Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2001-09-28
2004-08-31
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C359S016000, C257S069000, C257S072000, C257S351000
Reexamination Certificate
active
06784454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroluminescent display device, and in particular, to transistors constructing the circuit structure in the pixel section of an electroluminescent display device.
2. Description of the Related Art
An electroluminescence (hereinafter referred to as EL) display device which uses an EL element which is a self-illuminating element as an illumination element in each pixel has attracted a strong interest as an alternative display device for a display device such as a liquid crystal display device (LCD) and a CRT because the EL display device has advantages such as thin width and low power consumption, in addition to the advantage of being self-illuminating. Such an EL display device has thus been researched.
In particular, there is a high expectation for an active matrix type EL display device in which a switching element such as, for example, a thin film transistor for individually controlling an EL element is provided for each pixel and EL elements are controlled for each pixel, as a high resolution display device.
FIG. 1
shows a circuit structure for one pixel in an active matrix type EL display device having m rows and n columns. In the EL display device, a plurality of gate lines GL extend on a substrate in the row direction and a plurality of data lines DL and power supply lines VL extend on the substrate in the column direction. Each pixel has an organic EL element
50
, a switching TFT (first TFT)
10
, an EL element driving TFT (second TFT)
20
, and a storage capacitor Cs.
The first TFT
10
is connected to the gate line GL and data line DL, and is turned on by receiving a gate signal (selection signal) on its gate electrode. A data signal which is being supplied on the data line DL at this point is then held in the storage capacitor Cs connected between the first TFT
10
and the second TFT
20
. A voltage corresponding to the data signal is supplied to the gate electrode of the second TFT
20
via the first TFT
10
. The second TFT
20
then supplies a current, corresponding to the voltage value, from the power supply line VL to the organic EL element
50
. In this manner, the organic EL element in each pixel is illuminated at a brightness based on the data signal, and a desired image is displayed.
The organic EL element is a current-driven element which is illuminated by supplying a current to an organic emissive layer provided between a cathode and an anode. The data signal output onto the data line DL, on the other hand, is a voltage signal with an amplitude corresponding to the display data. Thus, conventionally, in order to accurately illuminate the organic EL element by such a data signal, in an organic EL display device, a first TFT
10
and a second TFT
20
are provided in each pixel.
The display quality and reliability of the organic EL display devices described above remain insufficient, and the characteristic variations in the first and second TFTs
10
and
20
must be dissolved. In particular, reduction in characteristic variation in the second TFT
20
for controlling the amount of current supplied from the power supply line VL to the organic EL element
50
is desired, because such variation directly causes variation in the illumination brightness.
Moreover, it is preferable to construct the first and second TFTs
10
and
20
from a polycrystalline silicon TFT which has quick operation speed and which can be driven by a low voltage. In order to obtain a polycrystalline silicon, an amorphous silicon is polycrystallized by laser annealing. Because of various reasons such as, for example, energy variation in the irradiating laser at the irradiation surface, the grain size of the polycrystalline silicon is not uniform. When grain size is not uniform, in particular around the TFT channel, there is a problem in that the on-current characteristic or the like of the TFT may also vary.
SUMMARY OF THE INVENTION
The present invention is conceived to solve the above problem, and one object of the present invention is to improve the characteristic by alleviating variations among TFTs, and at the same time, to efficiently provide a plurality of TFTs for controlling an element to be driven, in a device having an organic EL element or the like as the element to be driven.
According to one aspect of the present invention, there is provided a semiconductor device comprising n thin film transistors for controlling the power supplied to an element to be driven which operates based on the supplied power, where n is an integer equal to or greater than 2, provided between the element to be driven and a power supply line for supplying power to the element to be driven, wherein the number of contact points for electrically connecting the n thin film transistors and corresponding element to be driven is equal to or less than (n−1).
From the viewpoint of reliability of the power supply to the element to be driven and of variation prevention, provision of a plurality of element driving thin film transistors for supplying power to the element to be driven is highly effective. On the other hand, for a case where the element to be driven is, for example, an emissive element, the contact section is usually a non-illuminating region. Therefore, by setting the number of contacts between n thin film transistors for supplying power to the element to be driven and the element to be driven to be equal to or less than (n−1), it is possible to improve the reliability as a device, while simultaneously securing a maximum actual operation region (illumination region for an emissive element) for the element to be driven.
According to another aspect of the present invention, there is provided a semiconductor device comprising a thin film transistor for controlling the power supplied to an element to be driven, provided between the element to be driven which operates based on the supplied power and a power supply line for supplying power to the element to be driven, wherein the thin film transistor and corresponding element to be driven are electrically connected to each other by a wiring layer; and the contact position between the wiring layer and the thin film transistor is placed to be distant from the contact position between the wiring layer and the element to be driven.
By placing the contact position between the wiring layer and the thin film transistor distant from the contact position between the wiring layer and the element to be driven, the formation of the element to be driven, which in many cases is formed at a layer above the wiring layer, on a more flat surface is facilitated. The thin film transistor and the wiring layer are separated by an insulation layer and the contact between the thin film transistor and the wiring layer is achieved through a contact hole formed on the insulation layer. The connection between the wiring layer and the element to be driven is achieved through a contact hole formed on another insulation layer for insulating the wiring layer and the element to be driven. Therefore, when the contact hole for connecting the thin film transistor and the wiring layer is formed to overlap to the position of the contact hole for connecting the wiring layer and the element to be driven, the element to be driven, which is formed at the uppermost layer, would be formed on top of an uneven surface with a large height difference due to two (steps of) contact holes. When an emissive element, for example, an organic EL element which uses an organic compound in the emissive layer, is used as the element to be driven, electric field concentration or the like tends to occur at the layer which includes the organic compound if the flatness of the formation surface is not good, and a dark spot which cannot illuminate tends to be generated from such electric field concentration. Therefore, by placing at a distance the contact between the wiring layer and the element to be driven from the contact between the thin film transistor and the wiring layer, the flatness of the formation r
Cantor & Colburn LLP
Nelms David
Sanyo Electric Co,. Ltd.
Tran Mai-Huong
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