Active matrix organic light emitting diode (AMOLED) displays are now entering the market place. TFT active matrix allow OLED displays to be larger in size, higher in resolution and lower in power dissipation than is possible using a passive matrix. A number of TFT active matrix pixel circuits have been developed for luminance control while correcting for TFT parameter variations. The circuits and driving methods are reviewed. A new driving method is presented IntroductionThe high brightness and efficiencies of organic light emitting diodes (OLEDs) have created considerable interest for displays 1 . Initial OLED display products were first commercialized incorporating passive matrix addressing 2 . While passive matrix addressing simplifies the display fabrication, the number of rows are limited to few hundred. Since the OLED is on only while being addressed, high peak currents are required to obtain average brightness levels. Row line resistance, column line resistance, OLED resistance, OLED capacitance and OLED off current restrict display luminance, size, and format and reduces efficeincy 3,4 .Incorporating thin film transistors (TFTs), active matrix addressing is used to make large size and high resolution liquid crystal displays (LCDs). The impedance of the liquid crystal materials used is that of a capacitor that varies with applied voltage as the refractive index changes. Applied LC voltages alternate from frame to frame to avoid image sticking due to ion plating. The pixel circuit consists simply of a single transistor and storage capacitor. The data voltages and row signals allow alternating TFT terminal voltages that tend to stabilize transistor characteristics such as threshold voltage.OLEDs are somewhat difficult to drive uniformly with active matrix TFTs. The most important reasons are: (1) luminance is current, not voltage, dependent, (2) OLED capacitance is very high, (3) TFT dimensions are relatively large, (4) effective TFT gate to drain capacitance (Cgd) and gate to source capacitance (Cgs) are high and (5) TFT threshold voltages and mobilities may vary from one device to another. The drive TFT must provide current for an OLED to light up. The large TFT dimensions tend to limit transconductance such that OLED is driven a large percentage of a frame time. The large TFT dimensions also limit the number of TFTs that can fit in the available pixel area. Low temperature polysilicon (p-Si) has an advantage over amorphous silicon (a-Si) since mobility can be one to two orders of magnitude higher. Polysilicon transistor widths can be smaller. In addition, the terminal voltages can be lower giving more power efficient operation. The OLED capacitance is so large that the OLED current and voltage can't be driven in to equilibrium in a row time. This coupled with the large TFT terminal gate to source and drain capacitance may result in luminance that depends upon the previous OLED state. Variations in threshold voltage and mobility depending upon implementation may add to luminance variations. With p-Si, init...

A 20‐inch, largest OLED display in the world is demonstrated which is driven by “Super Amorphous Silicon” technology. It has been widely believed that the characteristics of amorphous silicon TFT is not sufficient to drive OLED display. This paper turns over the hypothesis to prove that amorphous silicon can be applied to the large active‐matrix driven displays. Novel approaches to enable amorphous silicon to drive bright and long life OLED display are shown to open a bright future to realize larger OLED televisions.

This paper describes the pixel and array circuitry, which includes a CMOS SRAM cell and a precision current source at each pixel, of a direct view VGA active matrix OLED‐on‐crystalline‐silicon display, developed jointly by IBM Research Division and eMagin Corporation. The display has applications for low‐power personal information appliances such as a computer wristwatch. The display obtains data and control signals directly from the memory bus of the appliance's processor.

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (Vt) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.

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