Abstract:
A driving scheme and pixel circuits therein for active matrix light-emitting device displays are provided. The driving scheme is structured to perform both line selection of and power delivery to the pixels via the same scan-power electrode, as opposed to conventional approach where scanning and drive are performed via separate access lines, thereby allowing more compact pixel design and better utilization of light emitting area. Furthermore, this driving scheme provides a dynamic reference for the active elements in a pixel, creating a different design concept and greater flexibility for pixel circuits. Embodiments of pixel circuits employing this driving scheme are exemplified in this disclosure.

Description:
CROSS REFERENCE  
       [0001]     This application claims the priority of U.S. Provisional Patent Application No. 60/522,045, filed on Aug. 6, 2004, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the pixel circuit and a driving method of an active matrix display comprising light-emitting devices which emits light by conducting a driving current through a light emitting thin film such as an organic semiconductor thin film, and thin film transistors for controlling the light emitting operation of the respective light emitting devices. A preferred embodiment of the present invention applies to light emitting devices formed with organic material, the organic light emitting diode (OLED). More specifically, the present invention provides a method and structure to address and deliver the driving power to a pixel using multi-functional access lines, thereby simplifying the array structure of a light emitting device display and the fabrication process thereof, and increasing the fill factor of light emitting area.  
         [0004]     2. Description of the Prior Art  
         [0005]     Organic light emitting diode displays have attracted significant interests in commercial application in recent years. Its excellent form factor, fast response time, lighter weight, low operating voltage, and prints-like image quality make it the ideal display devices for a wide range of application from cell phone screen to large screen TV. Passive OLED displays, with relatively low resolution, have already been integrated into commercial cell phone products. Next generation devices with higher resolution and higher performance using active matrix OLEDs are being developed. Initial introduction of active matrix OLED displays have been seen in such products as digital camera and small video devices. Demonstration of OLED displays in large screen sizes further propels the development of a commercially viable active matrix OLED technology. The major challenges in achieving such a commercialization include (1) improving the material and device operating life, and (2) reducing device variation across the display area. Several methods have been suggested to address the second issue by including more active switching devices in a pixel, or by switching of supply lines externally. A common theme of these solutions is an increase of device complexity. The present invention addresses the complexity issue by structuring the pixel so that a conventional scanning electrode is configured as a current supply electrode to the light emitting device in part of a cycle to deliver full drive power, without adding to the circuit any additional switching electrode or signals.  
         [0006]     Examples of using organic material to form an LED are found in U.S. Pat. No. 5,482,896, U.S. Pat. No. 5,408,109, and U.S. Pat. No. 5,663,573, and examples of using organic light emitting diode to form active matrix display devices are found in U.S. Pat. No. 5,684,365 and U.S. Pat. No. 6,157,356, all of which are hereby incorporated by reference.  
         [0007]     An active matrix OLED display ( FIG. 1 ) is typically structured with “SELECT” electrodes for row select, “DATA” electrodes for setting the pixel state, power electrodes VDD to drive the pixels, and a reference voltage. A basic pixel in an active matrix display also contains at least one transistor for data control, and at least a memory element to hold the data sufficiently long so a pixel remains stable in a frame. A circuit diagram for a basic pixel  100  in an active matrix OLED display is depicted in  FIG. 2  in further detail. An active matrix with pixel circuit structured similar to  FIG. 2  allows data to be written and retained in a storage capacitor  204  according to the data signal delivered in an address cycle, while the power supply VDD continuously drives OLED  205  through an n-channel transistor  201 , according to the data setting in capacitor  204 . The selection of pixels to receive data information is controlled by an n-channel transistor  203  that is controlled by the voltage on a select electrode connected to the gate of transistor  203 . An active matrix driving scheme allows the drive transistor  201  remain in a data state for an extended period of time after the address cycle, the peak current required for achieving a brightness level is reduced accordingly compared to a passive matrix. Its peak driving current does not scale with the resolution, making it suitable for high resolution applications. Stability of the display is also improved appreciably.  
         [0008]      FIG. 2   b  illustrates a similar construction of an active matrix light emitting device display with n-channel drive transistor  201   b , n-channel data control transistor  203   b , capacitor  204   b , and light emitting diode  205   b.    
