Canadian Patents Database / Patent 2438363 Summary

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(12) Patent Application: (11) CA 2438363
(54) English Title: A PIXEL CIRCUIT FOR AMOLED DISPLAYS
(54) French Title: CIRCUIT DE PIXELS POUR AFFICHAGES AMOLED
(51) International Patent Classification (IPC):
  • G09G 3/3208 (2016.01)
  • G09G 3/3225 (2016.01)
  • G09F 9/33 (2006.01)
  • H01L 51/00 (2006.01)
(72) Inventors :
  • NATHAN, AROKIA (Canada)
  • SAMBANDAN, SANJIV (Canada)
  • SAKARIYA, KAPIL (Canada)
  • SERVATI, PEYMAN (Canada)
(73) Owners :
  • IGNIS INNOVATION INC. (Canada)
(71) Applicants :
  • IGNIS INNOVATION INC. (Canada)
(74) Agent: GOWLING LAFLEUR HENDERSON LLP
(45) Issued:
(22) Filed Date: 2003-08-28
(41) Open to Public Inspection: 2005-02-28
(30) Availability of licence: N/A
(30) Language of filing: English

English Abstract




A pixel circuit for AMOLED displays is provided. The pixel circuit includes
active matrix organic light emitting diode (AMOLED) display, a drive TFT for
driving OLED and a circuit for compensating for the threshold voltage of the
drive
TFT. The circuit applies a rising gate voltage to compensate for the rising
threshold voltage of the drive TFT.


Note: Claims are shown in the official language in which they were submitted.


What is claimed is:
1. A pixel circuit for use in a display comprising:
an organic light emitting diode (OLED);
a pixel driver for driving the OLED, having a drive TFT; and
a compensation circuit for compensating for the shift of the threshold
voltage of the pixel driver.
2. The pixel circuit according to claim 1, wherein the pixel driver includes a
first stage circuit having first and second TFT transistors, and a second
stage
circuit which is provided between the first stage circuit and the drive TFT
and has
third and forth TFT transistors.
3. The pixel circuit according to claim 2, wherein the compensation circuit
includes a potential divider provided for the first and second stage circuits.
5. The pixel circuit according to claim 3, wherein the compensation circuit
further includes a storage capacitor provided parallel to the potential
divider.
6. The pixel circuit according to claim 5, wherein a signal line for
controlling
the charge of the storage capacitor.
17

Note: Descriptions are shown in the official language in which they were submitted.


CA 02438363 2003-08-28
A Pixel Circuit for AMOLED Displays
Field of the Invention
The present invention relates to a circuit for displays, more specifically to
a
pixel circuit for active matrix organic light emitting diode (AMOLED)
Displays.
Background of (and Summary) of the Invention
Amorphous silicon thin film transistors (a-Si:H TFT) are suitable for active
matrix organic light emitting diode (AMOLED) display backplanes due to their
low
leakage, good spatial uniformity, and the possibility of a low temperature
process.
A 2-TFT voltage driven circuit is the simplest and smallest AMOLED
pixel.circuit.
However, with this circuit, the OLED drive current drops over time due to
threshold
voltage shifts in the drive TFT. Hence better circuits are required to
compensate
for the decay in current through the OLED.
There are two main driving principles of AMOLED circuits: Voltage and
Current programming. Each drive scheme has unique advantages and
disadvantages when used for a-Si or p-Si TFT AMOLED pixel operation, briefly
described below.
Current Programmed Circuits
A self compensating 4-TFT current programmed circuit is developed to
overcome time dependent threshold voltage shifts described above and keep the
2


