ACTIVE MATRIX LIGHT EMITTING DIODE
PIXEL STRUCTURE AND CONCOMITANT
This application claims the benefit of U.S. Provisional 5 Application No. 60/044,174 filed Apr. 23, 1997, which is herein incorporated by reference.
This invention was made with U.S. government support under contract number F33615-96-2-1944. The U.S. government has certain rights in this invention. 10
The invention relates to an active matrix light emitting diode pixel structure. More particularly, the invention relates to a pixel structure that reduces current nonuniformities and threshold voltage variations in a "drive transistor" of the pixel structure and method of operating said active matrix 15 light emitting diode pixel structure.
BACKGROUND OF THE DISCLOSURE
Matrix displays are well known in the art, where pixels are illuminated using matrix addressing as illustrated in FIG. 20 1. A typical display 100 comprises a plurality of picture or display elements (pixels) 160 that are arranged in rows and columns. The display incorporates a column data generator 110 and a row select generator 120. In operation, each row is sequentially activated via row line 130, where the corre- 25 sponding pixels are activated using the corresponding column lines 140. In a passive matrix display, each row of pixels is illuminated sequentially one by one, whereas in an active matrix display, each row of pixels is first loaded with data sequentially. Namely, each row in the passive matrix 30 display is only "active" for a fraction of the total frame time, whereas each row in the active matrix display can be set to be "active" for the entire total frame time.
With the proliferation in the use of portable displays, e.g., in a laptop computer, various display technologies have been 35 employed, e.g., liquid crystal display (LCD) and lightemitting diode (LED) display. An important distinction between these two technologies is that a LED is an emissive device which has power efficiency advantage over nonemissive devices such as (LCD). Generally, an important 40 criticality in portable displays is the ability to conserve power, thereby extending the "on time" of a portable system that employs such display.
In a LCD, a fluorescent backlight is on for the entire „
& 45 duration in which the display is in use, thereby dissipating
power even for "off" pixels. Namely, all pixels in a LCD are illuminated, where a "dark" or "off" pixel is achieved by causing a polarized layer to block the illumination through that pixel. In contrast, a LED (or OLED) display only 5Q illuminates those pixels that are activated, thereby conserving power by not having to illuminate off pixels.
Although a display that employs an OLED pixel structure can as reduce power consumption, such pixel structure exhibits nonuniformity in intensity level over time. Namely, 55 the OLED structure will degrade with use, where it has been found that the turn-on voltage of an organic OLED increases over life, with the voltage increase dependent on the total time-integrated charge density through the OLED.
With use, the gate to source voltage (threshold voltage) of go the "drive transistor" M2 may vary, thereby causing a change in the current passing through the LED. This varying current contributes to the nonuniformity in the intensity of the display.
Another contribution to the nonuniformity in intensity of 65 the display can be found in the manufacturing of the "drive transistor". In some cases, the "drive transistor" is manu
factored from a material that is difficult to ensure uniformity of the transistors such that variations exist from pixel to pixel.
However, it has been observed that the brightness of the OLED is proportional to the current passing through the OLED. Therefore, a need exists in the art for a pixel structure and concomitant method that reduces current nonuniformities and threshold voltage variations in a "drive transistor" of the pixel structure.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, a current source is incorporated in a LED (OLED) pixel structure that reduces current nonuniformities and threshold voltage variations in a "drive transistor" of the pixel structure. The current source is coupled to the data line, where a constant current is initially programmed and then captured.
In another embodiment, the constant current is achieved by initially applying a reference voltage in an auto-zero phase that determines and stores an auto zero voltage. The auto zero voltage effectively accounts for the threshold voltage of the drive transistor. Next, a data voltage which is referenced to the same reference voltage is now applied to illuminate the pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
FIG. 1 depicts a block diagram of a matrix addressing interface;
FIG. 2 depicts a schematic diagram of an active matrix LED pixel structure of the present invention;
FIG. 3 depicts a schematic diagram of an alternate embodiment of the present active matrix LED pixel structure;
FIG. 4 depicts a schematic diagram of another alternate embodiment of the present active matrix LED pixel structure;
FIG. 5 depicts a block diagram of a system employing a display having a plurality of active matrix LED pixel structures of the present invention; and
FIG. 6 depicts a schematic diagram of an alternate embodiment of the active matrix LED pixel structure of FIG. 2.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
FIG. 2 depicts a schematic diagram of an active matrix LED pixel structure 200 of the present invention. In the preferred embodiment, the active matrix LED pixel structure is implemented using thin film transistors (TFTs), e.g., transistors manufactured using amorphous or poly-silicon. Similarly, in the preferred embodiment, the active matrix LED pixel structure incorporates an organic light-emitting diode (OLED). Although the present pixel structure is implemented using thin film transistors and an organic light-emitting diode, it should be understood that the present invention can be implemented using other types of transistors and light emitting diodes. For example, if transistors that are manufactured using other materials exhibit the