A new pixel circuit for "clamped inverter driving" was developed. Since a PMOS inverter is applied instead of the conventional CMOS for the pixel circuit, the circuit is simplified and is composed of only three TFTs. A simulation of gray-scale characteristics verified that this pixel circuit can compensate variation in V th and carrier mobility. A 2.5-inch OLED display based on the new pixel circuit was fabricated by LTPS processing and achieves 6-bit gray-scale and good uniformity. IntroductionSeveral designs of pixel circuits for organic light-emitting-diode (OLED) displays have been reported [1][2][3]. Regarding the driving scheme for OLED displays, it is known that the voltageprogramming method has advantages of high-speed writing and compatibility with existing driver ICs. However, this scheme has one problem that must be solved; namely, V th and carrier mobility of the TFTs in the pixel circuit must be compensated effectively. At SID'02 and '03, we proposed a "clamped inverter driving" method, which uses sweep-signal comparison to control the duration of light emission [4,5]. This method is a kind of voltage programming. It compensates V th and carrier mobility of a TFT effectively and achieves not only luminous characteristics of 6-bit gray-scale and good uniformity but also gamma correction and peak luminance. The pixel circuit for this driving method, however, requires more than four TFTs. Accordingly, in the present work, we designed a simplified pixel circuit with only three TFTs. Such a pixel circuit composed of fewer TFTs has advantages of high aperture ratio and high yield ratio. We applied it to a 2.5-inch OLED display and evaluated the display characteristics of the OLED panel. Figure 1 shows the configuration of the pixel circuit for the previously developed "clamped inverter driving" scheme, which controls the brightness of the OLED by means of changing the length of the "ON" and "OFF" states of the inverter. TFT T1 and TFT T2 compose the CMOS inverter, and TFT T3 and TFT T4 -Fig. 1 Configuration of previous pixel circuit Fig. 2 Timing chart for OLED driving V data =High Writing mode Illumination mode V data =Low V data =High V data =Low OLED ON OFF ON OFF Vdata Bright Dark Data-line voltage Sweep signal Data signal OLED T4 T3 T1 T2 Illumination line Reset line Data line C1 CMOS inverter Clamped inverter driving method

No abstract

A new pixel circuit for the “clamped inverter driving method” reported previously is developed. The pixel circuit is composed of four TFTs, one capacitor and five signal lines including power supply. A 3.5‐inch OLED display with the proposed new pixel circuit has been fabricated by LTPS processing and COG of an existing LCD driver chip. A vertical scanning circuit and selector switches are integrated in the panel. The proposed driving scheme is confirmed as OLED display to produce 6‐bit gray‐scale, gamma correction, and peak luminance with clear full‐color images.

A method for generating threshold voltage (V th ) of thin-film transistors (TFTs) is proposed. This method is applied to the simplest pixel circuit, namely, one composed of two TFTs, a drive TFT and an address TFT, and one capacitor [i.e., two TFTs and one capacitor configuration], of active-matrix organic lightemitting diode (AM-OLED) displays. It involves two steps. First, source voltage of the drive TFT is quickly and precisely detected under the condition that carriers, namely holes, in the drive TFT are detrapped. Second, a simple subtraction and bit-shift operation are executed using the detected source voltage of the drive TFT. A prototype 3.8-in 480 × 320-pixel AM-OLED display was developed to confirm that a V th -distribution image produced by this method corresponds to nonuniform images of AM-OLED displays. Furthermore, the AM-OLED display produced highuniformity images when the V th distribution was compensated by the proposed method.Index Terms-Active-matrix organic light-emitting diode (AM-OLED), hysteresis, simplest pixel circuit, V th distribution.

No abstract