U.S. patent application number 10/468319 was filed with the patent office on 2004-07-08 for pixel current driver for organic light emitting diode displays.
Invention is credited to Nathan, Arokia, Sakariya, Kapil, Servati, Peyman.
Application Number | 20040129933 10/468319 |
Document ID | / |
Family ID | 23024994 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040129933 |
Kind Code |
A1 |
Nathan, Arokia ; et
al. |
July 8, 2004 |
Pixel current driver for organic light emitting diode displays
Abstract
A pixel current driver comprises a plurality of thin film
transistors (TFTs) each having dual gates and for driving OLED
layers. A top gate of the dual gates is formed between a source and
a drain of each of the thin film transistors, to thereby minimize
parasitic capacitance. The top gate is grounded or electrically
tied to a bottom gate. The plurality of thin film transistors may
be two thin film transistors formed in voltage-programmed manner or
five thin film transistors formed in a current-programmed
.DELTA.V.sub.T-compensated manner. Other versions of the
current-programmed circuit with different numbers of thin film
transistors are also presented that compensate for .delta.V.sub.T.
The OLED layer are continuous and vertically stacked on the
plurality of thin film transistors to provide an aperture ratio
close to 100%.
Inventors: |
Nathan, Arokia; (Waterloo,
CA) ; Servati, Peyman; (Waterloo, CA) ;
Sakariya, Kapil; (Waterloo, CA) |
Correspondence
Address: |
PEARNE & GORDON LLP
1801 EAST 9TH STREET
SUITE 1200
CLEVELAND
OH
44114-3108
US
|
Family ID: |
23024994 |
Appl. No.: |
10/468319 |
Filed: |
January 23, 2004 |
PCT Filed: |
February 18, 2002 |
PCT NO: |
PCT/CA02/00173 |
Current U.S.
Class: |
257/40 ; 257/72;
257/E27.111 |
Current CPC
Class: |
H01L 27/283 20130101;
G09G 3/3241 20130101; G09G 2320/0223 20130101; H01L 27/3276
20130101; G09G 3/3233 20130101; G09G 2300/0804 20130101; H01L 27/12
20130101 |
Class at
Publication: |
257/040 ;
257/072 |
International
Class: |
H01L 035/24 |
Claims
We claim:
1. A pixel current driver for an organic light emitting diode
(OLED) having an OLED layer for emitting light, comprising: a
plurality of thin film transistors (TFTs) each having dual gates
and for driving the OLED layer, a top gate of the dual gates being
formed between a source and a drain of each of the thin film
transistors, to thereby minimize parasitic capacitance.
2. The pixel current driver as claimed in claim 1, wherein each of
the thin film transistor is an a-Si:H based thin film
transistor.
3. The pixel current driver as claimed in claim 1, wherein each of
the thin film transistor is a polysilicon-based thin film
transistor.
4. The pixel current driver as claimed in claim 3, wherein each of
the thin film transistors is a p-channel thin film transistor.
5. The pixel current driver as claimed in claim 2, wherein the dual
gates are fabricated in a normal inverted staggered TFT
structure.
6. The pixel current driver as claimed in claim 2, wherein the top
gate is grounded.
7. The pixel current driver as claimed in claim 2, wherein the top
gate is electrically tied to a bottom gate.
8. The pixel current driver as claimed in claim 2, wherein the
plurality of thin film transistors are two thin film
transistors.
9. The pixel current driver as claimed in claim 8, wherein the two
thin film transistors are formed in a voltage programmed
manner.
10. The pixel current driver as claimed in claim 2, wherein the
plurality of thin film transistors are five thin film
transistors.
11. The pixel current driver as claimed in claim 10, wherein the
five thin film transistors are formed in a current-programmed
.DELTA.V.sub.T-compensated manner.
12. The pixel current driver as claimed in claim 2, wherein the
OLED layer is continuously and vertically stacked on the plurality
of thin film transistors.
13. The pixel current driver as claimed in claim 2, wherein the
pixel current driver is a current mirror based pixel current driver
for automatically compensating for shifts in the Vth of each of the
thin film transistor in a pixel.
14. The pixel current driver as claimed in claim 10, wherein the
pixel current driver is for monochrome displays.
15. The pixel current driver as claimed in claim 10, wherein the
pixel current driver is for full color displays.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a an organic light emitting
diode display, and more particularly to an a pixel current driver
for an organic light emitting display (OLED), capable of minimizing
parasitic couplings between the OLED and the transistor layers.
