(12) United States Patent
`Yumoto
`
`USOO6859193B1
`(10) Patent No.:
`US 6,859,193 B1
`(45) Date of Patent:
`Feb. 22, 2005
`
`(54) CURRENT DRIVE CIRCUIT AND DISPLAY
`DEVICE USING THE SAME, PIXEL
`CIRCUIT, AND DRIVE METHOD
`(75) Inventor: Akira Yumoto, Kanagawa (JP)
`(73) Assignee: Sony Corporation (JP)
`
`* Y Not
`Otice:
`
`Subj
`y disclai
`h
`f thi
`ubject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`(21) Appl. No.:
`09/787,036
`(22) PCT Filed:
`Jul. 14, 2000
`(86) PCT No.:
`PCT/JP00/04763
`S371 (c)(1),
`(2), (4) Date: Aug. 13, 2001
`(87) PCT Pub. No.: WO01/06484
`PCT Pub. Date:Jan. 25, 2001
`Foreign Application Priority Data
`(30)
`Jul. 14, 1999
`(JP) ......................................... P11-200843
`(51) Int. Cl. .................................................. G09G 3/32
`(52) U.S. Cl. ....................... 345/82; 345/204; 315/169.3
`(58) Field of Search ............................ 345/82, 76, 204,
`345/55, 83, 84, 62; 315/169.3, 169.1, 169.4;
`313/498–500, 505
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,952,789 A * 9/1999 Stewart et al. ........... 315/169.4
`6,229,506 B1
`5/2001 Dawson et al. ............... 345/82
`6,307.322 B1 * 10/2001 Dawson et al. .......... 315/169.1
`6,501,466 B1 * 12/2002 Yamagishi et al. ......... 345/204
`
`6,583,775 B1 * 6/2003 Sekiya et al. ................. 345/76
`6,686,699 B2 * 2/2004 Yumoto .........
`... 315/169.3
`2003/O128200 A1
`7/2003 Yumoto ...................... 34.5/211
`
`FOREIGN PATENT DOCUMENTS
`
`11/1989
`7/1997
`10/1997
`
`JP
`1-27967O
`JP
`9-1973.13
`JP
`9-263810
`* cited by examiner
`Primary Examiner Lun-yi Lao
`(74) Attorney, Agent, or Firm-Rader Fishman & Grauer
`PLL. Ronald P. Kananen
`(57)
`ABSTRACT
`A display including a current drive circuit capable of Sup
`plying a desired current to a light-emitting element in each
`pixel Stably and accurately irrespective of the characteristic
`variations of active elements in the pixel, thereby providing
`a high-definition image. Each pixel is composed of a receiv
`ing transistor (TFT3) for receiving a signal current (1 w)
`from a data ine (data) when a Scanning line (ScanA) is
`Selected, a converting transistor (TFT1) for converting the
`current level of the received signal current (1 w) to a voltage
`level and holding the Voltage level, and a driving transistor
`(TFT3) for allowing a drive current having a current level
`corresponding to the held voltage level to flow through
`light-emitting element (OLED). The converting thin film
`transistor (TFT1) generates the converted voltage level at its
`gate by allowing the signal current (Iw) through its channel,
`and a capacitor (C) holds the Voltage level at the gate of the
`transistor (TFT1). The transistor (TFT2) allows the drive
`current having a current level corresponding to the Voltage
`level held by the capacitor (C) to flow through the light
`emitting element (OLED).
