PCT
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`(51) International Patent Classification 6 :
`WO 98/48403
`G09G 3/10
`
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`
`(11) International Publication Number:
`
`Al
`
`(43) International Publication Date:
`
`29 October 1998 (29.10.98)
`
`(21) International Application Number:
`
`PCT/US98/08367
`
`(22) International Filing Date:
`
`23 April 1998 (23.04.98)
`
`(81) Designated States: JP, KR, European patent (AT, BE, CH, CY,
`DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT,
`SE).
`
`(30) Priority Data:
`60/044,174
`09/064,696
`09/064,697
`
`23 April 1997 (23.04.97)
`22 April 1998 (22.04.98)
`22 April 1998 (22.04.98)
`
`US
`US
`US
`
`Published
`With international search report.
`Before the expiration of the time limit for amending the
`claims and to be republished in the event of the receipt of
`amendments.
`
`(71) Applicant: SARNOFF CORPORATION [US/US]; 201 Wash(cid:173)
`ington Road, CN-5300, Princeton, NJ 08543-5300 (US).
`
`(72) Inventors: DAWSON, Robin, Mark, Adrian; 184 Coppermine
`Road, Princeton, NJ 08540 (US). KANE, Michael, Gillis;
`44 Robin Drive, Skillman, NJ 08558 (US). HSU, James,
`Ya-Kong; 7107 Hana Road, Edison, NJ 08817 (US).
`HSUEH, Fu-Lung; 14 Kinglet Drive South, Cranbury, NJ
`IPRI, Alfred, Charles; 7 Cotswold Lane,
`08512 (US).
`Princeton, NJ 08540 (US). STEW ART, Roger, Green; 3
`Ski Drive, Neshanic Station, NJ 08853 (US).
`
`(54) Title: ACTIVE MATRIX LIGHT EMITTING DIODE PIXEL STRUCTURE AND METHOD
`
`(57) Abstract
`
`A LED pixel structure (200, 300,
`400, 600, 700) that reduces current
`nonuformities 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.
`
`DATA
`LINE
`
`220
`
`230
`
`210
`
`SELECT
`LINE
`
`295
`
`+Vro
`
`250 s
`G
`
`P1
`
`G
`
`D
`
`N1 270
`s
`
`Cs
`
`G
`
`G
`
`s
`
`240
`
`P4
`
`D
`
`s
`
`260
`
`P2
`
`D
`
`290
`Q.B)
`
`-
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`FOR THE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international applications under the PCT.
`
`AL
`AM
`AT
`AU
`AZ
`BA
`BB
`BE
`BF
`BG
`BJ
`BR
`BY
`CA
`CF
`CG
`CH
`Cl
`CM
`CN
`cu
`CZ
`DE
`DK
`EE
`
`Albania
`Annenia
`Austria
`Australia
`Azerbaijan
`Bosnia and Herzegovina
`Barbados
`Belgium
`Burkina Faso
`Bulgaria
`Benin
`Brazil
`Belarus
`Canada
`Central African Republic
`Congo
`Switzerland
`Celle d'Ivoire
`Cameroon
`China
`Cuba
`Czech Republic
`Gennany
`Denmark
`Estonia
`
`ES
`FI
`FR
`GA
`GB
`GE
`GH
`GN
`GR
`HU
`IE
`IL
`IS
`IT
`JP
`KE
`KG
`KP
`
`KR
`KZ
`LC
`LI
`LK
`LR
`
`Spain
`Finland
`France
`Gabon
`United Kingdom
`Georgia
`Ghana
`Guinea
`Greece
`Hungary
`Ireland
`Israel
`Iceland
`Italy
`Japan
`Kenya
`Kyrgyzstan
`Democratic People's
`Republic of Korea
`Republic of Korea
`Kazakstan
`Saint Lucia
`Liechtenstein
`Sri Lanka
`Liberia
`
`LS
`LT
`LU
`LV
`MC
`MD
`MG
`MK
`
`ML
`MN
`MR
`MW
`MX
`NE
`NL
`NO
`NZ
`PL
`PT
`RO
`RU
`SD
`SE
`SG
`
`Lesotho
`Lithuania
`Luxembourg
`Latvia
`Monaco
`Republic of Moldova
`Madagascar
`The fonner Yugoslav
`Republic of Macedonia
`Mali
`Mongolia
`Mauritania
`Malawi
`Mexico
`Niger
`Netherlands
`Norway
`New Zealand
`Poland
`Portugal
`Romania
`Russian Federation
`Sudan
`Sweden
`Singapore
`
`SI
`SK
`SN
`sz
`TD
`TG
`TJ
`TM
`TR
`TT
`UA
`UG
`us
`uz
`VN
`YU
`zw
`
`Slovenia
`Slovakia
`Senegal
`Swaziland
`Chad
`Togo
`Tajikistan
`Turkmenistan
`Turkey
`Trinidad and Tobago
`Ukraine
`Uganda
`United States of America
`Uzbekistan
`Viet Nam
`Yugoslavia
`Zimbabwe
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`WO98/48403
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`PCT/US98/08367
`
`ACTVE MATRIX LIGHT EMITTING DIODE PIXEL STRUCTURE AND METHOD
`
`-1-
`
`This application claims the benefit of U.S. Provisional Application
`5 No. 60/ 044, 174 filed April 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
`10 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
`15 a "drive transistor" of the pixel structure and method of operating said
`active matrix light emitting diode pixel structure.
