CA2507276C - 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
CA2507276C
CA2507276C CA002507276A CA2507276A CA2507276C CA 2507276 C CA2507276 C CA 2507276C CA 002507276 A CA002507276 A CA 002507276A CA 2507276 A CA2507276 A CA 2507276A CA 2507276 C CA2507276 C CA 2507276C
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transistor
connected
gate
pixel
thin film
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CA2507276A1 (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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    • 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 driver circuit for driving a colour pixel of a colour display and a pixel circuit having the pixel driver circuit are provided. The pixel driver circuit includes a data line, address lines, switch thin film transistors, feedback thin film transistors and drive thin film transistors. The pixel circuit includes organic light emitting diodes, which are driven by the pixel driver circuit.

Description

PIXEh CURRENT DRIVER FOR ORGANIC LIGHT EMITTING DIODE DISPLAYS
BACKGROUND OF THE INVENTION
l.Field of the Invention ' The present invention relates to a an organic light emitting diode display, and more particularly to a pixel current driver for an organic light emitting diaplay(OLED), capable of minimizing parasitic aoupling~ between the OLED and the transistor layers.

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 OZaED's demonstrated superiority over the LCD, there still rema~.n,.
several challenging issues related to encapsulation and lifetime, yield, color efficiency, and drive electronics, ail 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 7.ayer of backplane electronics, based on thin-film transistors (TFTs) fabricated using amorphous silicon (a-Si:B), 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,th~
excessive peak driving and leakage, currents associated with passive matrix addressing. This in turn increases the lifetime of the 07aED .
In active matrix OLED (AMOL$D) 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. The solution to having an aperture ratio that is invariant on scaling or on-pixel integration density is to vertically stack the O?~ED 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 (TFTa). 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. .

3 Summary of the Invention According to an aspect of the present invention, there is provided a pixel driver circuit for driving a colour pixel of a colour display, which includes: a first address line; a S data line; a first switch thin film transistor, a first node of the first switch transistor being connected to the data line and a gate of the switch transistor being connected to the first address line; a feedback thin film transistor, a first node and a gate of the feedback transistor being connected to a second node of the first switch transistor and a second node of the feedback transistor being connected to a ground potential; a second' switch thin film transistor, a , source of the second switch transistor being connected to a second node ofthe first switch transistor, a gate of the second switch transistor being connected to a second address line; a first drive thin film transistor, a gate of the first drive transistor being connected to a drain of the second switch transistor; a third switch thin film transistor, a source of the third switch 'transistor being connected to the second node of the first switch transistor, a gate of the third .switch transistor being connected to a third address line; a second drive thin film transistor, a gate, of the second,drive transistor being connected to the drain of the third switch transistor; a fourth switch thin film transistor, a source of the fourth switch transistor being connected to the second node of the first switch transistor, a gate of the fourth switch transistor being connected to a fourth address line; and a third drive thin film transistor, a gate of the third drive transitor being connected to the drain of the fourth switch transistor.
According to a further aspect .of the present invention, there is provided a pixel circuit, which includes: the pixel driver circuit; a first organic light emitting diode, a source of the first drive transistor being connected to the ground potential and a drain of the first drive transistor being

4 connected to the first organic light emitting diode; a second organic light emitting diode, a source of the second drive transistor being connected to the ground potential and a drain of the second drive transistor being connected to the second organic light emitting diode; and a third organic light emitting diode, a source of the third drive transistor being connected to the ground potential and a drain of the third drive transistor being connected to the third organic light emitting diode.
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.
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 30um and the width is 1600~Zm. The length and width of the transistors may change depending on the maximum drive current required by 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 ~VT-compensated manner, or four or The OLED layer is vertically stacked on the plurality of thin film transistors.
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
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:
Fig. 1 shows variation of required pixel areas with mobility for 2-T and 5-T pixel drivers;

5 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 normalized shift in threshold voltage;
Fig. il 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;

