon)TFT to design the protection circuit.
Simulation and analysis
Establishment of TFT source leakage breakdown model
When the TFT is in the working state, when the source and drain voltage Vds reaches a certain value, the drain current Ids begins to increase sharply in the process of continuing to increase the voltage, and the source and drain breakdown occurs. With the increase of the gate-source voltage Vgs, the value of Vds increases step by step.