`
`22 Claims, 19 Drawing Sheets
`
`
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`ScanA
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 1 of 19
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`US 6,859,193 B1
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`FIG. 1
`
`Vdd
`
`OLED
`
`TFT1
`
`TFT2
`
`data
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`SC3
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 2 of 19
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 3 of 19
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`US 6,859,193 B1
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`NIW80EL\/€)#7
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`BOIOH10BTEHOOAJLOTTE
`HO1000.NOOIWES
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`WT|B
`NHL
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 4 of 19
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`BOOMLOETE
`TWIEW BZ |
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`WT|B
`QN||VITISNI
`ELIVS) º
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`ZZZZZZZ |
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`N|WHOELW9#7
`BOOM!.10ETEEGOJLOETE
`HO1000N00|WES
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 5 of 19
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`US 6,859,193 B1
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`FIG.5
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`Vdd
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`OLED
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`TFT2
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 6 of 19
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 7 of 19
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`dSA
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`8XOA
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`|gueOS
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`ZgueOS
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`9NINNY/OS
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`BANG
`BNIT
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 8 of 19
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`US 6,859,193 B1
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`FIG.8
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`Wedd
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`OLED
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`TFT2a
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`TFT2b
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`CS
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`SCanA
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 9 of 19
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 10 0f 19
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`US 6,859,193 B1
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`F.G. 1
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`Vdd
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`CS
`SCanA
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`-
`- ScanB
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 11 0f 19
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`F.G. 13
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 12 0f 19
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`US 6,859,193 B1
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`F.G. 15
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`Vdd
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`OLED
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`TFT2
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`Vold
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`OLED
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`TFT2
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`FIG.16
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 13 of 19
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`US 6,859,193 B1
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`
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`F.G. 17A
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`F.G. 17B
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`FIG.17
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`Vdd
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`OLED
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`TFT2
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`H
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 14 Of 19
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`US 6,859,193 B1
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`Vdd
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`OLED
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`TFT2
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 15 of 19
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`US 6,859,193 B1
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`
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`Wild
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`OLED
`
`TFT2
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`FIG.2OA scanA
`FIG.20B scans
`FIG.20C scans'
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`FG.20
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`ONE FRAME PEREOD
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`L
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`L
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`U L J
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`amamam-amb
`EXTNGUISHING
`PERIOD
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 16 0f 19
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`US 6,859,193 B1
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`FIG.21
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`W
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`Vdd
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`CS-N-1
`A
`SCar
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`B
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`OLED
`
`-- SC3
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`
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`TFT3
`TFT1
`WIL
`100/10
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`TFT2
`WL=
`5/20
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 17 0f 19
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`US 6,859,193 B1
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`FIG.22
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`Vod
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`OLED
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`TFT2
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`FG.23
`CURRENT FLOWING
`THROUGH TFT1
`
`PRESENT
`INVENTION
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`SEGE
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`
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`CONVENTIONAL
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`EXAMPLE up
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`TIME
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 18 of 19
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`US 6,859,193 B1
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`FG.24
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`Vdd
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`OLED
`
`TFT2
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`FG.25
`
`DATA LINE
`POTENTIAL
`
`
`
`PRESENT
`INVENTION
`
`CONVENTIONAL
`EXAMPLE
`
`o
`VBLA - - - - - - - - - - - - - - - F - F
`Vth1 - - - - - - - - - - - - - - - - -1- - - - - - - - - -
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`NITIAL VALUE = 0
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`TIME
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`U.S. Patent
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`Feb. 22, 2005
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`Sheet 19 of 19
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`US 6,859,193 B1
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`FG.26
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`ScanA
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`TFT2
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`OLED
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`Wild
`(NEGATIVE POTENTIAL)
`
`FIG.27
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`US 6,859,193 B1
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`1
`CURRENT DRIVE CIRCUIT AND DISPLAY
`DEVICE USING THE SAME, PIXEL
`CIRCUIT, AND DRIVE METHOD
`
`2
`illustrated example, a source of the TFT2 is set at a reference
`potential (ground potential), an anode of the light emitting
`element OLED is connected to Vdd (power supply
`potential), and a cathode is connected to a drain of the TFT2.
`On the other hand, a gate of the TFT1 is connected to a
`Scanning line SCAN, the Source is connected to a data line
`DATA, and the drain is connected to the holding capacitor C
`and the gate of the TFT2.
