TFT pixel threshold voltage compensation circuit with short data programming time and low frame rate

Abstract

A pixel circuit is operable in initialization, data programming, a threshold compensation, and emission phases. The pixel circuit includes a drive transistor configured to control an amount of current to a light-emitting device during the emission phase depending upon a voltage applied to a gate of the drive transistor. A first ultra-low leakage oxide transistor is employed as a data switch device, and the data voltage is applied to the gate of the drive transistor through the first ultra-low leakage oxide transistor during the data programming phase. A second ultra-low leakage oxide transistor is employed as an initialization switch device. The second ultra-low leakage oxide transistor is in an on state during the initialization, data programming, and threshold compensation phases, and the initialization voltage is applied to the gate of the drive transistor through the second ultra-low leakage oxide transistor during the initialization phase.

Description

TECHNICAL FIELD[0001]The present invention relates to design and operation of electronic circuits for delivering electrical current to an element in a display device, such as for example to an organic light-emitting diode (OLED) in the pixel of an active matrix OLED (AMOLED) display device.BACKGROUND ART[0002]Organic light-emitting diodes (OLED) generate light by re-combination of electrons and holes, and emit light when a bias is applied between the anode and cathode such that an electrical current passes between them. The brightness of the light is related to the amount of the current. If there is no current, there will be no light emission, so OLED technology is a type of technology capable of absolute blacks and achieving almost “infinite” contrast ratio between pixels when used in display applications.[0003]Several approaches are taught in the prior art for pixel thin film transistor (TFT) circuits to deliver current to an element of a display device, such as for example an org...

AI Technical Summary

Benefits of technology

[0013]The present invention relates to pixel circuits that are capable of compensating the threshold voltage variations of the drive transistor with an ultra-short one horizontal time 1H of less than about 2 μs, which is shorter as compared to conventional configurations, with additionally removing the possible memory effects associated with the light-emitting device and the drive transistor from the previous frame. An ultra-short 1H time (<2 μs) is achieved via separation of threshold compensation of the drive transistor and data programming phases. The threshold compensation time is decided by the drive transistor characteristics and is difficult to reduce further without degrading the compensation accuracy. By separating the threshold compensation and data programming phases, a longer time can be allocated to threshold compensation for compensation accuracy while maintaining a short 1H time. As referenced above, the RC constant time required for charging the programming capacitor is determinative of the programming time, and such programming time can be reduced to ultra-short 1H times (<2 μs).

[0014]To achieve such results, a two-capacitor pixel circuit configuration is used, whereby a first capacitor is used for drive transistor threshold compensation, and a second capacitor is used to store the data voltage. The threshold compensation and data programming operations are independent of each other, and thus a short one horizontal time can be achieved by employing a short data programming phase.

[0015]Embodiments of the present application also use an ultra-low leakage oxide transistor, such as an IGZO transistor, as the data switch device, and this permits the stored data voltage to be retained longer on the second capacitor due to ultra-low leakage of the IGZO device. A conventional typical refresh rate is 60 Hz. By using an IGZO device as the data switch transistor, the refresh rate can be reduced to 30 Hz or lower for static images. With a lower refresh rate, the power consumption in turn is reduced.

[0017]An aspect of the invention, therefore, is a pixel circuit for a display device that employs a two capacitor configuration and an ultra-low leakage data switch transistor to isolate the data programming phase from the compensation phase, thereby enable a shorter 1H time as compared to conventional configurations, and further permitting a reduced refresh rate. The pixel circuit is operable in an initialization phase, a data programming phase, a threshold compensation phase, and an emission phase.

Problems solved by technology

The threshold compensation time is decided by the drive transistor characteristics and is difficult to reduce further without degrading the compensation accuracy.

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