Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-12-27
2004-03-23
Liang, Regina (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C315S169300
Reexamination Certificate
active
06710757
ABSTRACT:
This application incorporates by reference of Taiwan application Serial No. 090100392, filed on Jan. 8, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a method of driving an active matrix electro-luminescent display, and more particularly to a method of driving an active matrix electro-luminescent display for preventing threshold voltage shift of thin film transistors in the active matrix electro-luminescent display.
2. Description of the Related Art
Active matrix electro-luminescent (AMEL) displays are generally used for small size displays, e.g., 1.3″×1.2″, with high resolution. The AMEL displays employ organic light emitting diodes (O-LEDs) to generate optical signals. The brightness of an O-LED depends on the current flowing through itself. In addition, various types of transistors can be used as the active components to drive the O-LEDs. Among them, poly-Si thin film transistors (poly-Si TFT) are widely used. On the other hand, in thin film transistor liquid crystal displays (TFT-LCDs), amorphous Si thin film transistors (a-TFT) are widely used because of fewer masks for manufacturing, low film formation temperature, and low manufacturing cost. However, either poly-Si TFT or a-TFT has the problem that the conducting current decreases due to the threshold voltage shift after a long working time. This problem becomes serious especially while a-TFTs are used. Thus, AMEL displays rarely employ a-TFT.
Referring to
FIG. 1
, it shows a pixel array of O-LEDs for an AMEL display. The AMEL display has M scan lines and N data lines, forming a display of M×N pixels. A video sequence having a number of consecutive frames can be displayed in the AMEL display with the M×N pixels. Each pixel, denoted as P, has an O-LED, denoted as D, driven by thin film transistors Ta, Tb, and a capacitor C, wherein the source or drain of the transistor Ta is coupled to one of the data lines and the gate of the transistor Ta is coupled to one of the scan lines.
For example, in a pixel in
FIG. 1
, such as pixel P(
1
,
1
), or P
11
, the gate of a transistor Ta(
1
,
1
), or T
11
a
, is connected to a scan line, Scan(
1
), or S
1
, and the source (or drain) of the transistor Ta(
1
,
1
) is connected to a data line, Data(
1
), or D
1
, and the drain (or source) of the transistor Ta(
1
,
1
) is connected to capacitor C(
1
,
1
), or C
11
, and the gate of a transistor Tb(
1
,
1
), or T
11
b
. The drain of the transistor Tb(
1
,
1
) is connected to an O-LED D(
1
,
1
), or D
11
, while the source of the transistor Tb(
1
,
1
) is connected to a direct current (DC) voltage source V
DD
, wherein the transistor Tb(
1
,
1
) is an N-type transistor.
Referring to
FIG. 2
, it illustrates waveforms for driving the circuit shown in FIG.
1
. The time for the AMEL display to display a frame is defined as a frame time interval I. A conventional method for driving an AMEL display is as follows. Firstly, scan each of the scan lines sequentially. That is, apply a pulse with a positive voltage to the scan lines, Scan(
1
) to Scan(M), sequentially so as to turn on the transistors Ta of all of the pixels on each scan line. Simultaneously, as the transistors Ta are turned on, data signals representative of different required brightness are applied to the data lines associated with the pixels to emit light. In addition, different signal levels of the data signals correspond to the brightness for the pixels.
For example, at time t
1
, while a pulse
202
is applied to the scan line Scan(
1
) so as to turn on transistors Ta(
1
,
1
), Ta(
1
,
2
), and Ta(
1
,
3
), data signals with signal levels V(
1
,
1
), V(
1
,
2
), and V(
1
,
3
) are applied to data lines Data(
1
), Data(
2
), and Data(
3
), as shown in FIG.