[0003] 2. Description of the Prior Art
[0004] OLED displays have gained significant interest recently in
display applications in view of their faster response times, larger
viewing angles, higher contrast, lighter weight, lower power,
amenability to flexible substrates, as compared to liquid crystal
displays (LCDs). Despite the OLED's demonstrated superiority over
the LCD, there still remain several challenging issues related to
encapsulation and lifetime, yield, color efficiency, and drive
electronics, all of which are receiving considerable attention.
Although passive matrix addressed OLED displays are already in the
marketplace, they do not support the resolution needed in the next
generation displays, since high information content (HIC) formats
are only possible with the active matrix addressing scheme. Active
matrix addressing involves a layer of backplane electronics, based
on thin-film transistors (TFTS) fabricated using amorphous silicon
(a-Si:H), polycrystalline silicon (poly-Si), or polymer
technologies, to provide the bias voltage and drive current needed
in each OLED pixel. Here, the voltage on each pixel is lower and
the current throughout the entire frame period is a low constant
value, thus avoiding the excessive peak driving and leakage
currents associated with passive matrix addressing. This in turn
increases the lifetime of the OLED.
[0005] In active matrix OLED (AMOLED) displays, it is important to
ensure that the aperture ratio or fill factor (defined as the ratio
of light emitting display area to the total pixel area) should be
high enough to ensure display quality. Conventional AMOLED displays
are based on light emission through an aperture on the glass
substrate where the backplane electronics is integrated. Increasing
the on-pixel density of TFT integration for stable drive current
reduces the size of the aperture. The same happens when pixel sizes
are scaled down. The solution to having an aperture ratio that is
invariant on scaling or on-pixel integration density is to
vertically stack the OLED layer on the backplane electronics, along
with a transparent top electrode (see FIG. 2). In FIG. 2, reference
numerals S and D denote a source and a drain respectively. This
implies a continuous back electrode over the OLED pixel. However,
this continuous back electrode can give rise to parasitic
capacitance, whose effects become significant when the electrode
runs over the switching and other thin film transistors (TFTs).
Here, the presence of the back electrode can induce a parasitic
channel in TFTs giving rise to high leakage current. The leakage
current is the current that flows between source and drain of the
TFT when the gate of the TFT is in its OFF state.
SUMMARY OF THE INVENTION
[0006] Accordingly, it is an object of the present invention to
provide to a pixel current driver for an organic light emitting
display (OLED), capable of minimizing parasitic couplings between
the OLED and the transistor layers.
[0007] In order to achieve the above object, a pixel current driver
for OLED layer for emitting light according to the present
invention comprises a plurality of thin film transistors (TFTs)
each having dual gates and for driving the OLED layer. A top gate
of the dual gates is formed between a source and a drain of each of
the thin film transistors, to thereby minimize parasitic
capacitance.
[0008] Each of the thin film transistor may be an a-Si:H based thin
film transistor or a polysilicon-based thin film transistor.
[0009] The pixel current driver is a current mirror based pixel
current driver for automatically compensating for shifts in the Vth
of each of the thin film transistor in a pixel and the pixel
current driver is for monochrome displays or for full color
displays.
[0010] The dual gates are fabricated in a normal inverted staggered
TFT structure. A width of each of the TFTs is formed larger than a
length of the same to provide enough spacing between the source and
drain for the top gate. Preferably, the length is 30 .mu.m and the
width is 1600 .mu.m. The length and width of the transistors may
change depending on the maximum drive current required by the
circuit and the fabrication technology used. The top gate is
grounded or electrically tied to a bottom gate. The plurality of
thin film transistors may be two thin film transistors formed in
voltage-programmed manner or five thin film transistors formed in a
current-programmed .DELTA.V.sub.T-compensated manner, or four or
The OLED layer is vertically stacked on the plurality of thin film
transistors.
