Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-08-15
2004-05-11
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S366000
Reexamination Certificate
active
06734505
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film transistor including a so-called dual gate structure for driving the thin film transistor by using a plurality of gate electrodes and a method of manufacturing the thin film transistor. The present invention further relates to a display device including the thin film transistor, and a method of driving the display device.
An organic LED (light emitting diode) has a very high response speed and is a self-emitting device, and thus, it is expected that the application of the organic LED to a display device will allow providing an excellent flat display device having a wide viewing angle. The application of the organic LED to the flat display device replaces a liquid crystal display device. The above-mentioned organic LED is a current-driven element, and thus, the achievement of high-resolution display requires a continuous feed of a current through the organic LED element even during non-selection of a scanning line.
FIG. 9
is a diagram showing a circuit configuration for driving an organic LED, which has been heretofore proposed. The conventional circuit configuration shown in
FIG. 9
includes a switching thin film transistor, hereinafter referred to as TFT
80
, for generally performing switching, and a driver TFT
84
for driving an organic LED element
82
. Switching TFT
80
is driven to be turned on and off in accordance with a signal supplied through a scan line
86
, and thus supplies to driver TFT
84
a signal supplied through a signal line
88
.
Switching TFT
80
is connected to a gate electrode of driver TFT
84
so as to control conductance of driver TFT
84
. Under this control, driver TFT
84
supplies a current supplied from a supply line
90
to organic LED element
82
connected to a drain electrode of driver TFT
84
(to drive organic LED element
82
). Thus, driver TFT
84
drives organic LED element
82
. The conventional circuit shown in
FIG. 9
for driving organic LED element
82
also has a configuration in which a charge storage capacitor
92
is connected to a line
94
so as to ensure a stable supply of a current required for organic LED element
82
.
Driver TFT
84
supplies a current to organic LED element
82
in accordance with current levels determined by switching TFT
80
. In the conventional circuit, the current levels are used under control by which the current levels are changed in about four levels. Time division is used simultaneously with the controlled current levels, thus enabling more levels of control. The use of a change in the current levels allows control on brightness of the organic LED, thereby enabling gray level control on light emission of the organic LED element and enabling high-resolution display.
Gray level control on organic LED element
82
is, in principle, made possible in the following manner: a voltage applied to a gate in performing gray level control on the organic LED element is controlled by using the circuit configuration shown in FIG.
9
. The level of current flowing to organic LED element
82
is controlled by using the gate voltage. This relationship is shown in
FIG. 10
as a graph which is obtained by plotting a current I
oled
flowing to a different organic LED element versus a gate voltage of switching TFT
80
, in driving a typical organic LED element.
As shown in
FIG. 10
, the organic LED element varies in its current-voltage characteristics for each pixel because of various factors. Thus, the same gray level cannot be applied to the organic LED element stably even when a certain gate voltage V
gate
is applied to the organic LED element formed for one pixel each, as shown in FIG.
10
. Consequently, a problem exists: when the current level is changed only by changing the gate voltage by use of the circuit configuration shown in FIG.
9
and then gray level control is performed by direct use of the changed current level, the use of only this gray level control does not permit display of precise gray level throughout the whole screen of the display device.
A method of performing gray level control by changing a luminous area of the organic LED element through only on-off control by utilizing the characteristics that a maximum current value I
max
and a minimum current value I
min
do not vary pixel by pixel even if the current-voltage characteristics vary as shown in
FIG. 10
, that is, a so-called area gray level control method, has been also proposed in order to solve the above-mentioned problem.
FIG. 11
shows a drive circuit which has been heretofore proposed to implement the area gray level control method. In the drive circuit shown in
FIG. 11
, a plurality of organic LED elements
82
are arranged for each pixel
96
, and the drive circuit is adapted to be capable of controlling the gray level by controlling the number of organic LED elements
82
to emit light in the luminous area.
FIG. 12
is a plan view showing a configuration in which a plurality of organic LED elements
82
are arranged for each pixel for performing the conventional area gray level control. Organic LED elements
82
are configured in such a manner that a signal is supplied to these organic LED elements from a wiring
98
.
Although the method of controlling the gray level by means of the area gray level shown in
FIGS. 11 and 12
allows precise gray level control, the method has the problem that the brightness of a display part is decreased because of a decrease in filling rate of organic LED elements
82
per pixel. A driving current may be increased in order to increase the brightness of the display part; however, another problem arises, such that the increase in the driving current causes a reduction in a luminous life of the organic LED element or in a life of the driver TFT. Furthermore, since the need arises to perform on-off control on each organic LED element, it also becomes necessary to increase the number of driving elements such as the driver TFT. Thus, the above-described also leads to a decrease in the filling rate of the organic LED elements per pixel.
For driving the organic LED, a thin film transistor (TFT), as described above, is often used to drive the organic LED for display per pixel. The TFT for use in this case, particularly, the driver TFT, is required to be capable of providing a high level of current with stability. Various TFT structures have been heretofore proposed as TFT structures having stabilized current characteristics. A so-called dual gate TFT structure is disclosed in the gazette of Japanese Patent Laid-Open No. Hei 8(1996)-241997, for example. In this structure, gate electrodes are located oppositely to each other so as to sandwich an active layer with insulating layers interposed between the gate electrodes, and one of the gate electrodes is formed shorter than the other gate electrode in a source-drain direction.
The disclosed dual gate TFT exhibits excellent pentode characteristics, and thus stabilizes a drain current even if a power supply voltage varies. However, the dual gate TFT structure disclosed in the gazette of Japanese Patent Laid-Open No. Hei 8(1996)-241997 uses the gate electrode formed shorter than the active layer to form high-resistance portions on both ends of a silicon active layer, thereby achieving excellent pentode characteristics. Therefore, dual gate TFT structure is not sufficient to provide a high current level required for driver TFT of the organic LED.
A TFT including a plurality of gate electrodes, that is, a so-called dual gate TFT, and a display device including dual gate TFT are also disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963. Dual gate TFT disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963 comprises a second gate electrode formed to be narrower than a channel length, and thus stabilizes a threshold voltage while preventing a leakage current. However, dual gate TFT disclosed in the gazette of Japanese Patent Laid-Open No. 2000-243963 does not enable gray level control through discrete control on current levels while keeping a high curr
Suzuki Hiroshi
Tsujimura Takatoshi
Hogg William N.
Ngo Ngan V.
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