Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-06-09
2003-05-13
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C345S098000, C345S087000, C349S048000
Reexamination Certificate
active
06563270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a differential amplifier circuit utilizing thin film transistors, a buffer amplifier utilizing the same and a displays such as liquid crystal displays and electro-luminescence display utilizing the same.
2. Description of the Prior Art
Conventional semiconductors used for liquid crystal displays utilizing thin film transistors include amorphous silicon, high temperature polysilicon formed using a maximum processing temperature of 1000° C. or the like and low temperature polysilicon formed using a maximum processing temperature of 600° C. or less.
Recently, there are displays which utilize medium temperature (temperatures between the high and low temperatures) polysilicon formed using a maximum processing temperature in the range from 600° C. to 1000° C. and displays which utilize a mixture of amorphous silicon and low temperature polysilicon.
FIG. 5A
shows a liquid crystal display. Pixels
1001
are arranged in the form of a grid and are each connected to a data line
1005
and gate line
1004
. A signal from a terminal
1021
such as a computer is converted by a D-A converter
1020
from a digital signal to an analog signal which is sent to a video signal line
1022
. When the terminal
1021
is a television set which outputs an analog signal by itself, the D-A converter
1020
is not provided. The video signal line
1022
is connected to the source or drain of a switch transistor
1003
connected to each data line, and the switch transistor
1003
is opened when an open signal from a horizontal scan circuit
1010
is supplied to a switch gate line
1012
to supply a video signal to the data line
1005
.
The video signal supplied to the data line
1005
is supplied through a buffer amplifier
1002
to the pixel
1001
. The area of a pixel is shown in FIG.
5
B. The data line
1005
is connected to the source or drain of the pixel switch transistor, and the source or drain which is not connected to the data line is connected to a liquid crystal
1050
and a storage capacity
1040
.
The gate of the pixel transistor is connected to a gate line
1004
which is connected to a vertical scan circuit
1011
.
The semiconductor used for this pixel transistor is any of amorphous silicon, low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon and a mixture of amorphous silicon and low temperature polysilicon as described above.
Referring to a liquid crystal display utilizing amorphous silicon as a pixel transistor, amorphous silicon can not be used as a semiconductor for a buffer amplifier, switch transistor, horizontal scan circuit, vertical scan circuit or D-A converter because the field effect mobility of amorphous silicon is too small.
Therefore, the above-described circuits employ a monolithic IC utilizing a crystalline semiconductor such as single crystal silicon.
This results in a problem in that the surface area of regions on a liquid crystal display other than the display portion becomes large.
Further, cost reduction can not be achieved because of a high IC cost because a monolithic IC utilizing a crystalline semiconductor is used.
Under such circumstances, the use of polysilicon has been proposed. In the case of a liquid crystal display utilizing a low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon or a mixture of amorphous silicon and low temperature polysilicon, since the field effect mobility of such a material is 10 to 100 times greater than that of amorphous silicon, it can be used as a semiconductor for the buffer amplifier, switch transistor, horizontal scan circuit and vertical scan circuit. A bipolar transistor or the like utilizing single crystal silicon semiconductor is required for a D-A converter which converts a digital signal into an analog signal in a very short period of time.
In the case of a liquid crystal display utilizing a low temperature polysilicon, medium temperature polysilicon, high temperature polysilicon or a mixture of amorphous silicon and low temperature polysilicon, since the pixel switches, buffer amplifier, switch transistors, horizontal scan circuit and vertical scan circuit can be formed on a substrate using polysilicon, it is possible to reduce the surface area of regions of the liquid crystal display other than the display portion.
There is another advantage in that cost reduction can be achieve in an amount corresponding to a reduction in the IC cost because no monolithic IC utilizing a crystalline semiconductor is used.
FIGS. 6A and 6B
show conventional buffer amplifiers which can be formed using polysilicon.
FIG. 6A
shows a source follower formed using an n-channel type thin film transistor
1101
and a load resistor
1102
, which is a buffer amplifier having a gain of substantially
1
.
1103
designates an input terminal;
1106
designates an output terminal;
1104
designates a positive power source; and
1105
designates a positive power source.
FIG. 6B
shows a source follower formed using a p-channel type thin film transistor
1111
and a load resistor
1112
, which is a buffer amplifier having a gain of substantially
1
.
1113
designates an input terminal;
1116
designates an output terminal;
1114
designates a positive power source; and
1115
designates a negative power source.
These buffer amplifier can be very simply formed because they are formed using only one transistor. However, since the transistors have great variation and greatly fluctuate in characteristics in response to temperature changes, they become inoperable as a buffer amplifier for an increased number of tones because such variation hides some tones, although they work properly for a liquid crystal display having a small number of display tones.
Referring to tones, when a liquid crystal display is driven by 5 V, the voltage width for one tone where there are eight tone in total is 5 V/8 that is 625 mV. The voltage width will be 313 mV, 156 mV, 78 mV, 39 mV and 20 mV where there are 16, 32, 64, 128 and 256 tones, respectively.
The number of tones that one transistor can provide is limited to the range from 16 to 32 tones because of changes in characteristics and variation from one transistor to another.
Further, even if a differential circuit is formed using two. transistors, there is a problem inherent to polysilicon which has hindered variation in characteristics from being improved.
FIGS. 7A and 7B
illustrate the problem inherent to polysilicon.
FIG. 7A
is a plan view of an insulated gate type polysilicon thin film transistor (hereinafter referred to as polysilicon thin film transistor) comprised of a gate electrode
1202
, polysilicon
1201
, a source electrode
1203
, source contacts
1205
, a drain electrode
1204
and drain contacts
1206
. The polysilicon
1201
includes grain boundaries
1210
,
1211
and
1212
. The grain boundaries vary depending on the conditions under which the polysilicon has been crystallized, crystal nuclei, and their positions in the substrate rather than being formed uniformly.
FIG. 7B
shows a sectional view taken along the line X-X′ wherein the polysilicon
1201
, the gate electrode
1202
, a layer insulation film
1208
, the source electrode
1203
, the source contacts
1205
, the drain electrode
1204
and the drain contacts
1206
are shown on a substrate
1200
. The grain boundaries
1210
,
1211
and
1212
are formed in the form of cuts into the section as illustrated.
Carriers flowing from the source to the drain traverse the grain boundaries.
The problem is not significant when there is only one polysilicon thin film transistor. However, when a buffer amplifier or the like is formed by a plurality of transistors, the characteristics of the individual transistors vary slightly because the grain boundaries in the individual transistors vary. This is unsuitable for use in thin film transistor circuits such as differential amplifier circuits wherein transistors should have the same characteristics, and it has been only possible to configure bu
Philogene Haissa
Semiconductor Energy Laboratory Co,. Ltd.
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