         [0009]     Noticeable in both  FIGS. 2 and 2   b , the pixel circuits are structured as common-cathode where the cathode of the light emitting diode is connected to a common voltage line shared by other pixels. Where in  FIG. 2  the data signal is written into the capacitor referencing to a constant VDD, the actual drive control voltage, gate-to-source voltage (V GS ), that determines the current in the drive transistor is affected by the voltage across the light emitting diode according to, since the source terminal of the drive transistor floats on the light emitting diode. Where in  FIG. 2   b , the data signal is written into the capacitor being directly affected by voltage across the light emitting diode according to V GS =V DATA −V LED . In another word, in both of these examples, the gate voltage can not be directly referenced to the source of the transistor. The drive voltage is unavoidably interfered by the voltage across the light emitting diode. This illustrates a commonly know compromise between common-cathode structure and source-referencing scheme in a light emitting diode display pixel.  
         [0010]     As illustrate in the above example, the electrical current for producing light output flows through at least a control element that regulates the current. In a conventional light emitting device display, these control elements are fabricated on a thin film of amorphous silicon on glass. Power consumed in such control elements are converted to heat rather than yielding any light. To reduce such power consumption, polycrystalline silicon is preferred over amorphous silicon for its better mobility. More elaborated methods employing self-regulated multiple-stage conversions suitable for pixel circuit using polysilicon base material may be found in U.S. Pat. No. 6,501,466 and U.S. Pat. No. 6,580,408. These methods provide a current drive scheme while largely eliminated the impact from material and transistor non-uniformity typically associated with thin film polysilicon on glass. In these methods, typically a minimum of four transistors are required to achieve such self-regulated, multi-stage conversion to achieve a pixel-independent current drive for the display. An example of such methods is illustrated in  FIG. 3 . where four transistors  301 ,  302 ,  303 , and  307 , and 3 access lines, DATA, SELECT, and VDD, are used for each pixel with a storage capacitor  304  and an OLED  305 .  
         [0011]     The present invention provides a multi-functional scan-power electrode for pixel access that carries the conventional pixel select function and power delivery function on the same bus line, thereby allowing a reduction in display complexity. The pixel structure so constructed comprises a direct current path from scan-power electrode to the light emitting element, the turning-on and off of which are fully controlled by the voltage applied on said scan-power electrodes.  
       SUMMARY OF THE INVENTION  
       [0012]     This invention is used in the operation of an active matrix display comprising light emitting elements.  
         [0013]     As described in the background, the conventional pixel structures and the operating method of a light emitting device display involves a scanning electrode (or referred to as such different names as SELECT or GATE line) and a power supply electrode (VDD). The scanning electrode connects to a pixel through the high impedance gates of control transistors in the pixel. Such scanning electrodes did not operate to supply a major part of the drive current to the light emitting device for producing light output, nor did such prior art pixel structures operate with any direct conducting path from a scanning electrode to the light emitting device.  
         [0014]     The present invention provides a method to operate a light emitting device display wherein the drive power is delivered via the same scanning electrodes that perform pixel selection for data input. A pixel circuit and drive method are provided to allow a scanning electrode (now named as scan-power electrode in this invention) performs a dual functions of selecting and controlling data input to the pixel, and delivering drive current to the light emitting element in the pixel. Furthermore, such direct current paths provided by the control circuit in a pixel in this invention is fully controlled by the signal voltages applied to said scan-power electrodes without relying on additional control signals operated on separate control lines. Alternating direct current paths are thus created via the same scan-power electrodes, and a drive current in full capacity is delivered to the light emitting elements, rather than assisting in part to reduce the resistance of another main power delivering electrode, without having to distinctively switching the power electrodes.  
         [0015]     In summary, the present invention provides pixel structures and a drive method to operate a light emitting device display by dual functional scan-power electrodes, where each of such scan-power electrodes operates to perform line selection in a data input cycle, and operates to deliver drive power in a drive cycle. The pixel circuits so provided possess both structural distinction and functional distinction. One structural distinction is that a direct current path from a scan-power electrode to a voltage source via a light emitting element is provided. Such direct current path is enabled or disabled depending on the voltage applied to said scan-power electrodes, and may be further modulated by the data information. No additional switching mechanism is involved in such operation. As another structural distinction is that the display so constructed operates without a separate power electrode for delivering drive current; the drive current is deliver in whole during a period of operation via a scan-power electrode. A functional distinction is that the drive current needed for a light emitting device according to data information is delivered in full capacity, not relying on any other means, via a scan-power electrode. Furthermore, the present invention demonstrates in a preferred embodiment a pixel configuration in common-cathode, common-source, with n-channel drive transistor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram of a prior art active matrix light emitting device.  
