CA2438577C - Pixel current driver for organic light emitting diode displays - Google Patents

Pixel current driver for organic light emitting diode displays Download PDF

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Publication number
CA2438577C
CA2438577C CA002438577A CA2438577A CA2438577C CA 2438577 C CA2438577 C CA 2438577C CA 002438577 A CA002438577 A CA 002438577A CA 2438577 A CA2438577 A CA 2438577A CA 2438577 C CA2438577 C CA 2438577C
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transistor
thin film
connected
gate
pixel
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CA2438577A1 (en
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Arokia Nathan
Peyman Servati
Kapil Sakariya
Anil Kumar
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Ignis Innovation Inc
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Ignis Innovation Inc
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Priority to PCT/CA2002/000173 priority patent/WO2002067327A2/en
Priority to CA002438577A priority patent/CA2438577C/en
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    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G09G3/3241Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/28Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part
    • H01L27/32Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part with components specially adapted for light emission, e.g. flat-panel displays using organic light-emitting diodes [OLED]
    • H01L27/3241Matrix-type displays
    • H01L27/3244Active matrix displays
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00Aspects of the constitution of display devices
    • G09G2300/08Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809Several active elements per pixel in active matrix panels
    • G09G2300/0842Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00Control of display operating conditions
    • G09G2320/02Improving the quality of display appearance
    • G09G2320/0223Compensation for problems related to R-C delay and attenuation in electrodes of matrix panels, e.g. in gate electrodes or on-substrate video signal electrodes
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L2251/00Indexing scheme relating to organic semiconductor devices covered by group H01L51/00
    • H01L2251/50Organic light emitting devices
    • H01L2251/53Structure
    • H01L2251/5307Structure specially adapted for controlling the direction of light emission
    • H01L2251/5315Top emission
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/02Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having at least one potential-jump barrier or surface barrier; including integrated passive circuit elements with at least one potential-jump barrier or surface barrier
    • H01L27/12Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having at least one potential-jump barrier or surface barrier; including integrated passive circuit elements with at least one potential-jump barrier or surface barrier the substrate being other than a semiconductor body, e.g. an insulating body

Abstract

A pixel current driver based on a current mirror comprises a plurality of thin film transistors (TFTs) driving OLED layers. The plurality of thin film transistors may be four thin film transistors formed in a current-programmed .DELTA.V 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 T.
The OLED layer are continuous and vertically stacked on the plurality of thin film transistors to provide an aperture ratio close to 100%.

Description

,_. ~ 02438577 2004-07-27 '') J.
WO 02!067327 PCT/CA02/00173 PIXEL CURRENT DRIVER FOR ORGANIC LIGHT EMITTING DIODE DISPLAYS
BACKGROUND OF THE INVENTION
!.Field of the Invention The present invention relates to an organic light emitting diode disglay, and more particularly to w a pixel current driver for an organic light emitting diode (OLED), _ _ _ _ _ _._. _._. _- _._ _ ._ _._ . _ _____._ _

2.Description of the Prior Art 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 2o 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 CHIC) 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:Ii), polycrystalline silicon (poly-Si), or polymer technologies, to provide the bias voltage and drive current needed in each OLED based 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.
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 10- TFT integration for stable drive current reduces the size of the aperture. The same happens when pixel sizes are scaled down. One 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
Accordingly, it is an object of the present invention to provide a pixel current driver for an organic light emitting 34 display (OLED).
To achieve the above object, a pixel current driver for the OLED layer, according to an aspect of the present invention, comprises a plurality of thin, film transistors (TFTs) .