`In order to operate the pixel, first, when the Scanning line
`SCAN is brought to a selected State and a data potential Vw
`representing the brightness information is applied to the data
`line DATA, the TFT1 becomes conductive, the holding
`capacitor C is charged or discharged, and the gate potential
`of the TFT2 coincides with the data potential Vw. When the
`Scanning line SCAN is brought to an unselected State, the
`TFT1 becomes OFF and the TFT2 is electrically separated
`from the data line DATA, but the gate potential of the TFT2
`is stably held by the holding capacitor C. The current
`flowing through the light emitting element OLED via the
`TFT2 becomes a value in accordance with a gate/Source
`Voltage Vgs, and the light emitting element OLED continu
`ously emits the light with a brightness in accordance with the
`amount of the current supplied through the TFT2.
`When the current flowing between the drain and source of
`the TFT2 is Ids, this is the drive current flowing through the
`OLED. Assuming that the TFT2 operates in the saturated
`region, Ids is represented by the following equation.
`
`Here, Cox is the gate capacity per unit area and is given
`by the following equation:
`(2)
`Cox=eo'erfd
`In equation (1) and equation (2), Vth indicates a threshold
`value of the TFT2, it indicates a mobility of a carrier, W
`indicates a channel width, L indicates a channel length, eO
`indicates a permittivity of vacuum, er indicates a dielectric
`constant of the gate insulating film, and d is a thickness of
`the gate insulating film.
`According to equation (1), Ids can be controlled by the
`potential Vw written into the pixel. As a result, the bright
`ness of the light emitting element OLED can be controlled.
`Here, the reason for the operation of the TFT2 in the
`Saturated region is as followS. Namely, this is because, in the
`Saturated region, Ids is controlled by only the Vgs and does
`not depend upon the drain/Source Voltage Vds. Therefore,
`even if Vds fluctuates due to variations in the characteristics
`of the OLED, a predetermined amount of the drive current
`Ids can be passed through the OLED.
`AS mentioned above, in the circuit configuration of the
`pixel shown in FIG. 1, when written by Vw once, the OLED
`continues emitting light with a constant brightness during
`one Scanning cycle (one frame) until next rewritten. If large
`number of Such pixels are arranged in a matrix as in FIG. 2,
`an active matrix type display device can be configured. AS
`shown in FIG. 2, in a conventional display device, Scanning
`lines SCAN-1 through SCAN-N for selecting pixels 25 in a
`predetermined Scanning cycle (for example a frame cycle
`according to an NTSC standard) and data lines DATA giving
`brightness information (data potential Vw) for driving the
`pixels 25 are arranged in a matrix. The Scanning lines
`SCAN-1 through SCAN-N are connected to a scanning line
`drive circuit 21, while the data lines DATA are connected to
`a data line drive circuit 22. By repeating the writing of VW
`from the data lines DATA by the data line drive circuit 22
`while Successively Selecting the Scanning lines SCAN-1
`
`TECHNICAL FIELD
`The present invention relates to a current drive circuit for
`driving an organic electroluminescence (EL) element or
`other light emitting element controlled in brightness by a
`current, a display device providing a light emitting element
`driven by this current drive circuit for every pixel, a pixel
`circuit, and a method for driving a light emitting element. In
`more detail, the present invention relates to a current drive
`circuit for controlling an amount of the current Supplied to
`a light emitting element by an insulating gate type field
`effect transistor or other active element provided in each
`pixel and a So-called active matrix type image display device
`using the Same.
`
`15
`
`BACKGROUND ART
`In general, in an active matrix type image display device,
`an image is displayed by arranging a large number of pixels
`in a matrix and controlling a light intensity for every pixel
`in accordance with given brightness information. When
`25
`using a liquid crystal as an electro-optical Substance, the
`transmittance of each pixel varies in accordance with a
`Voltage written into the pixel. In an active matrix type image
`display device using an organic electroluminescence (EL)
`material as the electro-optical Substance as well, the basic
`operation is similar to that of the case where a liquid crystal
`is used. However, unlike a liquid crystal display, an organic
`EL display is a So-called Self-luminescent type having a light
`emitting element for every pixel, So has the advantages of a
`better Visual recognition of the mage in comparison with a
`liquid crystal display, no need for back light, and a fast
`response Speed. The brightnesses of individual light emitting
`elements are controlled by the amount of current. Namely,
`this display is largely different from a liquid crystal display
`in the point that the light emitting elements are current
`driven types or current controlled types.