         [0017]      FIG. 2  is a schematic diagram of a prior art pixel circuit in an active matrix light emitting device.  
         [0018]      FIG. 3  is a schematic diagram of a prior art pixel circuit in an active matrix light emitting device.  
         [0019]      FIG. 4  is a pixel circuit in a preferred embodiment of the present invention using a p-channel transistor for scan select, and an n-channel transistor for drive.  
         [0020]      FIG. 5  is a pixel circuit in another embodiment of this invention.  
         [0021]      FIG. 6  is a schematic diagram of a pixel circuit in another embodiment of the present invention.  
         [0022]      FIG. 7  is a schematic diagram of an active matrix light emitting device display in the present invention.  
         [0023]      FIG. 8  is a schematic diagram of a pixel circuit utilizing two same-type transistors in the present invention. Data reference is the high-side voltage Vref.  
         [0024]      FIG. 9  is a pixel circuit in an embodiment utilizing two same-type transistors in the present invention, and having a capacitor structure formed with part of an adjacent scan-power electrode.  
         [0025]      FIG. 10  is a schematic diagram of a pixel circuit, a variation from  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Preferred embodiments of the present invention are herein described using organic light emitting diodes as illustration. Examples of using organic material to form an LED are found in U.S. Pat. No. 5,482,896 and U.S. Pat. No. 5,408,109, and examples of using organic light emitting diode to form active matrix display devices are found in U.S. Pat. No. 5,684,365 and U.S. Pat. No. 6,157,356, all of which are hereby incorporated by reference.  
         [0027]     As evidenced in the prior art, the conventional method of constructing and operating a light emitting device display involves a scanning electrode (or referred to as SELECT line, GATE line, or other names carrying similar meaning) and a power supply electrode (VDD). The scanning electrode interacts with a pixel through high impedance gates of switching elements in the pixel and does not participate in delivering of drive current to the light emitting device.  
         [0028]     The present invention provides a method to drive light emitting device in an active matrix display without explicit power electrodes. This method is made possible by constructing specific pixel circuits so that the drive current is delivered via a scanning electrode without interfering with the scanning operation performed by a scanning electrode. A pixel so constructed utilizes a scan-power electrode that delivers drive current while inhibiting data input in one period, and enables data input according a scanning signal in another period. A pixel so constructed comprises direct current paths from a scan-power electrode, the enabling and inhibiting of which are fully controlled by voltage signals applied to the scan-power electrode.  
         [0029]     In the description of this invention, a direct current path represents a path that conducts direct electrical current when enabled, and is enabled in at least one of the operation periods of a display. A direct current in a pixel circuit is an electrical current not ended on or via a capacitor in said circuit; such current thus shall not be entirely from charging and discharging of capacitive elements in a pixel or other transient charging current. The capacitive elements in a pixel circuit include explicit capacitor structures such as parallel conductive plates, and parasitic capacitive components such as those arising from capacitive coupling between the input gate or base and the body of a transistor. The small control current into the high-impedance control nodes of a switching device (for example, gate current in an MOS device or base current in a bipolar transistor) is also excluded from the consideration of direct current. In other words, a direct current path so defined is a current path capable of conducting a sustained and significant current under biasing conditions consistent with operation of a display. Specifically, the conductions merely due to leakage, charging or discharging the parasitic elements in transient state, or a path via or ended on a capacitor are excluded from the definition of direct current path. An integrated circuit contains various types of parasitic components, such as gate-to-source capacitance, drain-to-substrate junction capacitance, and junction leakage paths. For example, the overlapping between the gate and the source region of an MOS transistor forms a parasitic gate-to-source capacitor, which is inherently connected to the gate and source in its own structure. Transient current and leakage current may arise from conduction through such components. These parasitic conducting paths are excluded from being a valid direct current path in this definition. On the other hand, a direct current path as defined may be a conduction path that is modulated by a transistor according to the gate voltage of the transistor. A direct current path so defined is thus a structure that may comprise transistors (via source-drain or emitter-collector), resistors, and diodes, connected in a manner that allows current flow in at least part of the operation.  