6 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 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(RaB) 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 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 ~.~»100ema/Ve. 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 (u~-lcm'/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 IOmA/cm' at 10V operation~to provide a brightness of 100 cd/m' - the required luminance for most displays. For example, with an a-Si :H TFT mobiJ.ity of 0. 5cm'/Vs and channel f length of 25~Cm, this drive current requirement translates into required pixel area of 300 ~.m',. which adequately meets the requirements of pixel resolution and speed for, some 3 inch monochrome display applications. Figure 1, illustrates simulatson 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 uQ denotes a reference mobility whose value is in the range 0.1 to 1 cm'/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.
the drive current and frame rate are kept constant at 20~tA 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 txade-off between u~ and TFT aspect ratio, W/L(Wide/Length).
In terms of threshold voltage (V~) 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 AVT 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 cixcuit (IC) technology; but with capital equipment coats 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 g (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. 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.
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 say 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~tm to provide enough spacing between the source and drain for the top gate, and the width is made very large (1600pm) 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.~~ ' 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 tvp gate is f3.oating, 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 currant is relatively high particularly when the top gate is biased with a negative voltage . The lowest values of leakage current axe 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 perfoxmance 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 re7,iable 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 ~V~.-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~Zm), reduced leakage, lower supply voltage (20V), higher linearity (~30dB), and larger dynamic range (..40dB). Before dwelling on the operation of the 5-T driver, the operation of the relatively simple voyage-driven 2-T driver is described.. Fig. 58 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, 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 T, 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, Vad~", and Ia,ta are activated or deactivated simultaneously. When V,~eee is activated, it forces T,, and Ta to turn on. Tl immediately starts conducting but Tz does not since T3 and T, are off . Therefore, the voltages at the drain and source of T, become equal. The current flow . through T1 starts charging the gate capacitor of transistors T3 25_ and TS, very much like the 2-T driver. The current of these transistors start increasing and consequently T1 starts to conduct current . Therefore, Tl's share of I~"ka reduces and Ta ~ s share of Iota increases. This process continues until the gate capacitors-of T3 and TS charge (via Tl) to a voltage that forces the current of T3 to be I~ta.~At this time, the current of T1 is zero and the entire I~~ goes through Ta and T3. At the same time, T5 drives a current through the OLED, which is ideally equal to I~ta* (WS/W3) , which signifies a current gain. Now if Iota and V8~,8 are deactivated, Ta will turn off, but due to the. presence of eapacitances'in T, and T5, the current of these two devices cannot be changed easily, since the capaeitances keep the bias voltages constant. This forces T, to conduct the same current as that of T3, to enable the driver Ts 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.
The result of transient simulation for the 5-T driver circuit is shown in Fig. 7. Aa can be seen, the circuit has a 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, arid T4 are scaled down. This also provides an internal gain (WS/W3 = 8), which reduces the required input current to <2EcA 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 TF~'T models for both forward and reverse regimes. A much improved linearity (-30dB) in the transfer characteristics (I~~,/Io~n) 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 requixed supply voltage and hence improves the dynamic range. According to Figure 9, a good dynamic range (.-4odB) is observed for supply voltage of 20V and drive.
currents in the range IoLEnslO~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 ~particulaxly when the shift is smaller than l0% 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. y The pixel architectures are compatible to surface (top) emissive AMOhED 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 (-3odB), and high dynamic range (.-40dB) at low supply voltages (15-20V) compared to the similar designs. (27V). The results descr ~ed here illustrate the feasibility of using a-Si:H for 3-i ch mobile monochrome display applications on both glass and lastic 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 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 OhEDs 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 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 ttiost 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 Vtb 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 Vt,, increases and the drive current decreases based on the current-voltage equation below:
Ids = ( ~.CdxW / 2L ) (V9a -Vth) s (in Saturation region) In the display, this would mean that the brightness of the OLED would decrease over time, which is unacceptable.
Hence, Che 2-T circuits shown earlier are not practical for OLED displays as they do not compensate for any increase in Va, . .
The first current mirror based pixel driver circuit is presented, which automatically compensated for shifts in the r Vrh 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. ~A. (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 ' 14 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 mirrox. 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 T3 and T, do not have 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 Ts 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 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.
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.
' It is important to note that the dual-gate.TFTs are used in the above-mentioned circuits to enable vertical integration of the OhSD - layers rnii~h - 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 5 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 (13)

