Low Temperature a-Si:H Pixel Circuits for Mechanically Flexible AMOLED Displays
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Low Temperature a-Si:H Pixel Circuits for Mechanically Flexible AMOLED Displays Arokia Nathan, Denis Striakhilev, Peyman Servati, Kapil Sakariya, Anil Kumar, Karim S. Karim, and Andrei Sazonov Electrical and Computer Engineering, University of Waterloo Waterloo, Ontario N2L 3G1, CANADA [email protected] ABSTRACT This paper presents the first demonstration of amorphous silicon (a-Si:H) thin film transistor (TFT) circuit integration on plastic substrates. The circuits compensate for material shortcomings such as metastable threshold voltage (VT) shift and high contact resistance, to supply stable and predictable currents to drive organic light emitting diode (OLED) displays on plastic. Various drive circuits were fabricated and tested. The simplest pixel driver circuit possible is the two TFT voltage-programmed circuit, which is very susceptible to any shift in VT of the drive TFT. To overcome this problem, we have developed current-programmed pixel circuits based on the current mirror circuit family, which include the simple and cascode current mirrors. The OLED current in these circuits is virtually independent of any VT shift or mobility variation in the drive TFT. Results show that the circuits provide higher linearity and dynamic range than currently available pixel circuits while minimizing the pixel area. INTRODUCTION Organic light-emitting diodes (OLEDs) have the potential to replace LCDs for flat panel display applications due to their many performance advantages like lower power usage, faster response time, lower cost fabrication, and a process that is ideal for mechanically flexible displays on plastic [1]. Large-area passive matrix flat panel displays on both rigid and flexible substrates have already been demonstrated [2][3], while the development of active matrix OLED (AMOLED) displays remains a challenge. Amorphous silicon is the technology of choice for AMOLED displays on plastic due to its maturity and low cost. However, the challenge lies in creating good electronic grade material at a low process temperature. In addition, the a-Si:H TFT is affected by VT shifts over time, leading to a gradual decrease in the OLED drive current. This paper demonstrates a-Si:H circuit integration as a building block for stable AMOLED backplane electronics on plastic substrates. THIN FILM TRANSISTORS ON PLASTIC Tri-layer inverted staggered TFTs incorporating low temperature a-Si:H and a-SiNx films were fabricated using a wet etching-based process with 5 photolithographic steps, on both plastic and glass substrates. [4]. 50 µm thick polyimide foil was used as a substrate. The TFTs on glass were intended to serve as a reference. First, the plastic substrate was coated on both sides with ~500 nm thick silicon nitride film (Figure 1a) before the deposition of the gate metal. Next, 120 nm Al was deposited and patterned with a gate metal mask (Figure 1b). After that, a tri-layer comprising thick silicon nitride gate insulator, intrinsic amorphous silicon channel layer and silicon nitride channel passivation
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