.,. CA 02438363 2003-08-28
OLED drive current constant. Figure 1 shows the 4-TFT current-programmed
pixel circuit 10, which has been previously published.
Voltage programmed circuits
Figure 2 shows the voltage-programmed pixel circuit 20, which is
programmed by chargeldischarge [Source: Joon-Chul Goh, Choong-Ki Kim, Jin
Jang, "A Novel Pixel Circuit for Active Matrix Organic Light Emitting Diodes,"
SID
03 Digest]. The circuit 20 has switches Sw1, Sw2 and Sw3, a storage capacitor
CST, an OLED and a drive.TFT (DTFT). During the programming stage, the
switches Sw1, Sw2 and Sw3 are on, allowing the storage capacitor CST to charge
up to a value of the supply voltage, while maintaining the source of the drive
at a
voltage corresponding to the data. In order to ensure that the OLED does not
have
any current through it, the cathode is pulsed to reverse bias the OLED. After
the
capacitor (and hence the gate of the drive TFT) has been charged to the supply
voltage, the signal TNO is turned off. The capacitor now discharges to the
value of
the data voltage above the threshold voltage of the TFT, hence ensuring
immunity
to the threshold voltage shift.
Figure 3 shows the voltage-programmed circuit 30, which is programmed
by modifying OLED characteristics [Source: Joo-Han Kim and Jerzy Kanicki, "
200dpi 3-a-Si:H Tfts Voltage Driven AM PLEDs," SID 03 Digest). The circuit 30
has TFT T1-T3 and a capacitor CST. The programming of the TFT occurs when
the SCAN signal goes high during which time the data is fed through the
capacitor
CST. The TFT T2 acts as an active resistor and forces the drive transistor to
operate in the linear region. Thus when there is a reduction in the OLED
current
3


CA 02438363 2003-08-28
with time, the drain voltage of the drive transistor, which is seen at node A,
increases, thereby increasing the current through the drive. This helps in
partial
compensation of the threshold voltage shift.
Figure 4 shows the voltage-programmed pixel circuit 40, which is
programmed by capacitive coupling [Source: James L.Sanford and Frank R.
Libsch, "TFT AMOLED Pixel Circuits and Driving Methods," SID 03 Digest].
Having opposite polarity device terminal voltages has been known to reduce the
stress and increase life. Hence the circuit 40 operates by switching polarity
across
the TFT. The threshold voltage shift compensation takes place in a manner very
similar to that of circuit 20. Initially the coupling capacitor is discharged
to the
threshold voltage of T3. This happens when AZ goes high thus turning on the
switching TFT T2. When the row line signal goes high, the data is written onto
the
capacitor.
Figure 5 shows the pixel circuit 50 of UDC Corp, which is programmed by
optoelectronic feedback, and has been previously published. The circuit 50 has
three TFTs Tup, TdOWn and Tdrwe . The three TFTs Tup, Tdown and Tdr~"e form a
latch
whose state will remain unchanged unless new data enters the pixel through the
select transistor. Optical feedback from the OLED to the TFT acting as a photo-

detector (Tup) ensures that the charge on the storage capacitance maintains a
steady current through the OLED.
Table 1 shows the summary of these pixel circuits 10-50.
4


CA 02438363 2003-08-28
Table 1 - Summary of Pixel Circuits
Circuit 10 Circuit Circuit Circuit Circuit
- 4 20 - 30 - 40 - 50 -


TFT current ProgrammedProgrammed ProgrammedProgrammed


grog, circuitby charge)by modifyingby capacitiveby


dischargeOLED coupling optoelectronic


characteristics feedback


FunctionalityOvercompensationOLED currentIncomplete Assumes Sensitive
to


drawback due to differentialinstabilitycompensationperfect ambient
OLED light


VT sft. (longexists (280~0~ with no
term because off


effect) of TFT current
in


path.


Lifetirae Supply voltageAuxiliaryThe Active Supply Supply voltage
TFT


Bottleneck in the resistor voltage
driving


path


Layout 92620Nm 92620Nm 86412Nm 81562Nm 92620~m
Area


Power/Pixel226NW 190~IW 168NW 152NW 142NW


per prog.


cycle


No of pins4 5 4 5 4


Array LevelSlow prog. Slow prog.Negligible NegligibleNegligible
because


Implicationof line capacitancedue to


capacitance
of


TFTs in
the


line


Other None CapacitorCapacitor Capacitor OE TFT;


Devices Capacitor


Hence, it is desirable to provide a new pixel circuit for AMOLED which meets
the
following specifications:
~ Perfect functionality - 100% threshold voltage shift compensation -
independence from external parameters like temperature, ambient
lighting etc.
~ High Lifetime - in excess of 10000hrs and stability for large range of
operation
5