`In the same way as a liquid crystal display, in an organic
`EL display as well, there are a simple matrix and an active
`matrix drive methods. The former is simple in structure, but
`makes it difficult to realize a large sized, high definition
`display, So the active matrix method is being vigorously
`developed. The active matrix method controls the current
`flowing through the light emitting element provided in each
`pixel by an active element (generally a thin film transistor,
`one type of the insulating gate type field effect transistor,
`hereinafter sometimes referred to as a “TFT") provided
`inside the pixel. An organic EL display of this active matrix
`method is disclosed in for example Japanese Unexamined
`Patent Publication (Kokai) No. 8-234683. One pixel’s worth
`of an equivalent circuit is shown in FIG. 1. The pixel is
`comprised of a light emitting element OLED, a first thin film
`transistor TFT1, a second thin film transistor TFT2, and a
`holding capacitor C. The light emitting element is an organic
`electroluminescence (EL) element. An organic EL element
`has a rectification property in many cases, So is Sometimes
`referred to as an OLED (organic light emitting diode). In the
`figure, the Symbol of a diode is used to indicate the light
`emitting element OLED. However, the light emitting ele
`ment is not always limited to an OLED and may be any
`element controlled in brightness by the amount of the
`current flowing through it. Also, a rectification property is
`not always required in the light emitting element. In the
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`through SCAN-N by the scanning line drive circuit 21, an
`intended image can be displayed. In a simple matrix type
`display device, the light emitting element contained in each
`pixel emits light only at an instant of Selection. In contrast,
`in the active matrix type display device shown in FIG. 2, the
`light emitting element of each pixel 25 continues to emit
`light even after finishing being written. Therefore, in par
`ticular in a large sized, high definition display, there is the
`advantage that the level of the drive current of the light
`emitting elements can be lowered in comparison with the
`Simple matrix type.
`FIG. 3 schematically shows a sectional structure of the
`pixel 25 shown in FIG. 2. Note, only OLED and TFT2 are
`represented for facilitating the illustration. The OLED is
`configured by Successively Superposing a transparent elec
`trode 10, an organic EL layer 11, and a metal electrode 12.
`The transparent electrode 10 is separated for every pixel,
`acts as the anode of the OLED, and is made of a transparent
`conductive film for example ITO. The metal electrode 12 is
`commonly connected among pixels and acts as the cathode
`of the OLED. Namely, the metal electrode 12 is commonly
`connected to a predetermined power Supply potential Vdd.
`The organic EL layer 11 is a composite film obtained by
`Superposing for example a positive hole transport layer and
`an electron transport layer. For example, Diamyne is vapor
`deposited on the transparent electrode 10 acting as the anode
`(positive hole injection electrode) as the positive hole trans
`port layer, Alq3 is vapor deposited thereon as the electron
`transport layer. Further, a metal electrode 12 acting as the
`cathode (electron injection electrode) is grown thereon. Note
`that, Alq3 represents 8-hydroxy quinoline aluminum. The
`OLED having such a laminate structure is only one example.
`When a voltage in a forward direction (about 10V) is applied
`between the anode and the cathode of the OLED having such
`a configuration, injection of carrierS Such as electrons and
`positive holes occurs and luminescence is observed. The
`operation of the OLED can be considered to be the emission
`of light by excisions formed by the positive holes injected
`from the positive hole transport layer and the electrons
`injected from the electron transport layer.