[0011] With the above structure of an a-Si:H current driver
according to the present invention, the charge induced in the top
channel of the TFT is minimized, and the leakage currents in the
TFT is minimized so as to enhance circuit performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above objects and features of the present invention will
become more apparent by describing in detail preferred embodiments
thereof with reference to the attached drawings in which:
[0013] FIG. 1 shows variation of required pixel areas with mobility
for 2-T and 5-T pixel drivers;
[0014] FIG. 2 shows a pixel architecture for surface emissive
a-Si:H AMOLED displays;
[0015] FIG. 3 shows a cross section of a dual-gate TFT
structure;
[0016] FIG. 4 shows forward and reverse transfer characteristics of
dual-gate TFT for various top gate biases;
[0017] FIG. 5A and FIG. 5B show an equivalent circuit for a 2-T
pixel driver and its associated input-output timing diagrams;
[0018] FIG. 6A and FIG. 6B show an equivalent circuit for a 5-T
pixel driver and its associated input-output timing diagrams;
[0019] FIG. 7 shows transient performance of the 5-T driver for
three consecutive write cycles;
[0020] FIG. 8 shows input-output transfer characteristics for the
2-T pixel driver for different supply voltages;
[0021] FIG. 9 shows input-output transfer characteristics for the
5-T pixel driver for different supply voltages;
[0022] FIG. 10 shows variation in OLED current as a function of the
normalized shift in threshold voltage;
[0023] FIG. 11 shows a 2-T polysilicon based pixel current driver
having p-channel drive TFTs;
[0024] FIG. 12 shows a 4-T pixel current driver for OLED
displays;
[0025] FIG. 13 shows a 4-T pixel current driver with a lower
discharge time;
[0026] FIG. 14 shows a 4-T pixel current driver without non-linear
gain;
[0027] FIG. 15 shows a 4-T pixel current driver that is the
building block for the full color circuit; and
[0028] FIG. 16 shows a full color (RGB) pixel current driver for
OLED displays.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] Although amorphous Si does not enjoy equivalent electronic
properties compared to poly-Si, it adequately meets many of the
drive requirements for small area displays such as those needed in
pagers, cell phones, and other mobile devices. Poly-Si TFTs have
one key advantage in that they are able to provide better pixel
drive capability because of their higher mobility, which can be of
the order of .mu..sub.FE.about.100 cm.sup.2/Vs. This makes poly-Si
highly desirable for large area (e.g. laptop size) VGA and SVGA
displays. The lower mobility associated with a-Si:H TFTs
(.mu..sub.FE.about.1 cm.sup.2/Vs) is not a limiting factor since
the drive transistor in the pixel can be scaled up in area to
provide the needed drive current. The OLED drive current density is
typically 10 mA/cm.sup.2 at 10V operation to provide a brightness
of 100 cd/m.sup.2--the required luminance for most displays. For
example, with an a-Si:H TFT mobility of 0.5 cm.sup.2/Vs and channel
length of 25 .mu.m, this drive current requirement translates into
required pixel area of 300 .mu.m.sup.2, which adequately meets the
requirements of pixel resolution and speed for some 3 inch
monochrome display applications. FIG. 1 illustrates simulation
results for the variation of the required pixel size with device
mobility calculated for two types of drivers, which will be
elaborated later, the 2-T and the 5-T drivers, wherein .mu..sub.0
denotes a reference mobility whose value is in the range 0.1 to 1
cm.sup.2/Vs. For instance, the area of the pixel for the 2-T driver
(see FIG. 5A) comprises of the area of the switching transistors,
area of the drive transistor, and the area occupied by
interconnects, bias lines, etc. In FIG. 1, the drive current and
frame rate are kept constant at 10 .mu.A and 50 Hz, respectively,
for a 230.times.230 array. It is clear that there is no significant
savings in area between the 2-T and 5-T drivers but the savings are
considerable with increasing mobility. This stems mainly from the
reduction in the area of the drive transistor where there is a
trade-off between .mu..sub.FE and TFT aspect ratio, W/L
(Wide/Length).
[0030] In terms of threshold voltage (V.sub.T) uniformity and
stability, both poly-Si and a-Si:H share the same concerns,
although in comparison, the latter provides for better spatial
uniformity but not stability (.DELTA.V.sub.T). Thus the inter-pixel
variation in the drive current can be a concern in both cases,
although clever circuit design techniques can be employed to
compensate for .DELTA.V.sub.T hence improving drive current
uniformity. In terms of long term reliability, it is not quite
clear with poly-Si technology, although there are already products
based on a-Si:H technology for displays and imaging, although the
reliability issues associated with OLEDs may yet be different. The
fabrication processes associated with a-Si:H technology are
standard and adapted from mainstream integrated circuit (IC)
technology, but with capital equipment costs that are much lower.
One of the main advantages of the a-Si:H technology is that it has
become low cost and well-established technology, while poly-Si has
yet to reach the stage of manufacturability. The technology also
holds great promise for futuristic applications since good
as-deposited a-Si:H, a-SiN.sub.x:H, and TFT arrays can be achieved
at low temperatures (.ltoreq.120.degree. C.) thus making it
amenable to plastic substrates, which is a critical requirement for
mechanically flexible displays.
