Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2001-11-09
2004-12-14
Eckert, II, George C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S066000, C257S072000, C257S500000
Reexamination Certificate
active
06831299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a semiconductor device having an analog buffer circuit composed of TFTs (thin film transistors) including polycrystalline semiconductor layers. Also, the present invention relates to a semiconductor device as an image display device.
2. Description of the Related Art
In recent years, the demand for information communications equipment is increased in accordance with the outspread of information communications. Here, a display device for displaying an image is essential for the information communications equipment. As the display device, there are a liquid crystal display device using liquid crystal, an EL (electroluminescence) display device using an EL element and the like. However, in accordance with the attempt to upsize a display portion and to make it have higher resolution, an active matrix display device in which a TFT is arranged in each pixel is becoming the mainstream.
FIG. 8
is a block diagram of an active matrix display device. A source signal line driver circuit and a gate signal line driver circuit are located around a pixel portion. The pixel portion, the source signal line driver circuit, and the gate signal line driver circuit are integrally formed on a substrate. Signals outputted from the source signal line driver circuit are inputted to source signal lines and transmitted to respective pixels. Also, signals outputted from the gate signal line driver circuit are inputted to gate signal lines and transmitted to respective pixels. The pixel portion is constructed by using liquid crystal, an EL element or the like. Here, an example of a structure of a pixel in the case where EL element is used will be shown in FIG.
15
.
Note that an EL element in this specification includes both an element for producing luminescence (fluorescence) from a singlet state and an element for producing luminescence (phosphorescence) from a triplet state.
The gate electrode of a switching TFT is connected with a gate signal line. One of a source region and a drain region is connected with a source signal line and the other is connected with one electrode of a capacitor and a gate electrode of an EL driver TFT. The other electrode of the capacitor, which is not connected with the switching TFT, is connected with a power source supply line. One of the source region and the drain region of the EL driver TFT is connected with the power source supply line and the other is connected with an EL element.
A method of driving a pixel with the above structure will be briefly described.
In the pixel of which gate signal line is selected, an analog signal voltage inputted from the source signal line is applied to the capacitor and the gate electrode of the EL driver TFT through a switching TFT which becomes to be in a conduction state. By this applied voltage, a current flows from the power source supply line to the EL element or in its reverse direction through the EL driver TFT. The EL element emits light with an intensity corresponding to the flowed current.
In order to miniaturize the display device and reduce a manufacturing cost, manufacturing a pixel portion and a driver circuit portion (source signal line driver circuit and gate signal line driver circuit) on a single substrate is attempted. At this time, TFTs composing the pixel portion and the driver circuit portion are manufactured using polycrystalline semiconductor layers.
Here, a structure of the source signal line driver circuit for outputting analog signals to the source signal lines will be described. Note that the source signal line driver circuit for outputting analog signals to x (x is natural number) source signal lines is assumed. As a drive method of the source signal line driver circuit, a point sequential drive and a line sequential drive are exemplified.
First, the point sequential drive will be described. In the point sequential drive, signals are inputted to the source signal lines in succession one by one. A block diagram of the source signal line driver circuit of the point sequential drive is shown in FIG.
9
.
The source signal line driver circuit is composed of a shift register
901
, an analog signal input line
903
and switching circuits
904
(SW.
1
to SW.x), and outputs signals to source signal lines S
1
to Sx.
In accordance with sampling signals from the shift register
901
, an analog signal voltage inputted from the analog signal input line
903
is outputted to the source signal lines S
1
to Sx in succession through the switching circuits
904
(SW.
1
to SW.x).
At this time, when a length of an effective horizontal scan period is indicated by a symbol H
1
(about 80% of a horizontal scan period) and the number of source signal lines (the number of pixels in a transverse direction) is indicated by a symbol N, a period which can be used for inputting a signal to one source signal line becomes H
1
/N.
This drive method has such an advantage to be able to simplify the structure of the driver circuit. However, in the display device having a large display portion and one having a high resolution, since N becomes larger, a signal output period per pixel H
1
/N is shortened, and thus cannot be sufficiently set. Therefore, the line sequential drive which will be described next is mainly made.
A block diagram of the source signal line driver circuit with the line sequential drive is shown in FIG.
10
.
The source signal line driver circuit shown in
FIG. 10
is composed of a shift register
101
, an analog signal input line
103
, a signal transfer line
106
, retaining capacitors
105
and
108
, first switching circuits (SW
1
1
to SW
1
x)
104
, second switching circuits (SW
2
1
to SW
2
x)
107
, and analog buffer circuits (AB.
1
to AB.x)
109
. In accordance with sampling signals from the shift register
101
, an analog signal inputted from the analog signal input line
103
is sampled and retained in the retaining capacitors
105
through the first switching circuits
104
. After the signals corresponding to one line is retained, these signals are retained in the next retaining capacitors
108
through the second switching circuits
107
in accordance with a signal inputted to the signal transport line
106
. Here, the retained signals corresponding to one line are simultaneously outputted to the source signal lines S
1
to Sx. Here, while the signals are outputted to the source signal lines S
1
to Sx, that is, immediately after the signals are outputted to the second switching circuits
107
, signals corresponding to next one horizontal line are retained in succession from the analog signal input line into the retaining capacitors
105
through the first switching circuits
104
.
According to this drive method, in the source signal line driver circuit, output signals corresponding to one horizontal line are retained first, and then simultaneously outputted to the source signal lines. Thus, even in the case of a display device having a large number of pixels, a period for outputting the signals to the source signal lines can be sufficiently set.
Here, when a large size panel is used, a load applied to per source signal line is increased. In order to reduce the influence of rounding of a signal due to the load, a signal amplifying circuit is required. Thus, in the block diagram shown in
FIG. 10
, the analog buffer circuits (AB.
1
to AB.x)
109
are located as signal amplifying circuits before the signals are outputted to the source signal lines. An example of the analog buffer circuit is shown in FIG.
5
.
In
FIG. 5
, the analog buffer circuit is composed of a differential circuit
5501
, a current mirror circuit
5502
and a constant current source
5503
. The differential circuit
5501
is composed of TFTs
5505
and
5506
. The current mirror circuit
5502
is composed of TFTs
5507
and
5508
. The constant current source
5503
is composed of a TFT
5504
.
The gate electrodes of the TFTs
5507
and
5508
are connected with each other. One of the source regi
Costellia Jeffrey L.
Eckert II George C.
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
LandOfFree
Semiconductor device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3330581