`
`ID
`
`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
`25 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.
`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). In a LCD, a fluorescent backlight is on for the
`35 entire duration in which the display is in use, thereby dissipating power
`
`30
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`even for "off' pixels. 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
`5 reduce power consumption, such pixel structure may exhibit
`nonuniformity in intensity, which is attributable to two sources, threshold
`voltage drift of the drive transistor and transistor nonuniformity due to
`manufacturing. 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.
`
`10
`
`15
`
`~
`
`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 an alternate 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
`25 applied to illuminate the pixel.
`In an another alternate embodiment, a resistor is incorporated in a
`LED (OLED) pixel structure to desensitize the dependence of the current
`passing through the OLED to the threshold voltage of the drive transistor.
`
`30
`
`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;
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`5
`
`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;
`FIG. 6 depicts a schematic diagram of an alternate embodiment of
`the active matrix LED pixel structure of FIG. 2; and
`FIG. 7 depicts a schematic diagram of an alternate embodiment of
`an active matrix LED pixel structure of the present invention.
`To facilitate understanding, identical reference numerals have been
`15 used, where possible, to designate identical elements that are common to
`the figures.
`
`10
`
`DETAILED DESCRIPTION
`FIG. 2 depicts a schematic diagram of an active matrix LED pixel
`2D 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).
`25 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
`30 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,
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`5
`
`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
`10 260. 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 (Vt) 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.
`
`15
`
`25
`
`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 P4 (240) is turned "On", where the voltage on the anode side of
`the OLED 290 is transmitted to the gate of the transistor P2 (260).
`Concurrently, transistor Pl (250) is also turned "ON" so that the constant
`current from the data line 220 flows through both the transistor P2 (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
`30 230 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 Nl
`(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
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`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
`5 completes the load or write cycle for the data.
`
`Continuous Illuminating Phase:
`When the select line is set "High", both transistors of Pl (250) and
`P4 (240)are turned "Off' and the transistor Nl (270) is turned "On".
`10 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-
`15 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
`20 approximately .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
`25 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
`30 1 NMOS transistor with 2.5 lines. (select line, data line-current source
`and VDD 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
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`15
`
`NMOS transistor Nl is replaced with a PMOS P3 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
`5 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 V sg of transistor 260 and by
`supplying a constant current source through the OLED 290. In fact, the
`10 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 OLEO 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 +Vnn line is
`coupled to the source of transistor 365 and one terminal of capacitor 355.
`25 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
`OLEO 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
`30 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.
`
`a>
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`Auto-Zero:
`When auto-zero line 330 and the illuminate line 340 are set to "Low",
`transistor P2 (375 )and P3 (370) are turned "On" and the voltage on the
`5 drain side of transistor Pl (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 P2
`10 375 is turned "Off'. The pixel circuit now settles to a threshold of the
`transistor Pl 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 Pl 365 on the capacitor Cc 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 Pl (365) regardless of its threshold voltage
`variation. Finally, Auto Zero line 330 is set to "High", which isolates the
`gate of transistor Pl 365. The purpose of auto-zero is henceforth
`accomplished.
