Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-04-22
2001-05-08
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S092000
Reexamination Certificate
active
06229506
ABSTRACT:
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 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. An important distinction between these two technologies is that a LED is an emissive device which has power efficiency advantage over non-emissive devices such as (LCD). 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 fluorescent backlight is on for the entire 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 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, 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 the “drive transistor” M
2
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 the display can be found in the manufacturing of the “drive transistor”. In some cases, the “drive transistor” is manufactured 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.
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Dawson Robin Mark Adrian
Hsu James Ya-Kong
Hsueh Fu-Lung
Ipri Alfred Charles
Kane Michael Gillis
Burke William J.
Chow Dennis-Doon
Sarnoff Corporation
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