Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-03-03
2001-09-18
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C315S337000, C345S074100, C345S076000, C345S211000, C345S212000
Reexamination Certificate
active
06291941
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to field emission displays (FEDs) and to a method for reducing emission to grid during turn on and turn off of a field emission display.
BACKGROUND OF THE INVENTION
Flat panel displays have recently been developed for visually displaying information generated by computers and other electronic devices. These displays can be made lighter and require less power than conventional cathode ray tube displays. One type of flat panel display is known as a cold cathode field emission display a field emission display (FED).
A cold cathode FED uses electron emissions to illuminate a cathodoluminescent screen and generate an image. A single pixel
10
of a prior art FED is shown in FIG.
1
A. The FED pixel
10
includes a substrate
11
formed with a conductive layer
12
. An array of emitter sites
13
are formed on the conductive layer
12
. Although each pixel
10
typically contains many emitter sites (e.g., 4-20 for a small display and several hundred for a large display), for simplicity only one emitter site
13
is shown in
FIG. 1A. A
grid
15
is associated with the emitter sites
13
and functions as a gate electrode. The grid
15
is electrically isolated from the conductive layer
12
by an insulating layer
18
. The grid
15
/conductive layer
12
/substrate
11
subassembly is sometimes referred to as a baseplate.
Cavities
23
are formed in the insulating layer
18
and grid
15
for the emitter sites
13
. The grid
15
and emitter sites
13
are in electrical communication with a power source
20
. The power source
20
is adapted to bias the grid
15
to a positive potential with respect to the emitter sites
13
. When a sufficient voltage differential is established between the emitter sites
13
and the grid
15
, a Fowler-Nordheim electron emission is initiated from the emitter sites
13
. The voltage differential for initiating electron emission is typically on the order of 20 volts or more.
Electrons
17
emitted at the emitter sites
13
collect on a cathodoluminescent display screen
16
. The display screen
16
is separated from the grid
15
by an arrangement of electrically insulating spacers
22
. The display screen
16
includes an external glass face
14
, a transparent electrode
19
and a phosphor coating
21
. Electrons impinging on the phosphor coating
21
cause the release of photons
25
which forms the image. The display screen
16
is the anode in this system and the emitter sites
13
are the cathode. The display screen
16
is biased by the power source
20
(or by a separate anode power source) to a positive potential with respect to the grid
15
and emitter sites
13
. The potential at the display screen
16
is termed herein as anode. In some systems the potential at the display screen
16
is on the order of 1000 volts or more.
One method of addressing the emitter sites
13
for use in video displays is taught by Crost et al. in U.S. Pat. No. 3,500,102. In this method the emitter sites
13
are electrically connected and placed parallel to additional rows of emitter sites. The grids
15
associated with the emitter sites
13
are electrically connected in parallel columns which are orthogonal to the emitter rows. The emitter sites
13
associated with each pixel
10
of the FED are uniquely defined by the intersection point of a specific emitter row and a specific grid column. Electrically addressing a row while simultaneously addressing a column activates a specific pixel
10
.
Another method for addressing the emitter sites
13
for use in video displays is disclosed by Casper et al. in U.S. Pat. No. 5,210,472. In this method, a common grid electrode is employed with respect to all of the pixels in the display. Addressing of the pixels within the display as taught by Casper et al. is accomplished with row and column electrodes which provide access for the emitter sites
13
to a source of electrons.
One problem in a FED that occurs during the turn on process (i.e., power up) is the emission of electrons from the emitter sites
13
to the grids
15
. Emission to grid during turn on is illustrated in FIG.
1
B. During the turn on process, electrons
26
emitted from the emitter sites
13
can go directly to the grid
15
rather than to the display screen
16
. This situation can lead to overheating of the grids
15
. The emission to grid can also affect the voltage differential between the emitter sites
13
and grids
15
. In addition, desorped molecules and ions can be ejected from the grid
15
causing excessive wear of the emitter sites
13
. Electron emission to grid can also lead to electrical arcing
30
between the grid
15
and the conductive layer
12
or emitter sites
13
. In addition, electrons
26
emitted from the emitter sites
13
can strike the spacers
22
causing a charge build up on the spacers
22
.
All of these problems decrease the lifetime, performance and reliability of the FED. Electron emission to grid is particularly a problem in consumer electronic products, such as camcorders, televisions and automotive displays, which are typically turned on and off many times throughout the useful lifetime of the product.
One reason for the electron emission to grid, is that electron emission may have commenced from the emitter sites
13
before the large voltage potential (V
Anode
) has been established at the display screen
16
. Typically, the display screen
16
is a relatively large, relatively high voltage structure which requires some period of time to reach full potential across its entire surface. In addition, the display screen
16
operates at a significantly higher voltage than any other component of the FED. Some period of time is required to ramp up to this operating voltage. Consequently, the display screen
16
can be at a low enough positive potential to allow electron emission to grid
15
to occur, as illustrated in FIG.
1
B. Although this situation may only occur for a relatively short period of time, it can cause system problems as outlined above.
A related situation can also occur during turn on of the display screen
16
and grid
15
if the emitter sites
13
are not electrically controlled. If the emitter sites
13
are not limited during power on, an uncontrolled amount of emission can occur causing the same problems as outlined above.
In addition, a similar situation exists during the turn off process for the FED cell
10
(i.e., power off). If power to the large positive potential at the display screen
16
is lost prior to termination of electron emission from the emitter sites
13
, then electron emission to grid, as illustrated in
FIG. 1B
, can occur.
In view of these problems associated with field emission displays, it is an object of the present invention to provide an improved method for controlling field emission displays to prevent electron emission to grid during turn on and turn off. It is yet another object of the present invention to provide an improved control circuit for an FED adapted to reduce electron emission to grid during turn-on and turn off. Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved method and an improved control circuit for reducing electron emission to grid during turn on and turn off of a field emission display are provided. The control circuit includes a threshold detector, a level shifter, a pass transistor and a pull down transistor. In an illustrative embodiment the threshold detector can be a logical inverter. The inverter receives an input signal (V
1
) that is proportional to the anode voltage (V
Anode
) and provides an inverted output signal. An output low of the inverter, corresponding to a high voltage at the anode, enables the pass transistor and grid, provided that a voltage supply for the grid is above a certain level. An output high of the inverter, corresponding to a low voltage at the anode, enables the pull down transistor and discharges the grid
Cathey, Jr. David A.
Voshell Thomas W.
Zimlich David A.
Gratton Stephen A.
Micro)n Technology, Inc.
Philogene Haissa
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