3 Each of the thin film transistor may be an a-Si:H
based thin film transistor or a polysilicon-based thin film transistor.
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 colour displays.
The circuits are fabricated using normal inverted staggered TFT structures. Preferably, the length is 30~un and the width is 1600um. 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 plurality of thin film transistors may be four thin film transistors formed in a current-programmed oVT-compensated manner. The OLED layer is vertically stacked on the plurality of thin film transistors.
With the above structure on an a-Si:H current driver according to an aspect of 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
The above objects and features of the present invention will become more apparent by describing in detail preferred

4 embodiments thereof with reference to the attached drawings in which:
Fig. 1 shows variation of required pixel areas with mobility for 2-T and 5-T pixel drivers;
Fig. 2 shows a pixel architecture for surface emissive a-Si:H AMOLED displays;
Fig. 3 shows a cross section of a dual-gate TFT structure;
Fig. 4 shows forward and reverse transfer characteristics of dual-gate TFT for various top gate biases;
Fig. 5A and Fig. 5B show an equivalent circuit for a 2-T
pixel driver and its associated input-output timing diagrams;
Fig. 6A and Fig. 6B show an equivalent circuit for a 5-T
pixel driver and its associated input-output timing diagrams;
Fig. 7 shows transient performance of the 5-T driver for three consecutive write cycles;
Fig. 8 shows input-output transfer characteristics for the 2-T pixel driver for different supply voltages;
Fig. 9 shows input-output transfer characteristics for the

5-T pixel driver for different supply voltages;
Fig. 10 shows variation in OLED current as a function of the normalised shift in threshold voltage;
Fig. 11 shows a 2-T polysilicon based pixel current driver having p-channel drive TFTs;
Fig. 12 shows a 4-T pixel current driver for OLED displays;

Fig. 13 shows a 4-T pixel current driver with a lower discharge time;
Fig. 14 shows a 4-T pixel current driver without non-linear 5 gain;
Fig. 15 shows a 4-T pixel current driver that is the building block for the full color circuit; and Fig. 16 shows a full color(RGB) pixel current driver for OLED displays.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although amorphous Si does not enjoy equivalent electronic properties compared to poly-Si, it adequately meets many of the drive requirements fox 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 ~.FE~100cm2/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 (~,FE-lcm2/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 lOmA/cmz at 10V operation to provide a brightness of 100 cd/m2 - the required luminance for most displays. For example, with an a-Si:H TFT mobility of 0.5cmz/Vs and channel length of 25~,m, this drive current requirement translates into required pixel area of 300 ~m2, which adequately meets the requirements of pixel resolution and speed for some 3 inch monochrome display applications. Figure 1 illustrates simulation results for the variation of the required pixel size with device mobility calculated for two types of drivers,

6 which will be elaborated later, the 2-T and the 5-T drivers, wherein ~,o denotes a reference mobility whose value is in the range 0.1 to 1 cm2/Vs. For instance, the area of the pixel for the 2-T driver (see Figure 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~.A and 50Hz, respectively, for a 230 x 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 ~,FE and TFT aspect ratio, W/L(Wide/Length).
In terms of threshold voltage (VT) 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 (OVT). 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 ~VT 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-SiNX:H, and TFT arrays can be achieved at low temperatures ,., j

7 PCT/CA02/00173 (s120'C) thus making it amenable to plastic substrates, which is a critical requirement~for mechanically flexible, displays.
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. Tn 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, Figure 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~.m to provide enough spacing between the source and drain for the top gate, and the width is made very large (1600um) 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 gave rise to a time-dependent-capacitance., - -Figure 4 shows results of static current measurements for four cases:~first when the top gate is tied to -lOV, second when the top gate is grounded, third when the top gate is floating, and lastly when the top gate is shorted to the

8 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 OV 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.
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 0/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.
The OLED drive circuits considered here are the well known voltage-programmed 2-T driver and the more sophisticated current-programmed ~VT-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~.m), reduced leakage, lower supply voltage (20V), higher linearity (~30dB), l