'~ CA 02438363 2003-08-28
~ Easy programming - preferably low number of pins, compatibility with
environment - hence preferably voltage programmed
~ Quick programming < 60ps
~ Low layout area
~ Low power consumption
Summary of the Invention
It is an object of the invention to provide a pixel circuit that obviates or
mitigates at least one of the disadvantages of existing systems.
In accordance with an aspect of the present invention, there is provided a
pixel circuit for use in a display which includes: an organic light emitting
diode
(OLED); a pixel driver for driving the OLED, having a drive TFT; and a
compensation circuit for compensating for the shift of the threshold voltage
of the
pixel driver.
Other aspects and features of the present invention will be readily apparent
to those skilled in the art from a review of the following detailed
description of
preferred embodiments in conjunction with the accompanying drawings.
Brief Description of the Drawings
The invention will be further understood from the following description with
reference to the drawings in which:
6

~
CA 02438363 2003-08-28
Figure 1 is a schematic diagram showing a 4-TFT current-programmed
pixel circuit;
Figure 2 is a schematic diagram showing a voltage-programmed pixel
circuit, which is programmed by chargeldischarge;
Figure 3 is a schematic diagram showing a voltage-programmed pixel
circuit, which is programmed by modifying the OLED characteristics;
Figure 4 is a schematic diagram showing a voltage-programmed pixel
circuit, which is programmed by capacitive coupling:
Figure 5 is a schematic diagram showing voltage-programmed pixel circuit,
which is programmed by optoelectronic feedback;
Figure 6 is a graph showing Threshold Voltage shift vs. Stress Voltage
characteristic;
Figure 7 is a block diagram showing a pixel circuit of the present invention;
Figure 8 is a schematic diagram showing a pixel circuit having 5 transistors;
Figure 9 is a graph showing optimization of power, stability, and area;
Figure 10 is a schematic diagram showing a first embodiment of the pixel
circuit of the present invention;
Figure 11 is a graph showing the simulation results of the transfer
characteristics of Figure 10;
Figure 12 is a graph showing transfer characteristics of Figure 10;
Figure 13 is a graph showing current vs. time characteristic of Figure 10;
Figure 14 is a graph showing drive current vs. time characteristics of Figure
10; and
7


CA 02438363 2003-08-28
Figure 15 is a graph showing drive current vs. time characteristics of Figure
10.
Detailed Description of the Preferred Embodiments)
A pixel circuit of the present invention is used in Amorphous-Silicon-based
Active-Matrix OLED Displays that is voltage programmed and driven. This
design:
~ Compensates for the Threshold-Voltage shift in amorphous silicon thin-film
transistors; .
~ Offers sufficient characteristics to drive an OLED pixel; and
~ Is able to be fabricated and integrated into an array.
Analysis of Threshold Voltage Shift
Figure 6 shows threshold voltage shift vs stress voltage characteristic for a
discrete a-Si TFT. To account for the non-linearity in the threshold voltage
shift
1S with time and gate voltage, a convenient empirical equation (1) is used for
the
design of the pixel circuit.
~Vt = A [exp (aV9) -1J[1-exp (-[i t)J ...(1)
Where ~Vt is the threshold voltage shift, V9 is the gate voltage applied, t is
the
time, a, [i and A are constants
At a given instance of time t = T, (1) is the following equation (2)
8


' CA 02438363 2003-08-28
OVt = n [exp (aVs) -1], where n = A [1-exp (-[i T)]
= n I(aVs) + (aVs)2/2! + (aVs)313! + ... ] ... (2)
Ignoring higher powers (as a « 1), the following equation (3) is obtained:
~Vt = n I(aVs)] _ CVs ... (3)
For relatively low values of the gate voltage, which are well within the
practical range of operation.
Principle of Compensation
Figure 7 shows a concept of the pixel circuit of the present invention. The
compensation circuit 102 in accordance with an embodiment of the present
invention applies a rising gate voltage to compensate for the rising threshold
voltage of the drive TFT.
The compensation technique of the present invention is now described in
detail. Figure 8 shows a pixel circuit 120 having 5 transistors M1-M4 and
Mdrive.
If alb = (N)n , c/d = (M)n and all the transistors have the same initial
threshold
voltage, we have the following equations:
Stab
Nn Nbiasl - UTo - Vo1 - ~Vbiasl )n - (Vin - UTo - Vin )n
Thus,
Vo1 = (Ubias1 - ~Ubias1) - (Vin - Vin) / N + (11 N - 1) VTo
_ (Vbiasl - Vin ~ N) - ~ (Vbias1 - Vin ~ N) + (1/ N - 1 ) VTo
9