`
`15
`
`2D
`
`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 Cc 350
`
`25 onto the gate of transistor Pl (365). Next, the select line is set "High".
`Thus, the VSG 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.
`
`30 Continuously Illuminating Data Phase During Deselect Row Phase:
`With the data voltage stored on the gate of transistor Pl (365), the
`illuminate line 340 is set to "Low", thereby turning "On" transistor P2 375.
`The current supplied by the transistor Pl 365, is allowed to flow through
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`the OLED 380. In sum, the transistor 365 behaves like a constant current
`source. This completes the Illumination phase.
`
`10
`
`FIG. 4 illustrates another alternate embodiment of the present
`5 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(cid:173)
`zero line 430 is coupled to the gate of transistor 465. One electrode of the
`15 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
`20 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
`
`25 amount "!iV", 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 Pl (460) VG(Pl) without
`
`dilution due to the floating node between the transistor P4 (445) and Cc
`
`30
`
`(450) coupling capacitor. When Auto Zero line 430 is then set to "Low", the
`transistor Pl (460) (drive transistor) is temporarily connected as a diode by
`closing the transistor P3 (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.
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`The pixel circuit is now allowed to settle to the threshold of transistor Pl
`460. Finally, Auto Zero line 430 is then set to "High", which isolates the
`gate of transistor Pl 460. The effect of this Auto Zero phase is to store on
`the capacitor Cc 450 a voltage (an auto-zero voltage) that represents the
`5 difference between the reference voltage on the data line and the transistor
`threshold voltage of Pl 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 Pl 460. The voltage VsG of the transistor 460 provides the
`
`transistor Pl (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 Pl 460. Since the voltage stored on the capacitor 450
`accounts for the threshold voltage of the transistor Pl 460, the overall
`overdrive voltage is now independent of the threshold voltage of the
`transistor Pl. The select line 420 is then set "High". This completes the
`load data phase.
`
`10
`
`15
`
`2D
`
`Continuously Illuminate Data During Deselect Row Phase:
`At the completion of the data loading phase, the gate of transistor Pl
`25 460 is now isolated except for its capacitive connections, where the
`overdrive voltage for driving the OLED is stored on capacitor C8 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 P3 (465) and P4 (445) are turned "Off', and the data voltage is
`kept in storage on VSG of transistor 460 as before. This source-to-gate
`
`30
`
`voltage VSG(Pl) 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.
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`In sum, FIG. 3 discloses a pixel structure that uses 4 PMOS
`
`transistors and 1 coupling capacitor with 3 l/2 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
`
`5 21/2 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 VSG(Pl)• The aforementioned two ( 2)
`
`15
`
`pixel structures can also be implemented in polysilicon NMOS and in
`10 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. 7 depicts a schematic diagram of an active matrix LED pixel
`structure 700 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 poly-silicon or
`amorphous silicon. Similarly, in the preferred embodiment, the active
`20 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.
`The present pixel structure 700 provides a uniform current drive in
`the presence of a large threshold voltage (Vt) nonuniformity. In other
`words, it is desirable to maintain a uniform current through the OLEDs,
`thereby ensuring uniformity in the intensity of the display.
`Referring to FIG. 7, pixel structure 700 comprises two PMOS
`transistors 710 and 720, a capacitor 730, a resistor 750 and a LED (OLED)
`740 (light element). A select line 770 is coupled to the gate of transistor 710.
`A data line 760 is coupled to the source of transistor 710. One terminal of
`resistor 750 is coupled to the source of transistor 720 and one electrode of
`the OLED 740 is coupled to the drain of transistor 720. Finally, the drain of
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`transistor 710, the gate of transistor 720 and one terminal of the capacitor
`730 are all coupled together.
`More specifically, when a row containing a pixel structure is
`selected to be the active row, a logical "high" level on the select line 770
`turns on transistor Ml 710, thereby allowing capacitor C 730 to be charged
`to a voltage Vg from the data line 760. After this row is deselected by a
`"low" level on the select line 770, which turns transistor Ml off, the voltage
`on the capacitor 730 is stored for the frame time. Since this voltage
`appears on the gate of transistor M2 720, it sets a current through
`transistor 720 that also passes through the OLED 740, which is located in
`the drain of the transistor 720.