         [0030]     A scan-power electrode represents an access line that is structured to perform both a scanning operation where a scanning signal is delivered to enable data input in selected pixels in one period of the operation, and a drive operation where a drive current is delivered to a light emitting device in another period of operation. A scanning electrode means a conventional access line that performs a conventional scanning (or select) operation only.  
         [0031]     An organic light emitting diode is used in most preferred embodiments wherever appropriate; the presence of such a device in such examples should not be construed as setting forth a limitation on the present invention directed for light emitting devices in general. The MOS devices are used as a preferred embodiment for the switching elements. Similar bipolar transistors perform equal functions as MOS devices.  
         [0032]     Preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings.  
         [0033]      FIG. 4  provides an example of a preferred embodiment according to the present invention, wherein a pixel circuit comprises a p-channel transistor  403  for scanning control, an n-channel transistor  401  for drive control, a capacitor  404  for storing data information, an organic light emitting diode (OLED)  405 , scan-power electrode  410 , a reference voltage source Vref  470 , and a data electrode. In the preferred configuration as shown, the cathode of OLED  405  is connected to Vref making this embodiment a common-cathode structure. The first terminal of capacitor  404  is connected to the gate of drive transistor  401 , and the second terminal of the capacitor  404  is connected to a source/drain terminal B of transistor  401 . The opposite source/drain terminal A of transistor  401  is connected to the scan-power electrode  410 . The definition of terminals A and B as being a source or drain terminals, or vise versa, is determined by the voltage setting on scan-power electrode  410 , dynamically. All voltage levels discussed herein reference to Vref  470 .  
         [0034]     In a preferred operation, Vref is set to be the lowest voltage (GND) in the system, and data information is formatted to positive values relative to GND. During a scanning cycle where data information is delivered through a data electrode to a pixel, the voltage on a scan-power electrode is set to GND. Since the gate of p-channel transistor  403  is at the lowest level, the p-channel transistor is turned on, allowing bidirectional current flow for data transfer between the data electrode and the storage capacitor  404 . The direction of such data current is determined by whether the incoming data voltage is more, or less, positive than the existing data voltage in the capacitor. In such event, n-channel transistor  401  is turned on due to positive voltage on its gate, thereby keeping the conducting channel open in transistor  401  and allowing a conducting path from the second terminal of capacitor  404  to the scan-power electrode, which is set at GND during this cycle. Such an operating configuration provides a low GND level reference to the capacitor during data input. Any data information formatted using low GND reference is therein properly registered to a pixel using the same fixed reference voltage, and stored in the capacitor  404 .  
         [0035]     The duration of such address cycle is typically set to approximately one N th  of the period assigned to refresh a display image. For example, for sixty frames per second refreshing rate on 100 horizontal lines, the addressing period is approximately 1/6000 of a second.  
         [0036]     In a drive cycle, a scan-power electrode  410  is switched to a drive voltage (VDD) by a driver connected to the scan-power electrode. As a preferred operation, VDD is set to be the most positive voltage level in the system. More detailed discussion and numerical examples for setting VDD are provided in the subsequent paragraphs. The p-channel transistor  403  is thus turned off by a VDD on it gate, isolating the capacitor  404  and the gate of transistor  401  from the data electrode. The data received in a scanning cycle in a pixel is thereby retained on capacitor  404 . A high positive voltage VDD on a scan-power electrode also provides a voltage source to bias transistor  401  into its operating point, and to forward bias OLED  405 . The drive current via transistor  401  is then regulated by the data information stored at capacitor  404 , which is connected to the gate of  401 .  
         [0037]     Note that in a drive cycle, transistor  401  is in a source-follower configuration with its gate-to-source voltage (V GS ) maintained by the capacitor ( 404 ) voltage, while in a scanning cycle, point B is brought to GND. The control voltage (V GS ) of  401  is thus a direct transfer of input signal without any influence from the forward characteristics of OLED  405 .  
         [0038]     In a preferred operating condition, the drive voltage VDD for a scan-power electrode is set to be equal to, or slightly higher than the sum of the maximum forward voltage of OLED and the dynamic range of input data. Such a VDD setting ensures the voltage drop from drain to source (V DS ) of  401  in a drive cycle is greater than V GS , forcing transistor  401  into its saturation region, thereby providing full current control through V GS  and eliminating any influence from variations of OLED. For a display comprising OLED operated in 4.5 to 8V forward voltage for light producing, and a data range between 0 and 3V, a proper setting for VDD will thus be about 11 to 12 volts. As another example, for a display comprising polymer LED that operates in 3 to 5.5V for light emitting, and a dynamic data range of 3.3V, a proper setting for VDD will then be about 9 to 10V.  
