Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-04-27
2003-03-25
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S092000, C330S257000, C330S288000
Reexamination Certificate
active
06538632
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a driving circuit for a semiconductor display device using thin-film transistors. More specifically, the invention relates to a thin-film transistor circuit which uses a differential amplifier circuit and a current mirror circuit and which is used in a driving circuit for an active-matrix type semiconductor display. The invention also relates to a semiconductor display device using the thin-film transistor circuits.
A technology for manufacturing thin-film transistors (TFT) using a semiconductor film formed on an inexpensive glass substrate has advanced rapidly in recent years. This is because there are growing demands for active matrix liquid crystal displays and EL displays. The active matrix liquid crystal display has a TFT disposed in each of several tens to several millions of pixel areas arranged in matrix and controls electric charges coming into or going out of individual pixel electrodes by the switching function of the TFTs.
FIG. 11
shows a configuration of a conventional active matrix liquid crystal display device. A shift register and a buffer circuit are generally called a driving circuit and are in recent years formed on the same substrate as the active matrix circuit. Denoted
1101
is a source signal line side driving circuit and
1102
a gate signal line side driving circuit.
Designated
1103
is an active matrix circuit (pixel matrix circuit) which has pixels TFT
1104
arranged in matrix. Each pixel TFT
1104
has its drain electrode connected to a pixel electrode. Between these pixel electrodes and counter electrodes is sandwiched and sealed liquid crystal. The pixel TFTs
1104
are each formed with an auxiliary capacitor
1106
to hold charge.
A technology is also known which uses quartz as a substrate and manufactures thin-film transistors using a polycrystalline silicon film.
Another technology is also known which utilizes a laser anneal method to manufacture thin-film transistors using crystalline silicon film on a glass substrate.
In the configuration shown in
FIG. 11
, a timing signal from the shift register circuit of the source signal line side driving circuit selects an image signal supplied to an image line, and the selected image signal is fed to the corresponding source signal line. Further, a timing signal from the gate signal line side driving circuit is supplied to the corresponding gate signal line (scan line). The image signal fed to the source signal line is written into the pixel electrode of the thin-film transistor selected by the timing signal from the gate signal line.
This operation is repeated by setting an appropriate timing to successively write information into each pixel arranged in matrix.
After image information for one screen (one frame) has been written, the writing of image information for the next screen is performed. In this way, images are displayed one after another. Normally, the writing of image information for one screen is performed 30 or 60 times a second.
FIG. 12
shows one example of the source signal line side driving circuit. Reference number
1200
represents a clock input terminal,
1201
a clock line,
1202
a start pulse input terminal,
1203
-
1205
shift registers,
1206
-
1211
inverter type buffers,
1212
a video signal input terminal,
1213
a video signal line,
1214
-
1216
and
1220
-
1222
switches,
1217
-
1219
and
1225
-
1227
storage capacitors,
1223
a transfer signal input terminal,
1224
a transfer signal input line,
1228
-
1230
analog buffers, and
1231
-
1233
source signal line connection terminals.
In the case of an analog gray scale, a gray scale signal entered into the source signal line side driving circuit uses a video signal which is continuous in time. In the case of a normally white mode (a display mode that displays a white color when the liquid crystal is not impressed with a voltage), a setting is made such that the displayed color approaches black as the absolute value of the voltage of the gradation signal increases. To the shift registers
1203
-
1205
, a start pulse is applied in synchronism from the start pulse input terminal
1202
, with the video signal and are shifted by a clock pulse entered from the clock pulse line. The outputs of the shift registers
1203
-
1205
are fed through the inverter type buffers
1206
-
1211
to a sampling circuit.
The sampling circuit comprises switches
1214
-
1216
and storage capacitors
1217
-
1219
.
Here, one example of a conventional circuit used as analog buffers
1228
-
1230
is shown in FIG.
13
. Designated
1301
is a terminal connected with a storage capacitor and used as a signal input terminal (IN). Denoted
1302
is a terminal connected with a source signal line and used as a signal output terminal (OUT). Reference numeral
1303
represents a constant current source,
1304
a constant voltage source,
1305
and
1306
P-channel TFTs, and
1307
and
1308
N-channel TFTs. In the analog buffer of
FIG. 13
, the differential circuit comprises P-channel TFTs and the current mirror circuit comprises N-channel TFTs.
The operation of the analog buffer of
FIG. 13
will be described. When the voltage of the input terminal (IN)
1301
of the differential circuit connected to the storage capacitor increases, the input current of the current mirror circuit connected to the opposite phase output of the input terminal (IN)
1301
decreases and the output current of the current mirror circuit also decreases correspondingly. On the other hand, the current of the same phase of the input terminal increases, causing the voltage of the output terminal (OUT)
1302
to rise to the same voltage level as the input terminal of the differential circuit. Therefore, the voltage of the source signal line connected to the output terminal (OUT)
1302
becomes equal to that of the input terminal.
In recent years, as the amount of information handled increases sharply, efforts have been made to increase the display capacity and enhance the resolution of the display. Examples of computer display resolutions for some standards are shown below in terms of pixel numbers.
Pixel number (horizontal×vertical): Standard
640×640: EGA
640×480: VGA
800×600: SVGA
1024×768: XGA
1280×1024: SXGA
Recent years have seen the spread of software even in a personal computer field that performs a plurality of displays which are different in nature. This gives rise to a trend that a growing number of displays are compatible with XGA and SXGA standards that have higher resolutions than VGA and SVGA.
The active matrix liquid crystal displays are very frequently used in the field of notebook type personal computer. In recent years, they have come to be used not only in the notebook type personal computer but often as the displays of desktop personal computers.
In addition to the display of data signals in personal computers, the active matrix liquid crystal displays with high resolutions have come to be used for displaying television signals.
The buffers or analog buffers in the active matrix liquid crystal display devices used for such displays are not useful if their current capacity is small, and thus are required to have a certain magnitude of current capacity. When buffers or analog buffers with a large current capacity are made using thin-film transistors, the TFTs with a large current capacity, i.e., with a large channel width, are necessary. However, the TFTs with a large channel width have variations in the crystallinity among devices, which in turn causes variations in threshold voltage among TFTs. Hence, there are necessarily variations in the characteristic of the buffer or analog buffer made of a plurality of TFTs. This means that buffers or analog buffers have characteristic variations among individual source signal lines, and these characteristic variations will lead to variations in voltage applied to the pixel matrix circuit, which in turn will cause display unevenness on the entire display screen.
When the TFT size (channel width) is too large, in some cases, o
Koyama Jun
Yamazaki Shunpei
Chang Kent
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
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