Abstract:
A LED pixel structure that reduces current nonuniformities and threshold voltage variations in a “drive transistor”of the pixel structure is disclosed. The LED pixel structure incorporates a current source for loading data into the pixel via a data line. Alternatively, an auto zero voltage is determined for the drive transistor prior to the loading of data.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/044,174 filed Apr. 23, 1997, which is herein incorporated by reference. 
    
    
     This invention was made with U.S. government support under contract number F33615-96-2-1944. The U.S. government has certain rights in this invention. 
    
    
     The invention relates to an active matrix light emitting diode pixel structure. More particularly, the invention relates to a pixel structure that reduces current nonuniformities and threshold voltage variations in a “drive transistor” of the pixel structure and method of operating said active matrix light emitting diode pixel structure. 
     BACKGROUND OF THE DISCLOSURE 
     Matrix displays are well known in the art, where pixels are illuminated using matrix addressing as illustrated in FIG. 1. A typical display  100  comprises a plurality of picture or display elements (pixels)  160  that are arranged in rows and columns. The display incorporates a column data generator  110  and a row select generator  120 . In operation, each row is sequentially activated via row line  130 , where the corresponding pixels are activated using the corresponding column lines  140 . In a passive matrix display, each row of pixels is illuminated sequentially one by one, whereas in an active matrix display, each row of pixels is first loaded with data sequentially. Namely, each row in the passive matrix display is only “active” for a fraction of the total frame time, whereas each row in the active matrix display can be set to be “active” for the entire total frame time. 
     With the proliferation in the use of portable displays, e.g., in a laptop computer, various display technologies have been employed, e.g., liquid crystal display (LCD) and light-emitting diode (LED) display. An important distinction between these two technologies is that a LED is an emissive device which has power efficiency advantage over non-emissive devices such as (LCD). Generally, an important criticality in portable displays is the ability to conserve power, thereby extending the “on time” of a portable system that employs such display. 
     In a LCD, a fluorescent backlight is on for the entire duration in which the display is in use, thereby dissipating power even for “off” pixels. Namely, all pixels in a LCD are illuminated, where a “dark” or “off” pixel is achieved by causing a polarized layer to block the illumination through that pixel. In contrast, a LED (or OLED) display only illuminates those pixels that are activated, thereby conserving power by not having to illuminate off pixels. 
     Although a display that employs an OLED pixel structure can as reduce power consumption, such pixel structure exhibits nonuniformity in intensity level over time. Namely, the OLED structure will degrade with use, where it has been found that the turn-on voltage of an organic OLED increases over life, with the voltage increase dependent on the total time-integrated charge density through the OLED. 
     With use, the gate to source voltage (threshold voltage) of the “drive transistor” M 2  may vary, thereby causing a change in the current passing through the LED. This varying current contributes to the nonuniformity in the intensity of the display. 
     Another contribution to the nonuniformity in intensity of the display can be found in the manufacturing of the “drive transistor”. In some cases, the “drive transistor” is manufactured from a material that is difficult to ensure uniformity of the transistors such that variations exist from pixel to pixel. 
     However, it has been observed that the brightness of the OLED is proportional to the current passing through the OLED. Therefore, a need exists in the art for a pixel structure and concomitant method that reduces current nonuniformities and threshold voltage variations in a “drive transistor” of the pixel structure. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, a current source is incorporated in a LED (OLED) pixel structure that reduces current nonuniformities and threshold voltage variations in a “drive transistor” of the pixel structure. The current source is coupled to the data line, where a constant current is initially programmed and then captured. 