9 and larger dynamic range (.-40dB) . Before discussing 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 Cs and the gate capacitance of the driver transistor TZ. Depending on the voltage on the data line, the capacitor charges up to turn the driver transistor TZ on, which then starts conducting to drive the OLED with the appropriate level of current. When the address line is turned off, T1 is turned off but the voltage at the gate of TZ remains since the leakage current of Tl 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.
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, Vaddress and Iota are activated or deactivated simultaneously. When Vaaaress is activated, it forces T,, and T2 to turn on. Tl immediately starts conducting but TZ
does not since T, and T4 are off. Therefore, the voltages at the drain and source of TZ become equal. The current flow through T1 starts charging the gate capacitor of transistors T, and TS, very much like the 2-T driver. The current of these transistors start increasing and consequently TZ starts to conduct current . Therefore, Tl' s share of Iota reduces and Tz' s share of Iota increases. This process continues until the gate capacitors of T3 and TS charge (via T1) to a voltage that forces - - the current of T3- to- be- Iota. At this time, -the eur-rent -of - T1 - is zero and the entire Iota goes through TZ and T3. At the same time, TS drives a current through the OLED, which is ideally equal to I~ta* (WS/W3) , which signifies a current gain. Now if Idata and vaaareeg are deactivated, TZ will turn off, but due to the presence of capacitances'in T3 and T5, the current of these two devices cannot be changed easily, since the capacitances keep the bias voltages constant. This forces T4 to conduct the same current as that of T3, to enable the driver TS to drive 5 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.
The result of transient simulation for the 5-T driver circuit is shown in Fig. 7. As can be seen, the circuit has a

10 write time of <70~,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 (WS/W3 = 8), which reduces the required input current to <2/~A for 10~,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 (~30dB) in the transfer characteristics (Idata/IoLSD) 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 TS) in the high current path, which in turn decreases the required supply voltage and hence improves the dynamic range. According to Figure 9, a good dynamic range (~40dB) is observed for supply voltage of 20V and drive currents in the range IpLEDSIO~A, which is realistic for high brightness. Figure 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

11 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.
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 (~30dB), and high dynamic range (~40dB) 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.
Polysilicon has higher electron and hole mobilities than amorphous silicon. The hole mobilities are large enough to allow the fabrication of p-channel TFTs.
The advantage of having p-channel TFTs is that bottom emissive OLEDs can be used along with a p-channel dxive TFT to make a very good current source. One such circuit is shown in Fig. 11. Tn 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.
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

12 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.
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 Vth 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 Vtn increases and the drive current decreases based on the current-voltage equation below:
Ids = ( ~.CoXW / 2L ) (Vgs -Vth) z (in Saturation region) 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 Vth~
The first current mirror based pixel driver circuit is presented, which automatically compensated for shifts in the Vth of the drive TFT in a pixel. This circuit is the 5-T
circuit shown in Fig. 6A.
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 Vth shift. The first circuit shown ~in Fig. 12 is a modification of the 5-T circuit of Fig. 6A. (Transistor T4 has been removed from the 5-T circuit). This circuit occupies _a smaller area than the 5-T circuit, and provides a higher WO 02/067327 . PCTICA02100173

13 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.
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, and T4 do not nave the same source voltages.
However, this gain is non-linear and may not be desirable in some cases.
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 T3 and T4 are at the same. voltage. It still maintains the lower capacitance discharge time, along with the other features of the circuit of Fig. 8.
Fig. 15 shows another version of the g-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.
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. .
The dual-gate TFTs are used in the above-mentioned circuits to enable vertical integration of the OLSD layers with minimum parasitic effects.
The above-mentioned circuits compensate for the Vth shift when . .: v ,

14 the circuits ccx~rise 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.

Claims (19)