`On the other hand, the TFT2 comprises a gate electrode 2
`formed on a Substrate 1 made of glass or the like, a gate
`insulating film 3 Superimposed on the top Surface thereof,
`and a Semiconductor thin film 4 Superimposed above the
`gate electrode 2 via this gate insulating film 3. This Semi
`conductor thin film 4 is made of for example a polycrystal
`line silicon thin film. The TFT2 is provided with a source S,
`a channel Ch, and a drain D acting as a passage of the current
`supplied to the OLED. The channel Ch is located immedi
`ately directly above the gate electrode 2. The TFT2 of this
`bottom gate Structure is coated by an inter-layer insulating
`film 5. A Source electrode 6 and a drain electrode 7 are
`formed above that. Above them, the OLED mentioned above
`is grown via another inter-layer insulating film 9. Note that,
`in the example of FIG. 3, the anode of the OLED is
`connected to the drain of the TFT2, So a P-channel thin film
`transistor is used as the TFT2.
`In an active matrix type organic EL display, generally a
`TFT (thin film transistor) formed on a glass substrate is
`utilized as the active element. This is for the following
`reason. Namely, an organic EL display is a direct viewing
`type. Due to this, it becomes relatively large in size. Due to
`restrictions of cost and manufacturing facilities, a usage of
`a single crystalline Silicon Substrate for the formation of the
`active elements is not practical. Further, in order to extract
`the light from the light emitting elements, usually a trans
`parent conductive film of ITO (indium tin oxide) is used as
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`4
`the anode of the organic EL layer, but ITO is frequently
`generally grown under a high temperature which an organic
`EL layer cannot endure. In this case, it is necessary to form
`the ITO before the formation of the organic EL layer.
`Accordingly, the manufacture process roughly becomes as
`follows:
`Referring to FIG. 3 again, first the gate electrode 2, gate
`insulating film 3, and Semiconductor thin film 4 comprised
`of amorphous Silicon are Successively Stacked and patterned
`on the glass substrate 1 to form the TFT2. In certain cases,
`the amorphous Silicon is Sometimes formed into polysilicon
`(polycrystalline Silicon) by heat treatment Such as laser
`annealing. In this case, generally a TFT2 having a larger
`degree of carrier mobility in comparison with amorphous
`Silicon and a larger current driving capability can be formed.
`Next, an ITO transparent electrode 10 acting as the anode of
`the light emitting element OLED is formed. Subsequently,
`an organic EL layer 11 is Stacked to form the light emitting
`element OLED. Finally, the metal electrode 12 acting as the
`cathode of the light emitting element is formed by a metal
`material (for example aluminum).
`In this case, the extraction of the light is started from a
`back Side (bottom Surface Side) of the Substrate 1, So a
`transparent material (usually a glass) must be used for the
`Substrate 1. In view of this, in an active matrix type organic
`EL display, a relatively large sized glass Substrate 1 is used.
`As the active element, ordinarily use is made of a TFT as it
`can be relatively easily formed thereon. Recently, attempts
`have also been made to extract the light from a front Side
`(top Surface side) of the Substrate 1. The Sectional structure
`in this case is shown in FIG. 4. The difference of this from
`FIG.3 resides in that the light emitting element OLED is
`comprised by Successively Superposing a metal electrode
`12a, an organic EL layer 11, and a transparent electrode 10a
`and an N-channel transistor is used as the TFT2.
`In this case, the Substrate 1 does not have to be transparent
`like glass, but as the transistor formed on a large sized
`substrate, use is generally still made of a TFT. However, the
`amorphous Silicon and polysilicon used for the formation of
`the TFT have a worse crystallinity in comparison with single
`crystalline Silicon and have a poor controllability of the
`conduction mechanism, therefore it has been known that
`there is a large variation in characteristics in formed TFTs.
`Particularly, when a polysilicon TFT is formed on a rela
`tively large sized glass Substrate, usually the laser annealing
`method is used as mentioned-above in order to avoid the
`problem of thermal deformation of the glass substrate, but it
`is difficult to uniformly irradiate laser energy to a large glass
`Substrate. Occurrence of variations in the State of the crys
`tallization of the polysilicon according to the location in the
`Substrate cannot be avoided.