' CA 02438363 2003-08-28
VTO is the initial (starting) threshold voltage of the TFTs.
Stage2:
Mn (Vbias2 - VTo - Vo2 - ~Vbias2 )n '- (Vo1 - VTo - ~Vo1 )n
Which is the same as,
Mn (Vbias2 - VTo - Vo2 - ~Vbias2 )n =
[ ~(Vbias1 - Vin l N) - i; (Vbiasl - Vin l N) + (1/ N - 1 ) VTo ) - VTo - ~
~(Vbiasl ' Vin l N) -
F, (Vbias~ - Vin l N) -~' (11N - 1 ) VTo) )n
As ~«1, we have the following reduced form:
Mn (Vbias2 - VTo - Vo2 - yVbias2 )n =
[ ~Vbiasl - Vin l N + (1l N - 1 ) VTo} - VTo - 2~ (Vbias1 - Vin l N) - ~ (1lN -
1 ) VTo ~n
Thus,
Vo2 = [Vbias2 - ~Vbias2~ -
[~biasl - Vin l N + ( 1l N -1 ) VTo~ - f, {2(Vbiasl - Vin l N) +' ( 1 /N -1 )
VTo})l M+ ( 1 /M -1 ) VTo
= [Vbias2 - l v bias1 - Vin l N + (1l N -1 ) VTo~/M) +
[12(Vbias1 - Vin lN) + ( 1 /N -1 ) VTo}lM -Vbias2~+ ( 1 /M -1 ) VTo
For achieving the desired purpose, we want Vo2 to be of the form (VoUt + ~
Vo~t).

' CA 02438363 2003-08-28
[Vbias2+ ( 1 /M -1 ) VTo - f Vbiasl - Vin / N +( 1 / N - 1 ) VTo}/M] _
[{2(Vbiasl - Vin IN) + (1/N - 1 ) VTo}lM - Vb;as2]
Subject to the following constraints, which ensure that, the transistors
operate in
saturation.
Vbias1 ' VTo 5 VDD
Vbias2 - VTo ~ VDD
Vin - VTo ~ Vo1
Vo1 - VTo <_ Vo2
The above conditions rewritten as follows:
1. [Vbias2+ ( 1 /M -1 ) VTo - ~Vbiasl - Vin / N + ( 1 / N - 1 ) VTo}lM] _
[~2(Vbiasl - Vin /N) + (1IN - 1 ) VTo}/M -Vbias2]
2. Vbiasl ' VTo s VDD
3. Vbias2 - VTo ~ VDD
4. [Vbiasl (1 + 1/M) - Vbiasz + VTo (1/N +1/MN -2lM - 1)] I (1/N + 1/NM) <_
Vin
5. V;n <_ [Vbias1 + ( 1 /N) VTo ] I ( 1 + 1 I N )
Solving the above set of conditions for different values of N and M, various
possible solutions are obtained as shown in Table 2. These solutions form
various
versions of the general circuit of Figure 8. These solutions are to be
optimised for
minimum layout area, minimum power consumption, and maximum stability.
11


CA 02438363 2003-08-28
xbn" ~ ~ c Y ~ , ~n_-~ .:. auc .a~:u'.'~c~.w~~~~_ w.:~ ~~fo ~~~ ~"~ 8 E~, >..
r ~ . . ~ .:_. ~ w ~t2 r -. ,~, - . .
f'a
a
- ~ r 'i:: _~ .J~t d ~.~"-~- ~ -.4> - ~ t~ ~,i'aV ~vi~v\il~ .. l_.~. ':.v s.~,


CA 02438363 2003-08-28
Optimization
The range of operation parameter:
This can be defined as 'Q'. As we need at least a 3 VTo range to operate the
drive,
D' = 3 - [lVlax(V;n) - f~tlin(Vin)]I V-ro
The layout area parameter:.
This is defined as 'a'.
a = ~ ;_~ t0 a(WIL)M; l min(WIL)
The power consumption t~arameter:
This can be defined as 'h': The power consumed by the compensation circuit 102
is the product of the supply voltage and the current. But the current is
dependent
on the minimum gate voltage in each stage, which in all the cases is
approximately V;n. Hence the supply voltage alone decides this parameter. The
supply voltage needs to be only as large as the largest bias voltage possible.
Hence we define h as follows:
A = maX(Vb;as) I V~d = maX(Vbias) > 1 dVTo
To find the optimum circuit we find the minimum value of P = (a 1~ l a).
Figure 9 shows optimization of power, stability and area (i.e. physical
dimensions
when it is fabricated) for the 5-transistor pixel circuit ~f Figure 8. In
Figure.9, P=-
40 is obtained as minimum value. a, A, 6 are selected to minimize P.
13


