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
2000-01-19
2004-08-24
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S204000, C257S274000, C257S338000, C257S368000
Reexamination Certificate
active
06781155
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroluminescence display device provided with an electroluminescence element and a thin film transistor.
2. Description of the Related Art
In recent years, electroluminescence (hereinafter referred to as EL) display devices employing EL elements have attracted attention as an alternatives to devices such as CRTs and LCDs. For example, EL display devices including a thin film transistor (hereinafter referred to as TFT) used as a switching element for driving the EL elements have been researched and developed.
FIG. 1
is an equivalent circuit diagram of a related art EL display device including an EL element and TFTS.
The EL display device shown in
FIG. 1
, which illustrates a portion near a gate signal line Gn in the n
th
row and a drain signal line Dm in the m
th
column, includes first and second TFTS
130
and
140
, and an organic EL element
160
.
The gate signal line Gn for supplying a gate signal and a drain signal line Dm for supplying a drain signal cross each other, and the organic EL element
160
and the TFTs
130
and
140
for driving the organic EL element
160
are provided near an intersection of these signal lines.
The first TFT
130
for switching operation includes a gate electrode
131
connected to the gate signal line Gn and receiving the gate signal, a drain electrode
132
connected to the drain signal line Dm and receiving the drain signal, and a source electrode
133
connected to a gate electrode
141
of the second TFT
140
.
The second TFT
140
for driving the organic EL element includes the gate electrode
141
connected to the source electrode
133
of the first TFT
130
, a source electrode
142
connected to an anode
161
of the organic EL element
160
, and a drain electrode
143
connected to a driving power supply
150
for supplying a current to the organic EL element
160
.
The organic EL element
160
includes the anode
161
connected to the source electrode
142
, a cathode
162
connected to a common electrode
164
, and a light emissive element layer
163
sandwiched between the anode
161
and the cathode
162
.
The device further includes a storage capacitor
170
having one electrode
171
connected between the source electrode
133
of the first TFT
130
and the gate electrode
141
of the second TFT
140
, and the other electrode
172
connected to a common electrode
173
.
A method of driving the circuit shown in the equivalent circuit diagram of
FIG. 1
will next be described with reference to signal timing charts in
FIG. 2
, in which signal timings of a signal VG(n)
1
supplied to the gate electrode
131
of the first TFT
130
in the n
th
row, a drain signal VD at the drain signal line Dm, and a signal VG(n)
2
supplied to the gate electrode
141
of the second TFT
140
in the n
th
row are illustrated in (a)-(c), respectively.
When the gate signal VG(n)
1
illustrated in FIG.
2
(
a
) is applied from the gate signal line Gn to the gate electrode
131
, the first TFT
130
is switched on. As a result, the drain signal VD illustrated in FIG.
2
(
b
) is supplied from the drain signal line Dm to the gate electrode
141
, which attains the same potential as the drain signal line Dm. A current corresponding to the value of a voltage applied to the gate electrode
141
is then supplied from the driving power supply
150
to the EL element
160
, which is caused to emit light.
It should be noted that in actual operation, while the first TFT
130
is in ON state, a current flows until the gate electrode
141
attains the same potential as the drain signal line Dm and electric charges are stored in a gate capacitor of the gate electrode
141
. After the first TFT
130
is turned off,the electric charges stored in the gate capacitor must be maintained, and also the potential of the gate must be retained as illustrated by the broken line in FIG.
2
(
c
).
In the above-described EL display device, however, a leakage current flows during the OFF period of the TFT. As a result, when the drain signal VD changes every horizontal period (IH) as illustrated in FIG.
2
(
b
), the potential VG(n)
2
of the gate electrode
141
cannot be maintained, but is changed as shown by the solid line in FIG.
2
(
c
).
More specifically, as illustrated by the solid line in FIG.
2
(
c
), (i) when the potential of the drain signal line Dm is lower than that supplied to the gate electrode
141
, a leakage current flows to the drain signal line Di through the first TFT
130
, decreasing the potential of the gate electrode
141
; and (ii) when the potential of the drain signal line Dm is higher than that supplied to the gate electrode
141
, a leakage current flows to the gate electrode
141
through the first TFT
130
, resulting in further storage of electric charges in the gate capacitor and in a higher potential of the gate electrode
141
.
In configuration (i) above, the organic EL element
160
receives a current larger than it is supposed to receive, leading to a higher luminance of the organic EL element. On the other hand, the element
160
will have a lower luminance in configuration (ii) above.
With either configuration, the device has a drawback in that it is difficult to cause each display pixel to emit light at the appropriate luminance when a large leakage current flows through the first TFT
130
as indicated by the solid line in FIG.
2
(
c
).
The second TFT serves to control the current supplied from the power supply for driving the organic EL element in accordance with the voltage applied to the gate of the second TFT, and supply it to the organic EL element. The second TFT has an active layer that includes an intrinsic or substantially intrinsic channel region overlapping its gate, and source and drain regions located on both sides of the channel region and having impurities doped therein.
However, when the second TFT is an n-channel transistor, a so-called saturation region of the drain current-drain voltage (Id-vd) characteristics, i.e. a region where the drain current Id is constant even though the drain voltage vd is increased, is extremely narrow (saturation characteristics are poor), as indicated by broken lines in FIG.
3
B. Consequently, the current value Id is increased with an increase in the value of Vd, and therefore a constant current cannot be obtained, but is affected by the voltage Vd, leading to a poor current controllability.
Especially when the TFT is formed of polycrystalline silicon, there exist grain boundaries of crystals, and electrons are trapped therein to form a potential barrier, thereby spreading a depletion layer. As a result, a strong electric field is applied to the grain boundaries at the edges of the drain electrode, whereby a collisional ionization phenomenon, in which accelerated electrons collide with lattices, occurs and the drain current is not saturated, but increased. This is a significant problem in the n-channel TFT, but rare in the p-channel TFT.
SUMMARY OF THE INVENTION
The present invention has been conceived in view of the above-described problems, and an object thereof is to provide an EL display device which creates a superior gradation (gray scale) display by, expressed in the terminology introduced above, suppressing a leakage current at the first TFT
130
to maintain the potential at the gate electrode
141
of the second TFT
140
, and improving current controllability of the second TFT
140
.
According to one aspect, the present invention provides an electroluminescence device which includes an electroluminescence element having a light emissive layer provided between first and second electrodes, a first thin film transistor receiving a selection signal at its gate to acquire a data signal, and a second thin film transistor provided between a driving power supply and the electroluminescence element and controlling power supplied from the driving power supply to the electroluminescence element in accordance with the data signal applied from the first thin film transistor. In this electr
Abraham Fetsum
Cantor & Colburn LLP
Sanyo Electric Co,. Ltd.
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