     In another embodiment, the constant current is achieved by initially applying a reference voltage in an auto-zero phase that determines and stores an auto zero voltage. The auto zero voltage effectively accounts for the threshold voltage of the drive transistor. Next, a data voltage which is referenced to the same reference voltage is now applied to illuminate the pixel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a block diagram of a matrix addressing interface; 
     FIG. 2 depicts a schematic diagram of an active matrix LED pixel structure of the present invention; 
     FIG. 3 depicts a schematic diagram of an alternate embodiment of the present active matrix LED pixel structure; 
     FIG. 4 depicts a schematic diagram of another alternate embodiment of the present active matrix LED pixel structure; 
     FIG. 5 depicts a block diagram of a system employing a display having a plurality of active matrix LED pixel structures of the present invention; and 
     FIG. 6 depicts a schematic diagram of an alternate embodiment of the active matrix LED pixel structure of FIG.  2 . 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 depicts a schematic diagram of an active matrix LED pixel structure  200  of the present invention. In the preferred embodiment, the active matrix LED pixel structure is implemented using thin film transistors (TFTs), e.g., transistors manufactured using amorphous or poly-silicon. Similarly, in the preferred embodiment, the active matrix LED pixel structure incorporates an organic light-emitting diode (OLED). Although the present pixel structure is implemented using thin film transistors and an organic light-emitting diode, it should be understood that the present invention can be implemented using other types of transistors and light emitting diodes. For example, if transistors that are manufactured using other materials exhibit the threshold nonuniformity as discussed above, then the present invention can be employed to provide a constant current through the lighting element. 
     Although the present invention is illustrated below as a single pixel or pixel structure, it should be understood that the pixel can be employed with other pixels, e.g., in an array, to form a display. Furthermore, although the figures below illustrate specific transistor configuration, it should be understood that the source of a transistor is relative to the voltage sign. 
     Referring to FIG. 2, pixel structure  200  comprises three PMOS transistors  240 ,  250 ,  260 , a NMOS transistor  270 , a capacitor  280  and a LED (OLED)  290  (light element). A select line  210  is coupled to the gate of transistors  240 ,  250  and  270 . A data line is coupled to the source of transistor  250  and a +V DD  line is coupled to the drain of transistor  270 . One electrode of the OLED  290  is coupled to the drain of transistors  240  and  260 . The source of transistor  240  is coupled to the gate of transistor  260  and to one terminal of capacitor  280 . Finally, the drain of transistor  250 , the source of transistor  270 , the source of transistor  260  and one terminal of the capacitor  280  are all coupled together. 
     The present pixel structure  200  provides a uniform current drive in the presence of a large threshold voltage (V t ) nonuniformity. In other words, it is desirable to maintain a uniform current across the OLED, thereby ensuring uniformity in the intensity of the display. 
     More specifically, the OLED pixel structure is operated in two phases, a load data phase and a continuous illuminating phase. 
     Load Data Phase 
     A pixel structure  200  can be loaded with data by activating the proper select line  210 . Namely, when the select line is set to “Low”, transistor P 4  ( 240 ) is turned “On”, where the voltage on the anode side of the OLED  290  is transmitted to the gate of the transistor P 2  ( 260 ). Concurrently, transistor P 1  ( 250 ) is also turned “ON” so that the constant current from the data line  220  flows through both the transistor P 2  ( 260 ) and the OLED  290 . Namely, the transistor  260  must turn on to sink the current that is being driven by the current source  230 . The current source  230  that drives the data line is programmed by external data. The gate to source voltage of transistor  260  (drive transistor) will then settle to a voltage that is necessary to drive the current. Concurrently, transistor N 1  ( 270 ) is turned “Off”, thereby disconnecting the power supply +V DD  from the OLED  290 . The constant current source  230  will also self-adjust the source-to-gate voltage to accommodate a fixed overdrive value (voltage) for transistor  260  and will compensate the threshold variation on the polysilicon TFT  260 . The overdrive voltage is representative of the data. In turn, the data is properly stored on the storage capacitor Cs  280 . This completes the load or write cycle for the data. 