CA 02438363 2003-08-28
Figure 10 shows the first embodiment of the pixel circuit 100. The pixel
circuit 100 has 2 signal lines; 3Vin and SEL. SEL turns on the pixel for
programming, and 3Vin is a voltage which represents the desired brightness of
the
pixel. The pixel circuit 100 also has 2 supply lines, \~dd and GND. The pixel
Circuit 100 also contains a storage capacitor Cs and a potential divider,
represented by the thick vertical bar.
The pixel circuit 100 of Figure 10 is a 5 TFT voltage programmed, a-Si or p-
Si TFT AMOLED pixel circuit.
Proarammina Time Analvsis
Following is a description of the programming of the pixel circuit 100, shown
in
Figure 10. The SEL signal goes on when the pixel is to be programmed. The
data for the pixel (3Vin) is delivered to the pixel, and stored in the
capacitor Cs.
The SEL signal then goes off. The charge in the capacitor Cs flows through the
potential divider, setting up the necessary bias voltages for M1, M2 and M3.
Once
the bias voltages are set yap, transistors M1, M2, M3 and M4 work together to
apply a voltage to the ga~ke of the drive TFT, which is equal to Vin plus the
threshold voltage shift of the drive TFT.
The threshold voltage shift in M1, M2, M3 and M~ worla; together to track and
compensate for the threshold voltage shift in the drive TFT, by providing the
appropriate gate voltage to the drive TFT.
During the programming
14


' CA 02438363 2003-08-28
2.2 R~;~e Cs <_ 32~Ss
Where, R,;"e is the line resistance.
For maintaining the charge, we need
0.11 R' Cs > 20ms
Where R' is the resistance of the potential divider. This can easily be made
high
by choosing a material with high resistivity such as a-Si:H itself and by
controlling
the length of the strip.
The circuit was tested using min size TFTs of 66!23.
Figure 11 shows simulation results of the transfer characteristics of Figure
10. The X-axis is the V'in supplied to the first stage. Figure 11 shows the
simulation results of the transfer characteristics of Figure 10. Figure 12
shows
transfer characteristics of Figure 10. Figure 13 shows current vs. time
characteristic of Figure 10. Figure 14 shows drive current vs. time
characteristics
of Figure 10. Figure 15 shows drive current (Idriv~e) vs. time characteristics
of
Figure 10.
According to the present invention, the pixel circuit meets the followings:
Functionality Drawback: None
Lifetime bottleneck: TFT M1 going into linear
Layout area: 109613pm2
Power consumption:


CA 02438363 2003-08-28
Dynamic power = 18pWlpixel
Static Power = 167pWJpixe!
Total Power consumption per programming = 185pW/pixel
~ther devices: Potential divider, capacitor
Number of Signal Lines: 4
/gray implicationso Negligible
While particular embodiments of the present invention have been shown
and described, changes and modifications may be made to such embodiments
without departing from the true scope of the invention.
16

A single figure which represents the drawing illustrating the invention.

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Admin Status

Title Date
Forecasted Issue Date Unavailable
(22) Filed 2003-08-28
(41) Open to Public Inspection 2005-02-28
Dead Application 2006-08-28

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Filing $300.00 2003-08-28
Registration of Documents $100.00 2003-12-30
Current owners on record shown in alphabetical order.
Current Owners on Record
IGNIS INNOVATION INC.
Past owners on record shown in alphabetical order.
Past Owners on Record
NATHAN, AROKIA
SAKARIYA, KAPIL
SAMBANDAN, SANJIV
SERVATI, PEYMAN
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.

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Abstract 2003-08-28 1 11
Description 2003-08-28 15 861
Claims 2003-08-28 1 30
Drawings 2003-08-28 15 246
Representative Drawing 2005-02-08 1 8
Cover Page 2005-02-08 1 31
Correspondence 2003-09-22 1 24