`More importantly, a resistor 750 is implemented in the present pixel
`structure. The resistor is coupled to the source of transistor 720 and
`serves as a negative feedback element. If an individual drive transistor
`15 has an unusually low threshold voltage, the transistor tends to pass more
`current to the OLED, but the additional current causes an additional
`voltage drop across the resistor 750, thereby reducing the current.
`A complementary effect occurs with a drive transistor having an
`unusually high threshold voltage. The overall effect is to reduce the
`20 nonuniformity in the current. It has been observed that resistors can be
`generally formed with much better resistance uniformity than the
`threshold voltage uniformity achieved with TFTs. One reason is that TFT
`threshold voltages are very sensitive to the trap density in the active silicon
`material, whereas the resistance of the doped layers used in resistors is
`215 much less sensitive to trap density. Measurements have shown that the
`percentage variation of resistance is very small across a polysilicon
`display wafer, and to the extent that resistance does vary, it is expected to
`be smoothly varying, unlike transistor thresholds.
`The current passing through the OLED 740 determines its
`30 brightness. However, it has been observed that when the pixel is
`implemented using TFTs, the threshold voltages of the TFTs can also vary
`over life as discussed above. In addition, there can be initial
`nonuniformities in the TFT threshold voltages. It should be noted that
`such nonuniformity with regard to transistor 710 is not a problem, since
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`its threshold voltage does not have a strong effect on the current that is
`established through the OLED. In contrast, variations in the threshold
`voltage of drive transistor 720 directly affect the current through the
`OLED.
`More specifically, the current, l 01Eo passing through the OLED in
`the present pixel structure can be expressed as:
`
`5
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`K' W
`IOLED = --(Vg - ½ -/OLEDR)
`2 L
`
`2
`
`(1)
`
`10 where K' is the intrinsic transconductance parameter of the transistor
`M2, Wand Lare its width and length, Vt is its threshold voltage, Vg is the
`
`voltage from the data line, and resistor R 750 has a value of lM in the
`preferred embodiment. However, the resistor value may range from l00K
`to l0M depending on the drive transistor characteristics. It has been
`15 observed that the present pixel structure may reduce the current variation
`to 1/3 of what is possible without the present resistor as discussed below.
`More specifically, it can be shown that with a resistor coupled to the
`source of transistor 720, the normalized sensitivity of the current through
`
`the diode to threshold voltage variations - -
`1oLED
`
`di
`OLED is:
`dV1
`
`-2/(V g- Vt+ IQLED R).
`
`(2)
`
`It is beneficial to increase the gate voltage V g as much as possible, but
`
`with the limitation that the transistor 720 must stay in saturation. By
`introducing a voltage drop across the resistor (IQLED R), the sensitivity to
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`25
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`threshold voltage variations can be reduced below what would be
`achievable without a resistor. Ultimately, the term OOLED R) cannot
`
`become larger than (V g - Vt), since such result would imply that
`
`transistor 720 was turned off. Therefore, the maximum reduction in
`30 sensitivity that can be achieved by placing a resistor in the source of
`transistor 720 is a factor of 2.
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`However, placing a resistor in the source permits the transistor 720
`width W to be increased, where such increase reduces standard deviation
`of the threshold voltage, a Vt. For a fixed maximum gate voltage, W can be
`
`increased, thereby deriving more benefit from the statistical reduction in
`5 a Vt. Thus, by putting a resistor in the source of the transistor 720, a
`
`reduction in current variation can be achieved through the combined
`
`effect of (1) reducing the sensitivity to threshold variations - -
`1 oLED
`
`d!OLED
`
`dVt
`
`(limited to a theoretical maximum benefit of 2X, or 50% reduction), and (2)
`reducing the threshold variation a Vt itself (no limit except for geometrical
`
`15
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`10 and capacitance constraints).
`FIG. 5 illustrates a block diagram of a system 500 employing a
`display 520 having a plurality of active matrix LED pixel structures 200,
`300,400,600 or 700 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 1/0 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
`20 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, 600 or 700). The pixel interface 522
`contains the necessary circuitry to drive the pixels 200, 300, 400, 600 or 700.
`For example, the pixel interface 522 can be a matrix addressing interface
`2.5 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.
`30 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 sh