We claim:
1. A pixel driver circuit for driving a colour pixel of a colour display, the circuit comprising:
a first address line;
a data line;
a first switch thin film transistor, a first node of the first switch transistor being connected to,the data line and a gate of the switch transistor being connected to the first address line;
a feedback thin film transistor, a first node and a gate of the feedback transistor being connected to a second node of the first switch transistor and a second node of the feedback transistor being connected to a ground potential;
a second switch thin film transistor, a source of the second switch transistor being connected to a second node of the first switch transistor, a gate of the second switch transistor being connected to a second address line;
a first drive thin film transistor, a gate of the first drive transistor being connected to a drain of the second switch transistor;
a third switch thin film transistor, a source of the third switch transistor being Connected to the second node of the first switch transistor, a gate of the third switch transistor being connected to a third address line;
a second drive thin film transistor, a gate, of the second drive transistor being connected to the drain of the third switch transistor;
a fourth switch thin film transistor, a source of the fourth switch transistor being connected to the second node of the first switch transistor, a gate of the fourth switch transistor being connected to a fourth address line; and a third drive thin film transistor, a gate of the third drive transitor being connected to the drain of the fourth switch transistor.
2. The pixel driver circuit according to claim 1, wherein at least one of the thin film transistor is an a-Si:H based thin film transistor.
3. The pixel driver circuit according to claim 1, wherein at least one of the thin film transistors is a polycrystalline silicon based thin film transistor.
4. The pixel driver according to claim 3, wherein the at least one of the thin film transistors is a p-channel thin film transistor.
5. The pixel driver circuit according to any one of claims 1-4, wherein at least one of the thin film transistors is a dual gate transistor.
6. The pixel driver circuit according to claim 5, wherein the dual gate is fabricated in a normal inverted staggered TFT
structure.
7. The pixel driver circuit according to claim 5 or 6, wherein a top gate of the dual gate is grounded.
8. The pixel driver circuit according to any one of claims 5-7, wherein a top gate of the dual gate is electrically tied to a bottom gate.
9. The pixel driver circuit according to any of claims 1-8, wherein the pixel driver circuit is provided for a full colour display.
10. A pixel circuit comprising:
a pixel driver circuit according to any one of claims 1-9;

a first organic light emitting diode, a source of the first drive transistor being connected to the ground potential and a drain of the first drive transistor being connected to the first organic light emitting diode;
a second organic light emitting diode, a source of the second drive transistor being connected to the ground potential and a drain of the second drive transistor being connected to the second organic light emitting diode; and a third organic light emitting diode, a source of the third drive transistor being connected to the ground potential and a drain of the third drive transistor being connected to the third organic light emitting diode.
11. The pixel circuit according to claim 10 further comprising:
a first capacitor connected between the gate of the first drive transistor and the ground potential a second capacitor connected between the gate of the second drive transistor and the ground potential; and a third capacitor connected between the gate of the third drive transistor and the ground potential.
12. The pixel circuit according to claim 10 or 11, wherein the first organic light emittingdiode is appropriate for emitting blue light, the second organic light emitting diode is appropriate for emitting green light and the third organic light emitting diode is appropriate for emitting red light.
13. The pixel circuit according to any one of claims 10-12, wherein the pixel circuit is provided for a full colour display.
CA002507276A 2001-02-16 2002-02-18 Pixel current driver for organic light emitting diode displays Expired - Fee Related CA2507276C (en)

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