[0031] To minimize the conduction induced in all TFTs in the pixel
by the back electrode, an alternate TFT structure based on a
dual-gate structure is employed. In a dual gate TFT (see FIG. 3), a
top gate electrode is added to the TFT structure to prevent the
OLED electrodes from biasing the a-Si:H channel area (refer to FIG.
2). The voltage on the top gate can be chosen such so as to
minimize the charge induced in the (parasitic) top channel of the
TFT. The objective underlying the choice of the voltage on the top
gate is to minimize parasitic capacitance in the driver circuits
and leakage currents in the TFTs so as to enhance circuit
performance. In what follows, the operation of the dual-gate TFT is
described, which will be central to surface emissive (100% aperture
ratio) AMOLED displays based on a-Si:H backplane electronics.
[0032] FIG. 3 illustrates the structure of a dual-gate TFT
fabricated for this purpose, wherein reference numerals S and D
denote a source and a drain respectively. The fabrication steps are
the same as of that of a normal inverted staggered TFT structure
except that it requires a sixth mask for patterning the top gate.
The length of the TFT is around 30 .mu.m to provide enough spacing
between the source and drain for the top gate, and the width is
made very large (1600 .mu.m) with four of these TFTs are
interconnected in parallel to create a sizeable leakage current for
measurement. A delay time is inserted in the measurement of the
current to ensure that the measurement has passed the transient
period created by defects in the a-Si:H active layer, which give
rise to a time-dependent capacitance.
[0033] FIG. 4 shows results of static current measurements for four
cases: first when the top gate is tied to -10V, second when the top
gate is grounded, third when the top gate is floating, and lastly
when the top gate is shorted to the bottom gate. With a floating
top gate, the characteristics are almost similar to that of a
normal single gate TFT. The leakage current is relatively high
particularly when the top gate is biased with a negative voltage.
The lowest values of leakage current are obtained when the top gate
is pegged to either 0V or to the voltage of the bottom gate. In
particular, with the latter the performance of the TFT in the
(forward) sub-threshold regime of operation is significantly
improved. This enhancement in sub-threshold performance can be
explained by the forced shift of the effective conduction path away
from the bottom interface to the bulk a-Si:H region due to the
positive bias on the top gate. This in turn decreases the effect of
the trap states at the bottom interface on the sub-threshold slope
of the TFT.
[0034] It should be noted that although the addition of another
metal contact as the top gate reduces the leakage current of the
TFT, it can potentially-degrade pixel circuit performance by
possible parasitic capacitances introduced by vertically stacking
the OLED pixel. Thus the choice of top gate connection becomes
extremely critical. For example, if the top gates in the pixel
circuit are connected to the bottom gates of the associated TFTs,
this gives rise to parasitic capacitances located between the gates
and the cathode, which can lead to undesirable display operation
(due to the charging up of the parasitic capacitance) when the
multiplexer O/P drives the TFT switch. On the other hand, if the
top gates are grounded, this results in the parasitic capacitance
being grounded to yield reliable and stable circuit operation.
[0035] The OLED drive circuits considered here are the well-known
voltage-programmed 2-T driver and the more sophisticated
current-programmed .DELTA.V.sub.T-compensated 5-T version (see
FIGS. 5A and 6A). The latter is a significant variation of the
previous designs, leading to reduced pixel area (<300 .mu.m),
reduced leakage, lower supply voltage (20V), higher linearity
(.about.30 dB), and larger dynamic range (.about.40 dB). Before
dwelling on the operation of the 5-T driver, the operation of the
relatively simple voltage-driven 2-T driver is described. FIG. 5B
shows input-output timing diagrams of the 2-T pixel driver. When
the address line is activated, the voltage on the data line starts
charging capacitor C.sub.s and the gate capacitance of the driver
transistor T.sub.2. Depending on the voltage on the data line, the
capacitor charges up to turn the driver transistor T.sub.2 on,
which then starts conducting to drive the OLED with the appropriate
level of current. When the address line is turned off, T.sub.1 is
turned off but the voltage at the gate of T.sub.2 remains since the
leakage current of T.sub.1 is trivial in comparison. Hence, the
current through the OLED remains unchanged after the turn off
process. The OLED current changes only the next time around when a
different voltage is written into the pixel.