         [0039]     Referring to  FIG. 4 , drive transistor  401  is considered having a varying configuration that is dynamically determined by the voltage on a scan-power electrode. In a drive cycle, terminal A of  401 , being connected to a scan-power electrode that is set at the most positive voltage level, is a drain terminal. Current flows from drain terminal A through source terminal B, and through OLED  405  to Vref. In contrast, in a scan (write) cycle, terminal A is at the lowest voltage set by scan-power electrode  410 . Any charging or discharging current directed toward capacitor  404  from the data electrode via transistor  403  is further directed toward terminal B. Transistor  401  remains in its on-state for any positive voltage accumulated on the capacitor. Terminal B thus operates as a drain when a charging current is directed into capacitor  404 , or when a discharging is taking place to drain the excess stored charge. Terminal A, being kept at GND in a scanning cycle, operates as source and providing a fix GND reference to point F of capacitor  404 , via transistor  401 . This dynamically varying configuration scheme creates the possibility for the circuit of  FIG. 4  to operate with an n-channel drive transistor in a common-cathode structure, without being influenced by OLED&#39;s characteristics.  
         [0040]     Associated with a display pixel circuits of  FIG. 4 , an external driver circuit capable of (1) driving the scan-power electrodes in the voltage range provided in the above discussion, and (2) providing current capacity to deliver the required current in full capacity via scan-power electrodes according to the data requirement may be connected to the scan-power electrodes to perform the operations. A preferred driver for driving scan-power electrodes in the present embodiment will then require a 10 volts output capacity to properly operate the display. As a second requirement, a driver operating the scan-power electrodes in the present invention is required to provide a current capacity to deliver drive current to the light emitting elements in all pixels in a row when each one is set to the brightest level. For example, given the highest brightness at 10 micro amp. per pixel and 640 pixels per row, a scan-power electrode driver requires at least an output capacity of about 6.4 mA per channel in its drive state to properly operate the display. An external row driver designed for this purpose thus needs to have the conventional voltage sequence, with enhanced current output capacity by including larger transistors or power transistors in its output stage. Such external driver circuit may be, as commonly practiced in display industry, attached to the display panel as an integral part of the display.  
         [0041]     As described in detail hereinabove, as a first perspective, the preferred embodiment comprises a scan-power electrode that controls the selection (scanning) of a pixel that involves data writing and data retaining by applying a first (scanning) signal and a second signal. The same scan-power electrode delivers drive current to the light emitting element during the period when the second (drive) signal is applied.  
         [0042]     As described hereinabove, the preferred embodiment in  FIG. 4  provides, as a second perspective, an illustration of the embodiment of a direct current path connecting said scan-power electrode and said reference voltage, via A-terminal and B-terminal of transistor  401 . Such a direct current path conducts a drive current in said drive cycle according to the voltage held in the capacitor  404 . It should be noted that various electrical elements may be further inserted in such a direct current path to further modify the operation. These further modifications does not violate the provision of a direct current path between a conventional scanning electrode and a voltage source to incorporate a drive function into the same scan-power electrode, as described in the present invention.  
         [0043]     The preferred embodiment of  FIG. 4  provides, as a third perspective, a demonstration of the functions of terminals A and B of transistor  401  as being drain and source vary in different operating cycles. The function of A and B terminals as being drain or source is not statically fixed at the time of design and implementation when connection and wiring are made, but rather depends on the operation voltage applied on said scan-power electrode. In this respect, it is more appropriate to refer to these terminals as second and third terminals (beside the gate terminal) that are dynamically assigned their functions (as drain or source) according to the operating voltages applied to the scan-power electrodes in different cycles in the operation.  
         [0044]     The embodiment of  FIG. 4  further provides, as a fourth perspective, a reference voltage equal to the scanning voltage for the second terminal (F) of capacitor  404  via transistor  401  during the time when such a voltage is applied on the scan-power electrode. This voltage repeats the same value each time when a scanning voltage is applied on said scan-power electrode, thereby providing a reference at a fixed voltage level for data writing. Terminal F of capacitor  404  is released to adjust itself according the operating point in a drive cycle where a voltage is determined by the current through OLED  405  and transistor  401 .  
