Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2000-11-13
2002-12-31
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S082000, C345S090000
Reexamination Certificate
active
06501466
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a display apparatus which employs a plurality of light emitting elements such as organic electro-luminescence elements that are controlled in their intensity by currents flowing through each picture element. This invention is particularly relates to a display apparatus of a so-called active matrix type display apparatus in which an amount of current supplied to each light emitting element is controlled with active elements such as insulated gate type field effect transistors equipped in each picture element. This invention further relates to a drive circuit to be applied to such active matrix type display apparatus, wherein leakage current of sub-threshold level flowing through the insulated gate type field effect transistors is effectively suppressed.
2. Description of the Related Art
Generally, in a picture display apparatus of an active matrix type, a plurality of picture elements are arranged in a matrix form, and a video image is displayed by controlling intensity of each picture element according to given intensity information of the video image. A transmission factor of each picture element changes according to an applied voltage to each picture element when a liquid crystal device is used as an electro-optic material. In the picture display apparatus of the active matrix type employing organic materials as the electro-optic materials, the operation thereof is similar to the operation of the liquid crystal device. However different from the liquid crystal display, an organic EL (Electro-Luminescence) display is a so-called self-radiation type display having a light emitting device at each picture element, so that the EL display has advantages over the liquid crystal device as follows. Namely, a visibility of a video image is higher, a back-light is not necessary and a response speed thereof is faster than that of the liquid crystal display. Intensity of the individual light emitting device of the organic EL (Electro Luminescence) display is controlled by the amount of drive current. Namely, the organic EL display is greatly different from the liquid crystal display in the point that the light emitting device is a current control type or a current drive type element.
Similar to the liquid crystal display, the organic EL display can possibly take both a simple matrix type and an active matrix type as the drive system. In the simple matrix type drive system, the construction thereof is simple, but it is difficult to apply a large-scale display and a high definition display. Accordingly the development for the active matrix system is more active than for the simple matrix type system. In the active matrix system, the current flowing through the light emitting device of each picture element is controlled with an active element (Thin Film type Transistor (TFT) which is one of an insulated gate type field effect transistor) fabricated in the picture element. An example of one picture element in the organic EL display of this active matrix system is depicted in
FIG. 6
as an equivalent circuit. Each picture element comprises a light emitting device OLED, a first thin film transistor TFT
1
, a second thin film transistor TFT
2
and a retention capacitor C. The light emitting device is an organic electro-luminescence (EL) element. The most of the organic Electro-luminescence device has a rectification characteristic so that the EL element can be called an OLED (Organic Light Emitting Diode) device, and in this
FIG. 6
, a sign of a diode device is applied to a sign for the light emitting device OLED. The light emitting device is not limited to the OLED device, and another type light emitting element can be applied if the intensity of such element is controlled by the drive current flowing through the element. In addition, as the light emitting device, the rectification characteristic is not always demanded. In the figure, a source electrode of the P-channel type transistor TFT
2
is connected to a Vdd (power potential), a cathode electrode of the light emitting device OLED is connected to ground potential and an anode electrode of the light emitting device OLED is connected to a drain electrode of the P-channel type transistor TFT
2
. On the other hand, a gate electrode of the N-channel type transistor TFT
1
is connected to a scanning line SCAN, a source electrode thereof is connected to a data line DATA and a drain electrode thereof is connected to both the retention capacitor C and a gate electrode of the transistor TFT
2
.
At first the scanning line SCAN is made in selected status in order to drive the picture element, then a data potential (signal voltage) Vw representing an intensity information is given to the data line DATA. Then the transistor TFT
1
is made ON, thereby the retention capacitor C charges or discharges and a gate potential of the transistor TFT
2
becomes the data potential Vw. After that, the scanning line SCAN is made in non-selected status, and the transistor TFT
1
is accordingly made OFF. In this case, the transistor TFT
2
is separated electrically from the data line DATA, but the gate potential of the transistor TFT
2
is maintained stable by virtue of the retention capacitor C. A current flowing through the light emitting device OLED by way of the transistor TFT
2
corresponds to a value of a gate-source voltage Vgs of the transistor TFT
2
, so that the light emitting device OLED continues to emit light with the intensity corresponding to the current amount supplied through the transistor TFT
2
.
By the way, a current Ids flowing between the drain-source of the transistor TFT
2
is a drive current to be supplied to the light emitting device OLED. When the transistor TFT
2
works in a saturation range, the drive current Ids is shown with a following expression.
Ids=&mgr;×Cox×W/L×
(
Vgs−Vth
)
2
/2
=&mgr;×
Cox×W/L
×(
Vw−Vth
)
2
/2 (1)
Where the Cox is a gate capacitance of an unit area, and the Cox is given with the following expression.
Cox
=∈
0
×∈
r/d
(2)
In these expressions (1) and (2), Vth shows a threshold voltage of the transistor TFT
2
, &mgr; shows a mobility of a carrier, the W shows a channel width, L shows a channel length, ∈
0
shows an electric constant, the ∈ r shows a relative permittivity of a gate insulator film and the d is a thickness of the gate insulator film.
According to the expression (1), the drive current Ids can be controlled by the data potential Vw to be applied to the picture element. As a result, the intensity of the light emitting device OLED can be controlled in accordance with the drive current Ids. The reason for operating the transistor TFT
2
in the saturation range is explained as follows. Namely the drive current Ids is controlled only by the gate-source voltage Vgs of the transistor TFT
2
in the saturation range, and the drive current Ids does not depend on the drain-source voltage Vds of the transistor TFT
2
. Namely, even if the drain-source voltage Vds of the transistor TFT
2
changes by characteristic dispersion of the light emitting device OLED, a predetermined amount of the drive current Ids can be stably supplied to the light emitting device OLED.
As above described, in the circuit structure of the picture element as shown in
FIG. 6
, once the light emitting device OLED is supplied the signal voltage Vw, the light emitting device OLED continues to emit light with a constant intensity during one scan cycle (one frame) until the writing voltage is renewed next. As shown in
FIG. 7
, the active matrix type display apparatus is constituted by arranging a plurality of the picture elements, such as depicted in
FIG. 6
, in a matrix form. In the conventional active matrix type display apparatus; as shown in
FIG. 7
, scanning lines SCAN-
1
to SCAN-N for selecting one picture element
25
with a predetermined scanning cycle (one frame of the NTSC standard) and data lines DATA for giving inte
Yamagishi Machio
Yumoto Akira
Kananen Ronald P.
Said Mansour M.
Shankar Vijay
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