[0036] Unlike the previous driver, the data that is written into
the 5-T pixel in this case is a current (see FIG. 6A). FIG. 6B
shows input-output timing diagrams of a 5-T pixel driver. The
address line voltage, V.sub.address and I.sub.data are activated or
deactivated simultaneously. When V.sub.address is activated, it
forces T.sub.1 and T.sub.2 to turn on. T.sub.1 immediately starts
conducting but T.sub.2 does not since T.sub.3 and T.sub.4 are off.
Therefore, the voltages at the drain and source of T.sub.2 become
equal. The current flow through T.sub.1 starts charging the gate
capacitor of transistors T.sub.3 and T.sub.5, very much like the
2-T driver. The current of these transistors start increasing and
consequently T.sub.2 starts to conduct current. Therefore,
T.sub.1's share of I.sub.data reduces and T.sub.2's share of
I.sub.data increases. This process continues until the gate
capacitors of T.sub.3 and T.sub.5 charge (via T.sub.1) to a voltage
that forces the current of T.sub.3 to be I.sub.data. At this time,
the current of T.sub.1 is zero and the entire I.sub.data goes
through T.sub.2 and T.sub.3. At the same time, T.sub.5 drives a
current through the OLED, which is ideally equal to
I.sub.data*(W.sub.5/W.sub.3), which signifies a current gain. Now
if I.sub.data and V.sub.address are deactivated, T.sub.2 will turn
off, but due to the presence of capacitances in T.sub.3 and
T.sub.5, the current of these two devices cannot be changed easily,
since the capacitances keep the bias voltages constant. This forces
T.sub.4 to conduct the same current as that of T.sub.3, to enable
the driver T.sub.5 to drive the same current into the OLED even
when the write period is over. Writing a new value into the pixel
then changes the current driven into the OLED.
[0037] The result of transient simulation for the 5-T driver
circuit is shown in FIG. 7. As can be seen, the circuit has a write
time of <70 .mu.s, which is acceptable for most applications.
The 5-T driver circuit does not increase the required pixel size
significantly (see FIG. 1) since the sizes of T2, T3, and T4 are
scaled down. This also provides an internal gain
(W.sub.5/W.sub.3=8), which reduces the required input current to
<2 .mu.A for 10 .mu.A OLED current. The transfer characteristics
for the 2-T and 5-T driver circuits are illustrated in FIGS. 8 and
9, respectively, generated using reliable physically-based TFT
models for both forward and reverse regimes. A much improved
linearity (.about.30 dB) in the transfer characteristics
(I.sub.data/I.sub.OLED) is observed for the 5-T driver circuit due
to the geometrically-defined internal pixel gain as compared to
similar designs. In addition, there are two components (OLED and
T.sub.5) in the high current path, which in turn decreases the
required supply voltage and hence improves the dynamic range.
According to FIG. 9, a good dynamic range (.about.40 dB) is
observed for supply voltage of 20V and drive currents in the range
I.sub.OLED.ltoreq.10 .mu.A, which is realistic for high brightness.
FIG. 10 illustrates variation in the OLED current with the shift in
threshold voltage for the 2-T and 5-T driver circuits. The 5-T
driver circuit compensates for the shift in threshold voltage
particularly when the shift is smaller than 10% of the supply
voltage. This is because the 5-T driver circuit is
current-programmed. In contrast, the OLED current in the 2-T
circuit changes significantly with a shift in threshold voltage.
The 5-T driver circuit described here operates at much lower supply
voltages, has a much larger drive current, and occupies less
area.
[0038] The pixel architectures are compatible to surface (top)
emissive AMOLED displays that enables high on-pixel TFT integration
density for uniformity in OLED drive current and high aperture
ratio. A 5-T driver circuit has been described that provides
on-pixel gain, high linearity (.about.30 dB), and high dynamic
range (.about.40 dB) at low supply voltages (15-20V) compared to
the similar designs (27V). The results described here illustrate
the feasibility of using a-Si:H for 3-inch mobile monochrome
display applications on both glass and plastic substrates. With the
latter, although the mobility of the TFT is lower, the size of the
drive transistor can be scaled up yet meeting the requirements on
pixel area as depicted in FIG. 1.
[0039] Polysilicon has higher electron and hole mobilities than
amorphous silicon. The hole mobilities are large enough to allow
the fabrication of p-channel TFTs.
[0040] The advantage of having p-channel TFTs is that bottom
emissive OLEDs can be used along with a p-channel drive TFT to make
a very good current source. One such circuit is shown in FIG. 11.
In FIG. 11, the source of the p-type drive TFT is connected to Vdd.