         [0045]     An additional benefit demonstrated in this embodiment is a common cathode configuration while using a preferred n-channel drive transistor without being affected by the characteristics of the light emitting element. This operating configuration is made possible by the connection of capacitor to the node F between the drive transistor and the light emitting device, and a dynamic setting of such common node F as described above, the n-channel drive circuit is operable while allowing the cathode of the light emitting device connected to a common electrode. Such operation was not possible in previous drive schemes unless the forward voltage drop of a light emitting device is taken as part of the gate voltage.  
         [0046]     It should also be noted that the operation of pixel circuit in  FIG. 4  may be extend to a pixel circuit where the light emitting element is a bi-directional device. By replacing the light emitting element  405  with a bi-directional device, the same analysis and description above may be applied. Such pixel circuit and its light emitting function operate equally well. An example of forming a bi-directional organic light emitting device is found in U.S. Pat. No. 5,663,573.  
         [0047]     In the embodiment of  FIG. 4 , the transistors may be thin film transistors formed on a layer of amorphous or polycrystalline silicon, or single crystal silicon substrate. The voltage reference is typically supplied through a continuous layer  470  of conducting material connected to each and every pixel. The organic light emitting diode may be formed with a stack of layers of small molecule or polymer organic materials. The data and scan-power electrodes are typically formed with coated conductive films and using standard photolithography-etch processing techniques.  
         [0048]     A variation from the embodiment of  FIG. 4  is provided in an example in  FIG. 10 , wherein an n-channel transistor  1003  and a p-channel transistor  1001  are used. In a preferred operation, Vref is set to a most positive voltage in the system; scan cycle for writing data is initiated by setting voltage high on a scan-power electrode, and driving power is enabled by setting voltage low on a scan-power electrode.  
         [0049]      FIG. 5  illustrates another alternative embodiment of  FIG. 4  wherein capacitor  504  uses Vref  570  as its fixed reference voltage. During a scan (write) cycle, a scan-power electrode is set low, turning on p-channel transistor  503 , and allowing data to be refreshed at the capacitor and gate of  501 . The scan-power electrode is set high for drive cycle, turning off transistor  503 , and forward biasing n-channel transistor  501 . The reference voltage for the capacitor is constant, and thus a faster response for writing data into the capacitor. In operating such circuit, the data voltage needs to be raised by an additional offset voltage approximately equal to the average onset voltage of OLED  505  to ensure transistor  501  is properly turned on and in its saturation region in a data input cycle. This embodiment operates in a similar manner as the pixel circuit provided in  FIG. 2 , but with one fewer access lines.  
         [0050]      FIG. 6  illustrates an embodiment of a pixel circuit in a common anode configuration, wherein the anodes of all OLEDs in a row are connected to a common electrode. The pixel circuit comprises a p-channel transistor  603 , an n-channel drive transistor  601 , a capacitor  604 , a light emitting diode  605 , a scan-power electrode  610 , and a voltage reference Vref  670 . Vref is set to be the lowest voltage in the system. The operation procedure is similar to that of the circuit in  FIG. 5 . Since the capacitor, and the data voltage, is referenced directly to the same voltage as the source node of the transistor, typically, no additional offset voltage is required for data format. A typical data range is the same as the dynamic range of transistor  601 , and references to voltage low.  
         [0051]      FIG. 7  is an embodiment of an active matrix OLED display, showing adjacent, n and n+1 rows, and m and m+1 columns. Pixel circuit of  FIG. 4  is inserted as block  700  for illustration. Comparing to  FIG. 3 , VDD electrode of  FIG. 3  is eliminated, thereby freeing more light emitting area.  
         [0052]     Further extensions of the present invention may be achieved by altering pixel bias direction, and by combining dynamic drive of adjacent pixels. Preferred embodiments and respective benefits of such extension are provided herein.  
         [0053]      FIG. 8  illustrates a preferred embodiment using an all n-channel devices in a pixel. The pixel circuit comprises an n-channel scan control transistor  803 , an n-channel drive transistor  801 , a capacitor  804 , an OLED  805  in a common anode configuration with its anode connected to a most positive voltage reference Vref  870 , and a scan-power electrode  810  performing dual operations. During a scan (write) cycle, a scan-power electrode is set high (same level as Vref) turning on n-channel transistor  803 , and disabling transistor  801  because its source and drain ends are at the same voltage. Data is written and stored in capacitor  804 . At the completion of data writing, voltage of the scan-power electrode is set low (to the lowest voltage in the system), turning off transistor  803 , thereby isolating a pixel from external data signal. Setting voltage low on a scan-power electrode enables power on transistor  801 , driving an output current according to the data voltage stored on the gate. Terminal B corresponds to the source, and terminal A is the drain. Gate-to-source control voltage V GS  is equal to the data voltage measured from Vref, i.e. (Vref−V Data ).  