2
. As the pulse
202
is applied to the scan line Scan(
1
), capacitors C(
1
,
1
), C(
1
,
2
), and C(
1
,
3
) are being charged so that voltages of nodes N(
1
,
1
), N(
1
,
2
), and N(
1
,
3
) approach the signal levels V(
1
,
1
), V(
1
,
2
), and V(
1
,
3
) and transistors Tb(
1
,
1
), Tb(
1
,
2
), Tb(
1
,
3
) are turned on. At the same time, current flows from the DC current source V
DD
through the transistors Tb(
1
,
1
), Tb(
1
,
2
), Tb(
1
,
3
), O-LEDs D(
1
,
1
), D(
1
,
2
), and D(
1
,
3
) so that the O-LEDs D(
1
,
1
), D(
1
,
2
), and D(
1
,
3
) of the pixels P(
1
,
1
), P(
1
,
2
), and P(
1
,
3
) emit light with different brightness. Since the signal levels V(
1
,
1
), V(
1
,
2
), and V(
1
,
3
) are different, the current flowing through the O-LEDs D(
1
,
1
), D(
1
,
2
), and D(
1
,
3
) are different. As a result, the brightness for the pixels P(
1
,
1
), P(
1
,
2
), and P(
1
,
3
) are different.
At time t
2
, although the voltage applied to the scan line Scan(
1
) is changed to a low level and the transistors Ta(
1
,
1
), Ta(
1
,
2
), and Ta(
1
,
3
) are turned off, the capacitor C(
1
,
1
), C(
1
,
2
), and C(
1
,
3
) store charges and nodes N(
1
,
1
), N(
1
,
2
), and N(
1
,
3
) maintain in a high level, the transistors Tb(
1
,
1
), Tb(
1
,
2
), Tb(
1
,
3
) are still in a turn-on state and the O-LEDs D(
1
,
1
), D(
1
,
2
), and D(
1
,
3
) continue to emit light. Thus, at time t
2
, the pixels P(
1
,
1
), P(
1
,
2
), and P(
1
,
3
) keeps in a state for displaying. After the frame time interval I for the current frame elapses, the state of the pixels will be changed.
During a frame time interval I, threshold voltage shift may occur in the transistors Tb and would degrade the display quality. To illustrate this phenomenon, a duty ratio for a transistor is defined as a ratio of the period during which a transistor is in a turn-on state during a frame time interval to the length of the frame time interval I. For example, during the frame time interval for one frame, the pixel P(
1
,
1
) is selected for displaying. As described above, the voltage across the capacitor C(
1
,
1
) keeps in the high level V(
1
,
1
) during the frame time interval and the gate of the transistor Tb(
1
,
1
) thus remains a high level and has a current flowing through it. At the same time, the O-LED D(
1
,
1
) emits light because of current flow through it. In this situation, the duty ratio for the transistor Tb(
1
,
1
) is one since the transistor Tb(
1
,
1
) remains turned on during the entire frame time interval. Unfortunately, threshold voltage shift may occur in that case. Besides, as will be explained below, the effect of threshold voltage shift occurred in the transistor Tb(
1
,
1
) may seriously degrade the display quality.
The cause of threshold voltage shift mentioned above is described as follows. If the transistor Tb(
1
,
1
) is an amorphous Si thin film transistor, its gate terminal is covered with an isolation layer of SiN formed at a low temperature. When the gate terminal remains in the high level state, the gate terminal will attract ions within the isolation layer of SiN and that will result in an increased voltage for the transistor Tb(
1
,
1
) to conduct. In other words, the threshold voltage for the transistor Tb(
1
,
1
) increases. In that case, as the capacitor C(
1
,
1
) applies a fixed voltage to the transistor Tb(
1
,
1
) the current flowing through the transistor Tb(
1
,
1
) decreases, thereby reducing the brightness for the O-LED D(
1
,
1
). The threshold voltage shift occurs in the transistor Tb with its duty ratio of one. Furthermore, the amount of brightness reduction for each pixel P is different since the voltage across the capacitor C associated with the transistor Tb of the pixel P is different. Thus, the brightness for the AMEL display may vary inconsistently and accordingly degrade the display quality. The problem due to threshold voltage shift may also occur in poly-Si TFT and degrades the display quality especially after the display is used for a long time.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method of driving an active matrix electro-luminescent (AMEL) display for preventing the effect of the thres
Chi Mei Optoelectronics Corp.
Liang Regina
Rabin & Berdo P.C.
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