Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2001-03-26
2002-11-05
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S157000
Reexamination Certificate
active
06475845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the construction of an electro-optical device. The present invention particularly relates to an active matrix type electro-optical device having a thin film transistor (TFT) made on an insulating substrate.
2. Related Background Art
In recent years, an EL display has been attracting attentions as a flat panel display that is to replace an LCD (liquid crystal display), and is actively researched. In the specification, the EL display has an EL element which is also called a light emitting device or a light emitting diode. Further, the EL (Electro Luminescence) includes triplet-based light emission or singlet-based light emission.
There are generally two types of driving system for the LCD display. One type is a passive matrix type used in an STN-LCD, etc. The other type is an active matrix type used in a TFT-LCD, etc. Similarly, there are generally two kinds of driving systems in the EL display. One type is a passive matrix type, and the other type is an active matrix type.
In the case of the passive matrix type, a wiring to serve as an electrode is arranged in each of upper and lower portions of an EL element. A voltage is sequentially applied to the wirings, and an electric current flows through the EL element so that the EL element is lighted.
In contrast to this, in the case of the active matrix type, each pixels has a TFT, and a signal can be held within each pixels.
FIGS. 15A and 15B
show a constructional example of the active matrix type electro-optical device used in the EL display.
FIG. 15A
is a view showing the construction of the entire circuit in which a pixel portion is arranged in the center of this circuit. A gate signal line side driving circuit for controlling the operation of a gate signal line is arranged to the left of the pixel portion. A source signal line side driving circuit for controlling the operation of a source signal line is arranged above the pixel portion. In
FIG. 15A
, a portion surrounded by a dotted line frame shows a circuit of one pixel.
FIG. 15B
shows an enlarged view of this circuit. In
FIG. 15B
, reference numeral
1501
designates a TFT (hereinafter called a switching TFT) functioning as a switching element when a signal is written into a pixel. In
FIG. 15B
, the switching TFT has a double gate structure, but may also have a single gate structure, a triple gate structure or a multi-gate structure having more than three gates. One of polarities of the TFT may be selected in accordance with a constructional form of the circuit. Reference numeral
1502
designates a TFT (hereinafter called an EL driving TFT) functioning as an element (an electric current control element) for controlling an electric current supplied to an EL element
1503
. In
FIG. 15B
, the TFT
1502
is arranged between an anode
1509
of the EL element
1503
and an electric current supply line
1507
. In an alternative constructional method, it is also possible to arrange the TFT
1502
between a cathode
1510
of the EL element
1503
and a cathode electrode
1508
. One of polarities of the TFT may be selected in accordance with the constructional form of the circuit. In this case, a system is common and often used in which a p-channel type TFT is used for the EL driving TFT, and the EL driving TFT is arranged between the anode
1509
of the EL element
1503
and the electric current supply line
1507
, since source grounding is preferable as the operation of a transistor, and there is a restriction in manufacture of the EL element
1503
. Reference numeral
1504
designates a holding capacitor for holding a signal (voltage) inputted from a source signal line
1505
. One terminal of the holding capacitor
1504
in
FIG. 15B
is connected to the electric current supply line
1507
, but there is also a case in which dedicated wiring is used. A gate terminal of the switching TFT
1501
is connected to a gate signal line
1506
, and a source terminal of this TFT
1501
is connected to the source signal line
1505
. A drain terminal of the EL driving TFT
1502
is connected to the anode
1509
of the EL element
1503
, and a source terminal of this TFT
1502
is connected to the electric current supply line
1507
.
An operation of the circuit of the active matrix type electro-optical device will next be explained with reference to
FIGS. 15A and 15B
. First, when the gate signal line
1506
is selected, a voltage is applied to a gate of the switching TFT
1501
, and the switching TFT
1501
attains a turned-ON state. Thus, a signal (voltage) of the source signal line
1505
is accumulated in the holding capacitor
1504
. The voltage of the holding capacitor
1504
becomes a voltage V
GS
between the gate and the source of the EL driving TFT
1502
so that an electric current according to the voltage of the holding capacitor
1504
flows through the EL driving TFT
1502
and the EL element
1503
. As a result, the EL element
1503
is lighted.
Luminance of the EL element
1503
, i.e., an electric current amount flowing through the EL element
1503
can be controlled by V
GS
. V
GS
is the voltage of the holding capacitor
1504
, and is a signal (voltage) inputted to the source signal line
1505
. Namely, the luminance of the EL element
1503
is controlled by controlling the signal (voltage) inputted to the source signal line
1505
. Finally, the gate signal line
1506
is set to a not-selected state, and the gate of the switching TFT
1501
is closed, and the switching TFT
1501
is set to a turned-OFF state. At that time, electric charges accumulated in the holding capacitor
1504
are held. Accordingly, V
GS
is held as it is, and an electric current according to V
GS
continuously flows through the EL driving TFT
1502
and the EL element
1503
.
The descriptions above are reported in SID99 Digest: P372: “Current Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFT”, ASIA DISPLAY 98: P217: “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, Euro Display99 Late News: P27: “3.8 Green OLED with Low Temperature Poly-Si TFT”, etc.
In the active matrix type electro-optical device, it is required that the pixel has a large holding capacity and high aperture ratio in view of display performance of this device. Since each pixel has the high aperture ratio, utilization efficiency of light is improved and a display unit can be saved in power and made compact.
In recent years, the pixel is reduced in size and an image with higher definition is required. Since the pixel size is reduced, regions for forming the TFT and wiring come to occupy increased area in one pixel, and the aperture ratio of the pixel is reduced.
Therefore, efficient layout of circuit elements that are required in the circuit construction of the pixel is indispensable to obtain a high aperture ratio of each pixel in the prescribed pixel size.
As mentioned above, a new pixel construction that has not conventionally been found is needed to realize the active matrix type electro-optical device having a high pixel aperture ratio with a reduced mask number.
SUMMARY OF THE INVENTION
The present invention is made to meet such a request, and an object of the present invention is therefore to provide an electro-optical device having a pixel realizing a high aperture ratio by using a pixel having a novel construction without increasing a mask number and a step number.
To solve the above problems of the prior art, the present invention has the following measures.
In the electro-optical device of the present invention, the attention is paid to the fact that, in the construction of a pixel portion of this device that a certain gate signal line has a constant electric potential in a period except for a period where this gate signal line is selected. The electro-optical device of the present invention is characterized in that, when a gate signal line in an i-th row is selected, one of the gate signal lines including the gate signal line
Dang Phuc T.
Fish & Richardson P.C.
Nelms David
Semiconductor Energy Laboratory Co,. Ltd.
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