         [0054]     In a preferred operating scheme, data voltage is formatted to be greater than the maximum operating forward voltage of OLED, or alternatively, Vref is set equal to, or slightly higher than the sum of the maximum gate voltage according to data format and the onset voltage of OLED. This bias condition ensures drain-to-source voltage drop V DS  is greater than V GS  in a drive cycle, thereby forcing transistor  801  into its saturation region.  
         [0055]     A common cathode pixel may be constructed by varying the embodiment of  FIG. 8 , replacing the two n-channel transistors with two p-channel transistors, reconnecting the second terminal of the storage capacitor to the scan-power electrode (with the first terminal remains to the gate of the drive transistor), and reversing the polarity of OLED, voltage source and data.  
         [0056]     Considering efficiency in area utilization, a favorable embodiment of storage capacitor in a pixel circuit is a capacitor formed with the scan-power electrode conductor as part of the capacitor structure. A typical example of this is a capacitor formed underneath a scan-power bus line along one side of a pixel, having a thin layer of dielectric material between the scan-power bus electrode and another conductive layer underneath. In such embodiments, one capacitor terminal is connected to one of the two adjacent scan bus lines.  FIG. 9  illustrates a preferred embodiment of such pixel circuit. This embodiment illustrates a common cathode configuration with two p-channel transistors  903  and  901 . In a pixel belonging to the n th  data electrode, the source of transistor  901  and the second terminal of capacitor  904  are connected to a preceding (n−1) th  scan-power electrode. The gate of scan control transistor  903  is connected to the n th  scan-power electrode. In a preferred operation, driving power for the n th  row is enabled by setting the voltage on n th  scan-power electrode high. During a data writing cycle for the n th  row, voltage on n th  scan-power electrode is set low, turning on p-channel transistor  903  and allowing data to be written and stored in capacitor  904 . A scan cycle is terminated by setting the voltage of n th  scan-power electrode high, turning off transistor  903  and isolating the capacitor from data electrode. Since the power source for drive transistor  901  and voltage reference for storage capacitor  904  are connected to the preceding scan-power electrode, the operation of drive transistor is not affected by a voltage swing on n th  scan-power electrode. The power of n th  row is only temporarily paused when the preceding (n−1) th  scan electrode is set low in its scan cycle.  
         [0057]     An all n-channel, common anode embodiment can be obtained by replacing the two p-channel transistors in  FIG. 9  by n-channel transistors, and reversing the polarity of LED, power source, and data voltage.  
         [0058]     The present invention is described hereinbefore with specific combinations of transistors and polarity of OLED in each embodiment. These embodiments illustrate a drive scheme and rules to implement pixels circuit within such scheme. Variances and extensions are expected to be derived from the present invention. For example, an implementation using three or four transistors in a pixel with light emitting element, utilizing the method of delivering driving current and performing scan selection with the same access lines (scan-power electrodes) will fall well within the teaching of the present invention. As another example, utilizing the fluctuation of voltage on a scanning electrode to achieve a dynamic configuration of same transistors so that drain and source terminals of a drive transistor are interchangeable according to the momentary bias configurations, as described in the embodiment of  FIG. 4 , in a three-transistor implementation, will also fall within the scope of the present invention.  
         [0059]     Various preferred operating conditions with preferred reference voltages (Vref), are described in detail in this disclosure. The operation ranges described herein for the present invention shall not be construed as limitations to this invention. For example, the embodiment in  FIG. 4  may be operated in a linear region by providing a positive offset voltage to the data signal explicitly or implicitly through reconnecting the second terminal of the storage capacitor to a scan-power electrode.  
         [0060]     Although various embodiments utilizing the principles of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other variances, modifications, and extensions that still incorporate the principles disclosed in the present invention. The scope of the present invention embraces all such variances, and shall not be construed as limited by the number of active elements, wiring options of such, or the polarity of a light emitting device therein.