We claim:
1. A pixel driver circuit comprising:
an address line;
a data line;
a plurality of thin film transistors (TFTs) forming a current mirror, the plurality of thin film transistors comprising:
a switch thin film transistor, a first node of the switch transistor being connected to the data line and a gate of the switch transistor being connected to the address line;
a feedback thin film transistor, a first node of the feedback transistor being connected to the data line and a gate of the feedback transistor being connected to the address line;
a reference thin film transistor, a drain of the reference transistor being connected to a second node of the feedback transistor, a gate of the reference transistor being connected to a second node of the switch transistor and a source of the reference transistor being connected to a ground potential; and a drive thin film transistor, a gate of the drive transistor being connected to the gate of the reference transistor, the pixel driver circuit driving a pixel through the drive thin film transistor.
2. A pixel driver circuit comprising:
an address line;
a data line;
a plurality of thin film transistors (TFTs) forming a current mirror, the plurality of thin film transistors comprising:
a switch thin film transistor, a first node of the switch transistor being connected to the data line and a gate of the switch transistor being connected to the address line;
a feedback thin film transistor, a gate of the feedback transistor being connected to the address line and a second node of the feedback transistor being connected to a ground potential;
a reference thin film transistor, a drain of the reference transistor being connected to a second node of the switch transistor, a gate of the reference transistor being connected to the second node of the switch transistor and a source of the reference transistor being connected to a first node of the feedback transistor; and a drive thin film transistor, a gate of the drive transistor being connected to the gate of the reference transistor, the pixel driver circuit driving a pixel through the drive thin film transistor.
3. A pixel driver circuit comprising:
an address line;
a data line;
a plurality of thin film transistors (TFTs) forming a current mirror, the plurality of thin film transistors comprising:
a switch thin film transistor, a first node of the switch transistor being connected to the data line and a gate of the switch transistor being connected to the address line;
a feedback thin film transistor, a first node of the feedback transistor being connected to the data line and a gate of the feedback transistor being connected to the address line;
a reference thin film transistor, a drain of the reference transistor being connected to a second node of the feedback transistor, a gate of the reference transistor being connected to a second node of the switch transistor and a source of the reference transistor being connected to a ground potential;
a diode-use thin film transistor, a drain and a gate of the diode-use transistor being connected to a potential, and a source of the diode-use transistor being connected to the second node of the feedback transistor; and a drive thin film transistor, a gate of the drive transistor being connected to the gate of the reference transistor, the pixel driver circuit driving a pixel through the drive thin film transistor.
4. The pixel driver circuit according to any of claims 1-3 wherein the thin film transistors include amorphous silicon.
5. The pixel driver circuit according to any of claims 1-3 wherein the thin film transistors include polycrystalline silicon.
6. The pixel driver circuit according to any of claims 1-5 wherein the thin film transistors each comprise a second gate.
7. A pixel circuit comprising:
an organic light emitting diode; and the pixel driver circuit according to any of claims 1-6 for driving the organic light emitting diode, the source of the drive thin film transistor being connected to the ground potential and the drain of the drive thin film transistor being connected to the organic light emitting diode.
8. The pixel circuit according to claim 7 further comprising a capacitor, the capacitor being connected between the gate of the drive transistor and the ground potential.
9. The pixel driver circuit according to any one of claims 1-6, wherein the pixel driven circuit is provided for driving a color pixel of a full colour display.
10. The pixel circuit according to claim 7 or 8, wherein the pixel circuit is provided for a full colour display.
11. The pixel driver circuit according to claim 5, wherein the polycrystalline silicon is p-type.
12. The pixel driver circuit according to any one of claims 1-3, wherein at least one of the thin film transistors is an a-Si:H based thin film transistor.
13. The pixel driver circuit according to any one of claims 1-3, wherein at least one of the thin film transistors is a polysilicon-based thin film transistor.
14. The pixel driver circuit according to claim 13, wherein the at least one of the thin film transistor is a p-channel thin film transistor.
15. The pixel driver circuit according to any of claims 1-5, wherein the thin film transistors are dual gate transistors, and the dual gates are fabricated in a normal inverted staggered TFT structure.
16. The pixel driver circuit according to claim 15, wherein a top gate of the dual gate is grounded.
17. The pixel driver circuit according to claim 15, wherein a top gate of the dual gate is electrically tied to a bottom gate of the dual gate.
18. The pixel driver circuit according to any of claims 1-6, wherein the pixel driver circuit is provided for a monochrome display.
19. The pixel circuit according to claim 7 or 8, wherein the pixel circuit is provided for a monochrome display.
CA002438577A 2001-02-16 2002-02-18 Pixel current driver for organic light emitting diode displays Expired - Fee Related CA2438577C (en)

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