Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-06-21
2004-05-11
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S076000, C345S080000, C345S082000
Reexamination Certificate
active
06734636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) pixel circuit, and more particularly, to a technique for driving the pixel circuit that minimizes stress effects of a TFT device that provides current to the OLED.
2. Description of the Prior Art
An organic light emitting diode (OLED) pixel may utilize any of a variety of organic materials that emit light when an electric current is applied thereto. An OLED display comprises a plurality of OLED pixels organized into an array.
One method to achieve a large size and large format OLED display is to use an active matrix thin film transistor (TFT) back plane. A head mount display and even a direct view display for a small mobile application may use polysilicon or crystalline silicon as a back plane. Due to investments in amorphous silicon flat panel technologies, there is interest in using amorphous silicon (a-Si) as opposed to polysilicon (p-Si) or crystalline (c-Si) silicon as a back plane technology to make a larger OLED display. Large area crystalline silicon back planes would not be as cost effective as amorphous or polysilicon.
Amorphous silicon does not have complimentary devices, as are available in polysilicon or crystalline silicon, for two reasons:
(1) only n-channel field effect transistors (NFETs) are available in amorphous silicon flat panel display (FPD) manufacturing due to fewer photolithographic steps, and hence lower costs, as compared to polysilicon and
(2) p-channel field effect transistors (PFETs), although possible to make, exhibit substantially lower mobility or charge transport due to drift (approximately a factor of 5 to 10), and hence lower current drive, than n-channel field effect transistors (NFETs). NFETs have an average mobility approximately 0.5 to 1.0 cm
2
/V/sec in conventional manufacturing lines.
Due to a manner in which OLEDs are processed, it is not normally possible to drive OLEDs with an NFET configured current source. In conventional active matrix addressing, voltage signals are written into each pixel to control brightness of each pixel. The mobility and the stability characteristics of threshold voltage and mobility of amorphous silicon are suitable for driving twisted nematic liquid crystal, which is electrically similar to a small capacitive load, where a driving voltage is applied with a duty cycle in the range of 0.1% to 0.001%. However, for driving OLEDs requiring continuous current for operation, the amorphous silicon operating voltages are non-zero for a substantially larger percentage of the time, e.g., duty cycles of up to 100%. The higher voltages and continuous current severely stresses the amorphous silicon TFT. In particular, a gate to source voltage stress causes a threshold voltage to vary due to trapped charging and other effects such as creation of defect states and molecular bond breakage at a gate insulator-to-semiconductor interface and in a semiconductor layer of the TFT.
As the TFT's threshold voltage varies, current though the TFT will vary. As the current varies so does brightness of the OLED since light output of the OLED is proportional to current. A human observer can detect a pixel to pixel light output variation of as little as 1%. A higher level of 5% luminance variation is typically considered to be unacceptable.
FIG. 1
is a schematic of a prior art pixel circuit
100
used in a small a-Si backplane display test vehicle. Circuit
100
includes NFETs Q
101
and Q
102
, a capacitor Cs
110
and an OLED
120
.
NFET Q
101
and Cs
110
store a pixel voltage. A high voltage level on a gate line
125
turns NFET Q
101
ON, thus providing a voltage from a data line
130
to Cs
110
. After a period of time, the gate voltage of NFET Q
102
is the same as the voltage on data line
130
, and voltage on gate line
125
is set low. NFET Q
102
operates as a voltage follower to drive OLED
120
. Current through OLED
120
is sourced from a supply voltage Vdd and returned to a supply voltage Vss. As OLED
120
is driven, a threshold voltage (Vt) of NFET Q
102
changes with time t. The voltage across OLED
120
is
Vdd−Vcs−Vgs(t)−Vss,
where:
Vcs=voltage across Cs
110
;
Vgs(t)=voltage gate-to-source of NFET Q
102
as function of time t; and
Vss=negative supply voltage or OLED cathode voltage
The current through OLED
120
or NFET Q
102
is proportional to (Vgs−Vt)
2
because NFET Q
102
is biased in its saturation or constant current regime in which the drain to source voltage is equal to or greater than Vgs−Vt. As a result, voltage across OLED
120
and current through OLED
120
changes as the threshold voltage (Vt) of NFET Q
102
changes. With different driving histories from pixel to pixel, pixel to pixel current and luminance vary. This is known as pixel differential aging. The threshold variation of NFET Q
102
, which requires continuous current for operation, is considered unacceptable for many applications. However, the stress of NFET Q
102
operating in its saturation regime is less than if NFET Q
102
was biased in its linear regime, the drain to source voltage <Vgs−Vt.
For use with a-Si TFT back planes, circuit
100
requires relatively low power and voltage since only one NFET, i.e., NFET
102
, is connected from power supply Vdd to OLED
120
, which is connected to supply voltage Vss. Since OLED
120
current passes through a single NFET, the voltage difference in power supplies Vdd and Vss is kept to a minimum, i.e., a maximum OLED
120
voltage and the drain to source voltage of NFET Q
102
for operation just into the saturation regime.
A circuit that is similar to circuit
100
replaces NFET Q
101
and NFET Q
102
with PFET Q
101
and PFET Q
102
, respectfully, which can be used with polysilicon or crystalline silicon technology. Instead of PFET Q
102
operating as a voltage follower, PFET Q
102
operates as a current source. PFET Q
102
's threshold voltage has an even greater impact on the current into OLED
120
since the current through OLED
120
is proportional to (Vcs−Vt)
2
where Vgs=Vcs. If crystalline silicon, which has a high transconductance, is used, then the Vgs voltage would have to be less than Vt in order to produce a current low enough to drive OLED
120
at brightness levels of the order 100/cd/m
2
since pixel dimensions are usually very small. Threshold voltage variations in the subthreshold regime have an even greater impact on drain current variations because there is an order of magnitude current change for every 60 millivolt change in threshold voltage, or as dictated by a transistor drain current-gate voltage inverse sub-threshold slope, or approximately 60 mV/decade of current.
To minimize stress effects of a TFT device that provides OLED current, current driving is used to write a voltage stored in a pixel circuit. Sony Corporation, 7-35 Kitashinagawa 6-chome, Shinagawa-ku, Tokyo 141-0001, Japan has shown a polysilicon current mirror pixel in a 13″ diagonal 800×600 color active matrix OLED (AMOLED) display. The Sony circuit was published by T. Sasaoka et al., “A 13.0-inch AM-OLED Display with top emitting structure and adaptive current mode programmed pixel circuit (TAC)”, in 2001 SID International Symposium Digest of Technical Papers, volume XXXII, p384-387. In the Sony circuit, data on its data line is in the form of current rather than voltage. However, the Sony circuit does not correct for threshold variation of an OLED driving transistor.
A four PFET transistor circuit for use with polysilicon was developed by Sarnoff Corporation, 201 Washington Road Princeton, N.J. 08543-5300, as described by R. M. A. Dawson et al., “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays”, in IEDM, p875-878, 1998. The Sarnoff circuit uses a data line current to directly set a current in a transistor that drives an OLED. However, the circuit requires polysilicon and uses two transistors in series between the OLED and
Libsch Frank Robert
Sanford James Lawrence
Ohlandt Greeley Ruggiero & Perle L.L.P.
Philogene Haissa
Trepp Robert M.
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