Therefore, Vgs, gate-to-source voltage, and hence the drive current
of the p-type TFT is independent of OLED characteristics. In other
words, the driver shown in FIG. 11 performs as a good current
source. Hence, bottom emissive OLEDs are suitable for use with
p-channel drive TFTs, and top emissive OLEDs are suitable for use
with n-channel TFTs.
[0041] The trade-off with using polysilicon is that the process of
making polysilicon TFTs requires much higher temperatures than that
of amorphous silicon. This high temperature processing requirement
greatly increases the cost, and is not amenable to plastic
substrates. Moreover, polysilicon technology is not as mature and
widely available as amorphous silicon. In contrast, amorphous
silicon is a well-established technology currently used in liquid
crystal displays (LCDs). It is due to these reasons that amorphous
silicon combined with top emissive OLED based circuit designs is
most promising for AMOLED displays.
[0042] Compared to polysilicon TFTs, amorphous silicon TFTs are
n-type and thus are more suitable for top emission circuits as
shown in FIG. 2. However, amorphous silicon TFTs have inherent
stability problems due to the material structure. In amorphous
silicon circuit design, the biggest hurdle is the increase in
threshold voltage V.sub.th after prolonged gate bias. This shift is
particularly evident in the drive TFT of an OLED display pixel.
This drive TFT is always in the `ON` state, in which there is a
positive voltage at its gate. As a result, its V.sub.th increases
and the drive current decreases based on the current-voltage
equation below:
Ids=(.mu.C.sub.oxW/2L)(V.sub.gs-V.sub.th).sup.2 (in Saturation
region)
[0043] In the display, this would mean that the brightness of the
OLED would decrease over time, which is unacceptable. Hence, the
2-T circuits shown earlier are not practical for OLED displays as
they do not compensate for any increase in V.sub.th.
[0044] The first current mirror based pixel driver circuit is
presented, which automatically compensated for shifts in the
V.sub.th of the drive TFT in a pixel. This circuit is the 5-T
circuit shown in FIG. 6A.
[0045] Four more OLED pixel driver circuits are presented for
monochrome displays, and one circuit for full colour displays. All
these circuits have mechanisms that automatically compensate for
V.sub.th shift. The first circuit shown in FIG. 12 is a
modification of the 5-T circuit of FIG. 6A. (Transistor T.sub.4 has
been removed from the 5-T circuit). This circuit occupies a smaller
area than the 5-T circuit, and provides a higher dynamic range. The
higher dynamic range allows for a larger signal swing at the input,
which means that the OLED brightness can be adjusted over a larger
range.
[0046] FIG. 12 shows a 4-T pixel driver circuit for OLED displays.
The circuit shown in FIG. 13 is a 4-T pixel driver circuit based on
a current mirror. The advantage of this circuit is that the
discharge time of the capacitor Cs is substantially reduced. This
is because the discharge path has two TFTs (as compared to three
TFTs in the circuit of FIG. 12). The charging time remains the
same. The other advantage is that there is an additional gain
provided by this circuit because T.sub.3 and T.sub.4 do not have
the same source voltages. However, this gain is non-linear and may
not be desirable in some cases.
[0047] In FIG. 14, another 4-T circuit is shown. This circuit does
not have the non-linear gain present in the previous circuit (FIG.
13) since the source terminals of T.sub.3 and T.sub.4 are at the
same voltage. It still maintains the lower capacitance discharge
time, along with the other features of the circuit of FIG. 8.
[0048] FIG. 15 shows another version of the 4-T circuit. This
circuit is does not have good current mirror properties. However,
this circuit forms the building block for the 3 colour RGB circuit
shown in FIG. 16. It also has a low capacitance discharge time and
high dynamic range.
[0049] The full colour circuit shown in FIG. 16 minimizes the area
required by an RGB pixel on a display, while maintaining the
desirable features like threshold voltage shift compensation,
in-pixel current gain, low capacitance discharge time, and high
dynamic range.
[0050] It is important to note that the dual-gate TFTs are used in
the above-mentioned circuits to enable vertical integration of the
OLED layers with minimum parasitic effects. But nevertheless the
circuit compensates for the Vth shift even if the simple
single-gate TFTs. In addition, these circuits use n-type amorphous
silicon TFTs. However, the circuits are applicable to polysilicon
technology using p-type or n-type TFTs. These circuits when made in
polysilicon can compensate for the non-uniformity of the threshold
voltage, which is a problem in this technology. The p-type circuits
are conjugates of the above-mentioned circuits and are suitable for
the bottom emissive pixels.
* * * * *