Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-09-28
2001-05-08
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S205000, C345S206000
Reexamination Certificate
active
06229508
ABSTRACT:
The invention relates to an active matrix light emitting diode pixel structure. More particularly, the invention relates to a pixel structure that improves brightness uniformity by reducing current nonuniformities in a light-emitting diode of the pixel structure and method of operating said active matrix 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. 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 corresponding 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 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 employed, e.g., liquid crystal display (LCD) and light-emitting diode (LED) display. Generally, an important 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 backlight is on for the entire duration in which the display is in use. Namely, all pixels in a LCD are illuminated, where a “dark” pixel is achieved by causing a polarized layer to block the illumination through that pixel. In contrast, a LED display only illuminates those pixels that are activated, thereby conserving power by not having to illuminate dark pixels.
FIG. 2
illustrates a prior art active matrix LED pixel structure
200
having two NMOS transistors N
1
and N
2
. In such pixel structure, the data (a voltage) is initially stored in the capacitor C by activating transistor N
1
and then activating “drive transistor” N
2
to illuminate the LED. Although a display that employs the pixel structure
200
can reduce power consumption, such pixel structure exhibits nonuniformity in intensity level arising from several sources.
First, it has been observed that the brightness of the LED is proportional to the current passing through the LED. With use, the threshold voltage of the “drive transistor” N
2
may drift, thereby causing a change in the current passing through the LED. This varying current contributes to the nonuniformity in the intensity of the display.
Second, another contribution to the nonuniformity in intensity of the display can be found in the manufacturing of the “drive transistor” N
2
. In some cases, the “drive transistor” N
2
is manufactured from a material that is difficult to ensure initial threshold voltage uniformity of the transistors such that variations exist from pixel to pixel.
Third, LED electrical parameters may also exhibit nonuniformity. For example, it is expected that OLED (organic light-emitting diode) turn-on voltages may increase under bias-temperature stress conditions.
Therefore, a need exists in the art for a pixel structure and concomitant method that reduces current nonuniformities due to threshold voltage variations in a “drive transistor” of the pixel structure.
SUMMARY OF THE INVENTION
The present invention incorporates a LED (or an OLED) pixel structure and method that improve brightness uniformity by reducing current nonuniformities in a light-emitting diode of the pixel structure. In one embodiment, a pixel structure having five transistors is disclosed. In an alternate embodiment, a pixel structure having three transistors and a diode is disclosed. In yet another alternate embodiment, a different pixel structure having five transistors is disclosed. In yet another alternate embodiment, an additional line is provided to extend the autozeroing voltage range. Finally, an external measuring module and various external measuring methods are disclosed to measure pixel parameters that are then used to adjust input pixel data.
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European Patent Office Search Report dated Feb. 1, 1999.
R.M.A. Dawson et al., “Design of an Improved Pixel for a Polysilicon Active-Matrix Organic LED Display,” Technical Digest for the 1998 Symposium of the Society for Information Display, pp. 11-14.
C. C. Wu et al., “Integration on Organic LEDs and Amorphous Si TFTs onto Flexible and Lightweight Metal Foil substrates,” IEEE Electron Device Letters, vol. 18, No. 12, Dec. 1997.
C. C. Wu et al., “Integration on Organic LEDs and Amorphous Si TFTs onto Unbreakable Metal Foil Substrates,” 1996 Int. Electron Devices Meeting Tech. Digest, pp. 957-959.
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Burke William J.
Hjerpe Richard
Huq Abhik A.
Nguyen Kevin M.
Sarnoff Corporation
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