     Continuous Illuminating Phase 
     When the select line is set “High”, both transistors of P 1  ( 250 ) and P 4  ( 240 ) are turned “Off” and the transistor N 1  ( 270 ) is turned “On”. Although the source voltage of the transistor  260  may vary slightly, the source-to-gate voltage of the transistor  260  controls the current level during the illumination cycle. The Vsg of transistor  270  across the capacitor  280  cannot change instantaneously. Thus, the gate voltage on transistor  260  will track with its source voltage such that the source-to-gate voltage is maintained the same throughout the entire Load and Illumination phases. The leakage current of polysilicon TFT and voltage resolution required for gray scale luminance of OLED will determine the size of storage capacitor needed for holding a valid data for a frame time. In the preferred embodiment, the capacitor is on the order of approximately 0.25 pf. Namely, the capacitor must be large enough to account for the current leakage of transistor  260 . This completes the pixel operation for the illumination phase. 
     It should be noted that each data/column line  220  has its own programmed constant current source  230 . During the illumination phase, the subsequent programmed current source on the data lines feeds through and loads the next rows of all pixels, while the pixels of previous rows are operating in the illumination phase for the whole frame time. Thus, this pixel structure of FIG. 2 requires only 3 PMOS transistors and 1 NMOS transistor with 2.5 lines. (select line, data line-current source and V DD  voltage supply which can be shared with adjacent pixels). Alternatively, FIG. 6 illustrates an implementation where the pixel structure of FIG. 2 is implemented with all PMOS transistors, which will provide economy for using either PMOS or NMOS processes only. The NMOS transistor N 1  is replaced with a PMOS P 3  transistor  610 . However, an additional line (control line)  620  is coupled to the gate of transistor  610  for addressing the additional PMOS transistor, thereby requiring a total of 3.5 lines, i.e., an additional voltage supply for controlling the additional PMOS gate. 
     In sum, the pixel structures of FIG.  2  and FIG. 6 are designed to compensate the threshold variation of both polysilicon TFT and the OLED by self-adjusting/tracking mechanism on Vsg of transistor  260  and by supplying a constant current source through the OLED  290 . In fact, the pixel structures of FIG.  2  and FIG. 6 are able to accomplish proper operation during both Load and Illumination phases with hard voltage supply. These pixel structures can be implemented to design high-quality OLED displays with good gray scale uniformity and high lifetime despite instabilities in either the OLED or the pixel polysilicon TFT. 
     FIG. 3 illustrates an alternate embodiment of the present active matrix pixel structure. In this alternate embodiment, the data line voltage is converted into a current within the pixel structure without the need of a voltage-to-current converter such as the implementation of a current source as discussed above in FIGS. 2 and 6. 
     Referring to FIG. 3, pixel structure  300  comprises four PMOS transistors ( 360 ,  365 ,  370 ,  375 ), two capacitors  350  and  355  and a LED (OLED)  380 . A select line  320  is coupled to the gate of transistor  360 . A data line  310  is coupled to the source of transistor  360  and a +V DD  line is coupled to the source of transistor  365  and one terminal of capacitor  355 . An auto-zero line  330  is coupled to the gate of transistor  370  and an illuminate line is coupled to the gate of transistor  375 . One electrode of the OLED  280  is coupled to the drain of transistor  375 . The source of transistor  375  is coupled to the drain of transistors  365  and  370 . The drain of transistor  360  is coupled to one terminal of capacitor  350 . Finally, the gate of transistor  365 , the source of transistor  370 , one terminal of the capacitor  350  and one terminal of the capacitor  355  are all coupled together. 
     More specifically, FIG. 3 illustrates a pixel structure  300  that is operated in three phases: 1) an auto-zero phase, 2) a load data phase and 3) an illuminating phase. 
     Auto-zero 
     When auto-zero line  330  and the illuminate line  340  are set to “Low”, transistor P 2  ( 375 ) and P 3  ( 370 ) are turned “On” and the voltage on the drain side of transistor P 1  ( 365 ) is transmitted to the gate and is temporarily connected as a diode. The data line  310  is set to a “reference voltage” and the select line  320  is set to “Low”. The reference voltage can be arbitrarily set, but it must be greater than the highest data voltage. 
