Accelerated Stress Testing of a-Si:H TFTs for AMOLED Displays
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Accelerated Stress Testing of a-Si:H TFTs for AMOLED Displays Kapil Sakariya, Clement K.M. Ng, I-Heng Huang, Afrin Sultana, Sheng Tao, and Arokia Nathan Electrical and Computer Engineering, University of Waterloo Waterloo, Ontario, N2L 3G1, Canada ABSTRACT In this work, we have developed a method for accelerated stress testing of TFT driver circuits for AMOLED display backplane applications. Based on high current and temperature stress measurements, acceleration factors have been retrieved, which can be used to significantly reduce the testing time required to guarantee a 20000-hour display backplane lifespan. INTRODUCTION Threshold voltage (VT) shift in amorphous silicon (a-Si:H) thin film transistors (TFTs) due to extended periods of electrical stress critically affects the performance and operational lifetime of a-Si:H based active matrix organic light-emitting diode (AMOLED) pixel circuits [1]. Unlike the ubiquitous voltage driven AMLCD, which requires the a-Si:H TFT to merely act as an analog switch, the challenge with using a-Si:H TFTs in AMOLED backplane electronics stems from the fact that OLEDs are current driven. The drive current provided by a TFT is very sensitive to the threshold voltage, which increases over time due to electrical stress. In order to compensate for the increase in VT, new current and voltage programmed pixel circuits [1][2][3] have been developed, which work by increasing the gate voltage of the TFTs to overcome the increase in VT, thereby keeping VGS-VT constant. These circuits continue to provide a stable drive current to the OLED until the critical voltages reach the supply voltage, at which point the compensation mechanism fails. Figure 1 shows an example of a current programmed VT-shift compensating circuit, and Figure 2 shows the compensation mechanism of that circuit.
V-sel VDD
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Fig. 1. A current programmed VT-shift compensating circuit [1].
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(Increasing to compensate for VT )
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(Increasing due to stress)
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Fig. 2. VT-shift compensation mechanism in the circuit of Fig. 1. It is generally accepted that a 20000-hour pixel circuit operational lifetime is required before AMOLED displays can be proven feasible. VT-shift compensating pixel circuits can theoretically provide stable operation for such lengths of time, however there is no quick way to experimentally verify a pixel circuit’s lifetime without performing thousands of hours of measurements. It is therefore desirable to develop an accelerated testing method, which can reliably predict the true long-term behaviour of the a-Si:H TFTs in the pixel circuits using shorter high-stress tests. Powell et al. [4] have modeled the VT-shift and shown that it increases with temperature when TFTs are subject to constant voltage bias for very short time durations. While these findings cannot directly be applied to TFTs in VT-shift compensating AMOLED pixel circuits, they do demonstra
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