Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-04-25
2003-08-26
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S063000, C345S077000
Reexamination Certificate
active
06611108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic device structure. In particular, the present invention relates to an active matrix electronic device having a thin film transistor (TFT) formed on an insulating body, and to a method of driving an active matrix electronic device.
2. Description of the Related Art
EL displays (also referred to as electroluminescence displays) have been gathering attention in recent years as flat panel displays, which are substitutes for LCDs (liquid crystal displays), and research into such displays is proceeding apace.
LCDs can roughly be divided into two types of driving methods. One is a passive matrix type used in an LCD such as an STN-LCD, and the other is an active matrix type used in an LCD such as a TFT-LCD. EL displays can also be similarly broken down roughly into two types. One is a passive matrix type, and the other is an active matrix type.
For the passive matrix type, wirings which become electrodes are arranged in portions above and below EL elements (also referred to as electroluminescence elements). Voltages are applied to the wirings in order, and the EL elements turn on due to the flow of an electric current. On the other hand, each pixel has a thin film transistor with the active matrix type, and a signal can be stored within each pixel.
A schematic diagram of an active EL display device is shown in
FIGS. 13A and 13B
.
FIG. 13A
is a schematic diagram of an entire circuit, and a substrate
1350
has a pixel portion
1353
in its center. Gate signal line driver circuits
1352
for controlling gate signal lines are arranged to the left and right of the pixel portion. The arrangement may also be on only one side, left or right, but considering such issues as operational efficiency and reliability, it is preferable to use both positions as shown in
FIG. 13A. A
source signal line driver circuit
1351
for controlling source signal lines is arranged above the pixel portion. One pixel portion circuit in the pixel portion
1353
of
FIG. 13A
is shown in FIG.
13
B. Reference numeral
1301
denotes a TFT which functions as a switching element during write in to the pixel (hereafter referred to as a switching TFT) in FIG.
13
B. Reference numeral
1302
denotes a TFT which functions as an element (electric current control element) for controlling electric current supplied to EL elements
1303
(hereafter referred to as an EL driver TFT). The EL driver TFT
1302
is arranged between an anode of the EL element
1303
and an electric current supply line
1307
in FIG.
13
B. It is also possible, as a separate structuring method, to arrange the EL driver TFT
1302
between a cathode of the EL element
1303
and a cathode electrode
1308
. However, from the fact that it is good for TFT operation to have a source region connected to ground, and from limitations on the manufacture of the EL elements
1303
, a method in which a p-channel TFT is used in the EL driver TFT
1302
and is arranged between the anode of the EL element
1303
and the electric current supply line
1307
is generally seen and often employed. Reference numeral
1304
denotes a storage capacitor for storing a signal (voltage) input from a source signal line
1306
. One terminal of the storage capacitor
1304
is connected to the electric current supply line
1307
in
FIG. 13B
, but a specialized wiring may also be used. A gate electrode of the switching TFT
1301
is connected to a gate signal line
1305
, and a source region of the switching TFT
1301
is connected to the source signal line
1306
. Further, a drain region of the EL driver TFT
1302
is connected to an anode
1309
of the EL element
1303
, and a source region of the EL driver TFT
1302
is connected to the electric current supply line
1307
.
The EL element has a layer (hereafter referred to as an EL layer) containing an organic compound in which electroluminescence (luminescence generated by the addition of an electric field) is obtained, an anode, and a cathode. As to the luminescence in the organic compound, there is emission of light when returning to a ground state from a singlet excitation state (fluorescence), and emission of light when returning to a ground state from a triplet excitation state (phosphorescence), and the electronic device of the present invention may use both types of light emission.
Note that all layers formed between the anode and the cathode are defined as EL layers in this specification. Specifically, layers such as a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer are included as EL layers. An EL element basically has a structure in which an anode, a light emitting layer, and a cathode are laminated in order. In addition to this structure, the EL element may also have a structure in which an anode, a hole injecting layer, a light emitting layer, and a cathode are laminated in order, or a structure in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, and a cathode are laminated in order.
Furthermore, an element formed by an anode, an EL layer, and a cathode is referred to as an EL element within this specification.
Circuit operation of an active matrix electronic device is explained next with reference to
FIGS. 13A and 13B
. First, a voltage is applied to the gate electrode of the switching TFT
1301
when the gate signal line
1305
is selected, and the switching TFT
1301
is placed in a conducting state. The signal (voltage) of the source signal line
1306
is thus stored in the storage capacitor
1304
. The voltage of the storage capacitor
1304
becomes a voltage V
GS
between the gate and the source of the EL driver TFT
1302
, and therefore the electric current in response to the storage capacitor
1304
voltage flows in the EL driver TFT
1302
and in the EL element
1303
. As a result, the EL element
1303
turns on. The brightness of the EL element
1303
, namely the amount of electric current flowing in the EL element
1303
, can be controlled by V
GS
of the EL driver TFT
1302
. V
GS
is the voltage stored in the storage capacitor
1304
, and is the signal (voltage) inputted to the source signal line
1306
. In other words, the brightness of the EL element
1303
is controlled by controlling the signal (voltage) of the source signal line
1306
. Finally, the gate signal line
1305
is unselected, the gate of the switching TFT
1301
is closed, and the switching TFT
1301
is placed in a non-conducting state. The electric charge stored in the storage capacitor
1304
continues to be stored at this point. V
GS
of the EL driver
1302
is therefore stored as is, and the electric current in response to V
GS
continues to flow in the EL driver TFT
1302
and in the EL element
1303
.
Information regarding the above explanation is reported upon in papers such as the following: “Current Status and Future of Light-Emitting Polymer Display Driven by poly-Si TFT”, SID99 Digest, p. 372; “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, ASIA DISPLAY 98, p. 217; and “3.8 Green OLED with Low Temperature Poly-Si TFT”, Euro Display 99 Late News, p. 27.
Analog gray scale methods and digital gray scale methods exist as methods of gray scale expression for an EL display. In the analog gray scale method, the value of V
GS
of the EL driver TFT
1302
is changed, the amount of electric current flowing in the EL element
1303
is controlled, and the brightness is changed in an analog manner. In the digital gray scale method, on the other hand, the voltage between the gate and the source of the EL driver TFT operates at only two levels: a range in which no electric current flows in the EL element
1303
(equal to or less than the turn on start voltage); and a range in which the maximum electric current flows (equal to or greater than the brightness saturation voltage). In other words, the EL element
1303
only take
Fish & Richardson P.C.
Lee Wilson
Semiconductor Energy Laboratory Co,. Ltd.
Wong Don
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