     Next, the illuminate line  340  is set to “High”, so that transistor P 2   375  is turned “Off”. The pixel circuit now settles to a threshold of the transistor P 1   365  (drive transistor), thereby storing a voltage (an auto-zero voltage) that is the difference between the reference voltage on the data line and the threshold voltage of the transistor P 1   365  on the capacitor C c    350 . This sets the gate voltage, or more accurately V SG  of transistor  365  to the threshold voltage of transistor  365 . This, in turn, will provide a fixed overdrive voltage on transistor P 1  ( 365 ) regardless of its threshold voltage variation. Finally, Auto Zero line  330  is set to “High”, which isolates the gate of transistor P 1   365 . The purpose of auto-zero is henceforth accomplished. 
     Load Data Phase 
     At the end of the Auto Zero phase, the select line was set “Low” and the data line was at a “reference voltage”. Now, the data line  310  is set to a data voltage. This data voltage is transmitted through capacitor C c    350  onto the gate of transistor P 1  ( 365 ). Next, the select line is set “High”. Thus, the V SG  of transistor  365  provides transistor  365  with a fixed overdrive voltage for providing a constant current level. This completes the load data phase and the pixel is for illumination. 
     Continuously Illuminating Data Phase During Deselect Row Phase 
     With the data voltage stored on the gate of transistor P 1  ( 365 ), the illuminate line  340  is set to “Low”, thereby turning “On” transistor P 2   375 . The current supplied by the transistor P 1   365 , is allowed to flow through the OLED  380 . In sum, the transistor  365  behaves like a constant current source. This completes the Illumination phase. 
     FIG. 4 illustrates another alternate embodiment of the present active matrix pixel structure. In this alternate embodiment, the data line voltage is also converted into a current within the pixel structure without the need of a voltage-to-current converter such as the implementation of a current source as discussed above in FIGS. 2, and  6 . 
     Referring to FIG. 4, pixel structure  400  comprises three PMOS transistors ( 445 ,  460 ,  465 ), two capacitors  450  and  455  and a LED (OLED)  470 . A select line  420  is coupled to the gate of transistor  445 . A data line  410  is coupled to the source of transistor  445  and a VSWP line is coupled to the source of transistor  460  and one terminal of capacitor  455 . An auto-zero line  430  is coupled to the gate of transistor  465 . One electrode of the OLED  470  is coupled to the drain of transistors  465  and  460 . The drain of transistor  445  is coupled to one terminal of capacitor  450 . Finally, the gate of transistor  460 , the source of transistor  465 , one terminal of the capacitor  450  and one terminal of the capacitor  455  are all coupled together. 
     More specifically, FIG. 4 illustrates a pixel structure  400  that is also operated in three phases: 1) an auto-zero phase, 2) a load data phase and 3) an illuminating phase. 
     Auto-zero (By VSWP) Phase 
     VSWP (voltage switching supply) is set to a “lower voltage” by the amount “ΔV”, where the lower voltage is selected such that the OLED  470  is trickling a small amount of current (depending on the OLED characteristic, e.g., on the order of nanoamp). The lower voltage is coupled through onto the gate of transistor P 1  ( 460 ) V G (P 1 ) without dilution due to the floating node between the transistor P 4  ( 445 ) and C c  ( 450 ) coupling capacitor. When Auto Zero line  430  is then set to “Low”, the transistor P 1  ( 460 ) (drive transistor) is temporarily connected as a diode by closing the transistor P 3  ( 465 ). The select line  420  is then set to “Low” and a “reference voltage” is applied on the data line  410 . The reference voltage can be arbitrarily set, but it must be greater than the highest data voltage. The pixel circuit is now allowed to settle to the threshold of transistor P 1   460 . Finally, Auto Zero line  430  is then set to “High”, which isolates the gate of transistor P 1   460 . The effect of this Auto Zero phase is to store on the capacitor C c    450  a voltage (an auto-zero voltage) that represents the difference between the reference voltage on the data line and the transistor threshold voltage of P 1   460 . This completes the auto-zero phase. 
