IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 39, NO. 9, SEPTEMBER 2004 1477 Amorphous Silicon Thin Film Transistor Circuit Integration for Organic LED Displays on Glass and Plastic Arokia Nathan, Senior Member, IEEE, Anil Kumar, Kapil Sakariya, Peyman Servati, Student Member, IEEE, Sanjiv Sambandan, Student Member, IEEE, and Denis Striakhilev, Member, IEEE Abstract—This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integra- tion of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages. Index Terms—Active matrix display, amorphous silicon, organic light emitting diode, thin film transistor. I. INTRODUCTION I NTEREST in hydrogenated amorphous silicon (a-Si:H) technology extends well beyond the active matrix liquid crystal display (LCD) [1] (routinely found in laptops) and stems from a variety of desired technological features, in- cluding low temperature manufacturing with few constraints on the substrate size, material, or topology. More significantly, the a-Si:H active matrix is emerging as a promising technology for back-plane electronics for a new generation of displays based on the organic light-emitting diode (OLED) on both rigid [2] and mechanically flexible [3] substrates. Interest in plastic is being driven by the need for lightweight, unbreakable, and foldable display screens for computers and cell phones as well as for a plethora of new generation applications such as electronic books, newspapers, and maps [4]. Manuscript received December 14, 2003; revised February 28, 2004. This work was supported by the University of Waterloo, the Natural Sciences and Engineering Research Council of Canada (NSERC), Communications and In- formation Technology Ontario (CITO), and IGNIS Innovation Inc. The authors are with the Department of Electrical and Computer Engi- neering, University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: anathan@uwaterloo.ca). Digital Object Identifier 10.1109/JSSC.2004.829373 The OLED has evolved since inception and is now competing with the LCD [5], [6]. The OLED is lightweight and durable with high brightness, good contrast, and low power consump- tion, making it ideal for portable devices. However, unlike the LCD, the OLED is a current driven device. Consequently, the 1-TFT pixel structure operating as a switch in LCDs cannot pro- vide the constant current and subsequently constant illumination in the OLED pixel. In fact, a minimum of one switching TFT to program and store the analog-voltage and one drive TFT to pro- vide the current is required for these pixels. Hence, AMOLED display pixels need a minimum of two TFTs to control the drive current. Although a-Si:H, by virtue of material structure, does not enjoy the same electronic properties, such as speed (as com- pared to poly-Si or crystalline Si), there is need for this mature technology in this newly emerging and significant application area. Hence, the question arises as to whether circuit techniques can be used to compensate for intrinsic material shortcomings to meet performance requirements. This paper will review pre- cisely these challenges. Specifically, it examines design con- siderations pertinent to on-panel TFT circuits, whose integra- tion requires nonconventional design solutions. Here, we have to deal with material nonuniformity and must supply stable and predictable currents to drive the OLED [7]. We show that a-Si:H driver circuits can satisfy the requirements of area and speed [8], despite its lower mobility as compared to poly-Si [9]. More im- portantly, it has a lower processing cost and has demonstrated fabrication of TFT circuits on plastic substrates [3]. The cir- cuits and pixel architecture demonstrated here are based on TFT structures [10] and physically based compact models that accu- rately predict both the static and dynamic behavior, and imple- mented in VerilogA hardware description language [11], [12]. The choice of VerilogA emanates from the relative ease of im- plementation of differential functions for the dynamic modeling of the TFT in the same. II. PIXEL ARCHITECTURE AND DESIGN Amorphous silicon TFTs have inherent stability problems due to the defect-state creation and charge trapping [13] in the active semiconductor and dielectric layers. In amorphous sil- icon circuit design, one of the biggest hurdles is the prolonged electrical stress induced change in threshold voltage [14]. This shift is most evident in the drive TFT of an OLED display pixel due to its continuous ON state operation. As a result, its 0018-9200/04$20.00 © 2004 IEEE