Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-10-27
2002-05-21
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S080000, C345S207000
Reexamination Certificate
active
06392617
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to active matrix display devices, and more particularly to drive circuitry that is located within each pixel of an active matrix display.
BACKGROUND OF THE INVENTION
Arrays of organic light emitting diodes (OLEDs) are being utilized to create two-dimensional flat panel displays. As compared to conventional light emitting diodes (LEDs), which are made of compound semiconductors, the low cost and ease of patterning OLEDs makes compact, high resolution arrays practical. OLEDs can be adapted to create either monochrome or color displays and the OLEDs may be formed on transparent or semiconductor substrates.
As is known in the art, arrays of OLEDs and LEDs are typically classified as passive matrix arrays or active matrix arrays. In a passive matrix array, the current drive circuitry is external to the array, and in an active matrix array, the current drive circuitry includes one or more transistors that are formed within each pixel. An advantage of active matrix arrays over passive matrix arrays is that active matrix arrays do not require peak currents that are as high as passive matrices. High peak currents are generally undesirable because they reduce the luminous efficiency of available OLEDs. Because the transparent conducting layer of an active matrix can be a continuous sheet, active matrix arrays also mitigate voltage drop problems which are experienced in the patterned transparent conductors of passive matrices.
FIGS. 1 and 2
are depictions of active matrix pixels that are known in the prior art. It should be understood that although individual active matrix pixels are shown for description purposes, the individual active matrix pixels shown in
FIGS. 1 and 2
are typically part of an array of pixels that are located closely together in order to form a display. As shown in
FIGS. 1 and 2
, each of the active matrix pixels includes an address line
102
and
202
, a data line
104
and
204
, an address transistor
106
and
206
, a drive transistor
108
and
208
, a storage node
110
and
210
, and an OLED
112
and
212
. The address lines allow the pixels to be individually addressed and the data lines provide the voltage to activate the drive transistors. The address transistors control the writing of data from the data lines to the storage nodes. The storage nodes are represented by capacitors, although they need not correspond to separate components because the gate capacitance of the drive transistors and the junction capacitance of the address transistors may provide sufficient capacitance for the storage nodes. As shown, the OLEDs are connected to a drive voltage (V
LED
) and the current that flows through the OLEDs is controlled by the drive transistors. When current is allowed to flow through the drive transistors, the OLEDs give off light referred to as a luminous flux, as indicated by the arrows
114
and
214
.
Referring to
FIG. 1
, PMOS transistors are preferred when the cathode of the OLED
112
is grounded, and referring to
FIG. 2
, NMOS transistors are preferred when the anode of the OLED
212
is connected to the supply voltage (V
LED
). Utilizing the PMOS and NMOS transistors as shown in
FIGS. 1 and 2
makes the gate to source voltages of the drive transistors
108
and
208
insensitive to voltage drops across the OLEDs, thereby improving the uniformity of the light
114
and
214
that is given off by the OLEDs.
The operation of the prior art active matrix pixels is described with reference to the active matrix pixel configuration shown in
FIG. 2
, although the same concepts apply to the active matrix pixel of FIG.
1
. The active matrix pixel shown in
FIG. 2
serves as an analog dynamic memory cell. When the address line
202
is high, the data line
204
sets the voltage on the storage node
210
, which includes the gate of the drive transistor
208
. When the voltage on the storage node exceeds the threshold voltage of the drive transistor, the drive transistor conducts causing the OLED
212
to emit light
214
until the voltage on the storage node drops below the threshold voltage of the drive transistor, or until the voltage on the storage node is reset through the address transistor
206
. The voltage on the storage node will typically drop due to leakage through the junction of the address transistor and through the gate dielectric of the drive transistor. However, with sufficiently low leakage at the address and drive transistors and high capacitance at the storage node, the current through the OLED is held relatively constant until the next voltage is set on the storage node. For example, the voltage is typically reset at a constant refresh interval as is known in the art. The storage node is represented as a capacitor in order to indicate that sufficient charge must be stored on the storage node to account for leakage between refresh intervals. As stated above, the capacitor does not necessarily represent a separate component because the gate capacitance of the drive transistor and the junction capacitance of the address transistor may suffice.
In the active matrix pixel of
FIG. 2
, the voltage on the storage node
210
determines the intensity of the light
214
that is generated by the OLED
212
. If the intensity-current relationship of the OLED and gate voltage-current relationship of the drive transistor
208
are known, according to one method, the desired intensity of light is generated by placing the corresponding voltage on the storage node. Setting the voltage on the storage node is typically accomplished by utilizing a digital to analog converter to establish the voltage on the corresponding data line
204
. In an alternative method, the storage node is first discharged by grounding the data line, and then the data line is set to the CMOS supply voltage (V
dd
). Utilizing the latter method, the address transistor
202
functions as a current source, charging the storage node until the storage node is isolated by setting the address line low. The latter method offers the benefit of not requiring a digital to analog converter on each data line. However, one disadvantage of the latter method is that the storage node capacitance within a single pixel is a non-linear function of the voltage when supplied by the gates and junctions of the transistors. Another disadvantage is that the storage node capacitance of each pixel varies among the pixels in an array.
As described above, in order to obtain the desired luminous flux from the OLED
212
of
FIG. 2
, the voltage on the data line
204
is adjusted to control the current through the drive transistor
208
. Unfortunately, current flow through the drive transistor also depends on characteristics of the drive transistor, such as its threshold voltage and transconductance. Large arrays of drive transistors, as required to make a high-resolution display, exhibit variations in threshold voltage and transconductance that often cause the drive currents of the OLEDs to differ for identical control voltages, which in turn causes a display to appear non-uniform. In addition, different OLEDs emit different intensities of light even when driven with identical currents. Furthermore, the light intensity for a specified drive current drops as an OLED ages and different OLEDs can degrade at different rates, again causing a display to appear non-uniform.
Active matrix pixels are preferably implemented with a silicon substrate instead of a transparent dielectric substrate because transparent dielectric substrates require the transistors to be built as thin film devices. It is difficult to obtain a tight distribution of threshold voltages in large arrays of thin-film transistors especially as more transistors are needed to make the luminous flux from each pixel insensitive to threshold variations. With a silicon substrate, addressing, driving, and other circuit functions, can be easily integrated, particularly if the substrate and process are compatible with CMOS technology. Although known active matrix pixel technology is compatible with old
Agilent Technologie,s Inc.
Saras Steven
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