     Load Data Phase 
     At the end of the Auto Zero phase, the select line was set “Low” and the data line was at a “reference voltage”. Next, the data line is then switched from a reference voltage to a lower voltage (data voltage) where the change in the data is referenced to the data. In turn, the data voltage (data input) is load coupled through capacitors  450  and  455  to the gate of transistor P 1   460 . The voltage V SG  of the transistor  460  provides the transistor P 1  ( 460 ) with a fixed overdrive voltage to drive the current for the OLED  470 . Namely, the data voltage will be translated into an overdrive voltage on transistor P 1   460 . Since the voltage stored on the capacitor  450  accounts for the threshold voltage of the transistor P 1   460 , the overall overdrive voltage is now independent of the threshold voltage of the transistor P 1 . The select line  420  is then set “High”. This completes the load data phase. 
     Continuously Illuminate Data During Deselect Row Phase 
     At the completion of the data loading phase, the gate of transistor P 1   460  is now isolated except for its capacitive connections, where the overdrive voltage for driving the OLED is stored on capacitor C S    455 . Next, the VSWP is returned to its original higher voltage (illuminate voltage). In turn, with VSWP rising, there is now sufficient voltage to drive the OLED for illumination. Namely, when select line  420  is set to “High”, both transistors P 3  ( 465 ) and P 4  ( 445 ) are turned “Off”, and the data voltage is kept in storage on V SG  of transistor  460  as before. This source-to-gate voltage V SG(P1)  is maintained in the same manner throughout the entire Illumination phase, which means the current level through the OLED will be constant. This completes the Illumination cycle. 
     In sum, FIG. 3 discloses a pixel structure that uses 4 PMOS transistors and 1 coupling capacitor with 3½ lines. (Auto-Zero line and VDDH voltage supply can both be shared). FIG. 4 discloses a pixel structure that uses only 3 PMOS transistors and 1 coupling capacitor with 2½ line. (VSWP switching power supply could be share with adjacent pixel) Both of these two pixel structures can compensate the threshold variation of both polysilicon TFT and OLED by illuminating and auto-zero trickling current mechanism on V SG(P1) . The aforementioned two (2) pixel structures can also be implemented in polysilicon NMOS and in amorphous NMOS design. 
     The two (2) pixel structures of FIG.  3  and FIG. 4 can be implemented to design high-quality OLED with good gray scale uniformity and high lifetime despite instabilities in either the OLED or the pixel polysilicon TFT. 
     FIG. 5 illustrates a block diagram of a system  500  employing a display  520  having a plurality of active matrix LED pixel structures  200 ,  300 ,  400  or  600  of the present invention. The system  500  comprises a display controller  510  and a display  520 . 
     More specifically, the display controller can be implemented as a general purpose computer having a central processing unit CPU  512 , a memory  514  and a plurality of I/O devices  416  (e.g., a mouse, a keyboard, storage devices, e.g., magnetic and optical drives, a modem and the like). Software instructions for activating the display  520  can be loaded into the memory  514  and executed by the CPU  512 . 
     The display  520  comprises a pixel interface  522  and a plurality of pixels (pixel structures  200 ,  300 ,  400  or  600 ). The pixel interface  522  contains the necessary circuitry to drive the pixels  200 ,  300 ,  400  or  600 . For example, the pixel interface  522  can be a matrix addressing interface as illustrated in FIG.  1 . 
     Thus, the system  500  can be implemented as a laptop computer. Alternatively, the display controller  510  can be implemented in other manners such as a microcontroller or application specific integrated circuit (ASIC) or a combination of hardware and software instructions. In sum, the system  500  can be implemented within a larger system that incorporates a display of the present invention. 
     Although the present invention is described using PMOS transistors, it should be understood that the present invention can be implemented using NMOS transistors, where the relevant voltages are reversed. Namely, the OLED is now coupled to the source of the NMOS drive transistor. By flipping the OLED, the cathode of the OLED should be made with a transparent material. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.