`As a result, it is not rare for the Vth (threshold value) to
`vary according to pixel by Several hundreds of mV, in certain
`cases, 1V or more, even in the TFTs formed on an identical
`Substrate. In this case, even if a same signal potential Vw is
`written with respect to for example different pixels, the Vth
`will vary according to the pixels. As a result, according to
`equation (1) described above, the current Ids flowing
`through the OLEDs will largely vary for every pixel and
`consequently become completely off from the intended
`value, So a high quality of image cannot be expected as the
`display. A similar thing can be said for not only the Vth, but
`also the variation of parameters of equation (1) Such as the
`carrier mobility A. Further, a certain degree of fluctuation in
`the above parameters is unavoidable not only due to the
`variation among pixels as mentioned above, but also varia
`tions for every manufacturing lot or every product. In Such
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`a case, it is necessary to determine how the data line
`potential Vw should be set with respect to the intended
`current Ids to be passed through the OLEDs for every
`product in accordance with the final State of the parameters
`of equation (1). Not only is this impractical in the mass
`production process of displayS, but it is also extremely
`difficult to devise countermeasures for fluctuations in char
`acteristics of the TFTs due to the ambient temperature and
`changes of the TFT characteristics occurring due to usage
`over a long period of time.
`DISCLOSURE OF THE INVENTION
`An object of the present invention is to provide a current
`drive circuit capable of Stably and accurately Supplying an
`Intended current to a light emitting element etc. of a pixel
`without being affected by variations in characteristics of an
`active element inside the pixel, a display device using the
`Same and as a result capable of displaying a high quality
`image, a pixel circuit, and a method for driving a light
`emitting element.
`In order to achieve the object, the following means were
`devised. Namely, a display device according to the present
`invention provides a Scanning line drive circuit for Succes
`Sively Selecting Scanning lines, a data line drive circuit
`including a current Source for generating a signal current
`having a current level in accordance with brightness infor
`mation and Successively Supplying the same to data lines,
`and a plurality of pixels arranged at interSecting portions of
`the Scanning lines and the data lines and including current
`driven type light emitting elements emitting light by receiv
`ing the Supply of the drive current. The characterizing
`feature is that each pixel comprises a receiving part for
`fetching the Signal current from the data line when the
`Scanning line is Selected, a converting part for converting a
`current level of the fetched signal current to a Voltage level
`and holding the same, and a drive part for passing a drive
`current having a current level in accordance with the held
`Voltage level through the light emitting element.
`Specifically, the converting part includes a conversion use
`insulating gate type field effect transistor provided with a
`gate, a Source, a drain, and a channel and a capacitor
`connected to the gate. The conversion use insulating gate
`type field effect transistor generates a converted Voltage
`level at the gate by passing the Signal current fetched by the
`receiving part through the channel. The capacitor holds the
`Voltage level created at the gate. Further, the converting part
`includes a Switch use insulating gate type field effect tran
`Sistor inserted between the drain and the gate of the con
`version use insulating gate type field effect transistor. The
`Switch use insulating gate type field effect transistor
`becomes conductive when converting the current level of the
`Signal current to the Voltage level and electrically connects
`the drain and the gate of the conversion use insulating gate
`type field effect transistor to create the voltage level with the
`Source as the reference at the gate, while the Switch use
`insulating gate type field effect transistor is shut off when the
`capacitor holds the Voltage level and Separates the gate of the
`conversion use insulating gate type field effect transistor and
`the capacitor connected to this from the drain.
`In one embodiment, the drive part includes a drive use
`insulating gate type field effect transistor provided with a
`gate, a drain, a Source, and a channel. This drive use
`insulating gate type field effect transistor receives the Volt
`age level held at the capacitor at its gate and passes a drive
`current having a current level in accordance with that
`through the light emitting element via the channel. A current
`mirror circuit is configured by direct connection of the gate
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`of the conversion use insulating gate type field effect tran
`Sistor and the gate of the drive use insulating gate type field
`effect transistor, whereby a proportional relationship is
`exhibited between the current level of the signal current and
`the current level of the drive current. The drive use insulat
`ing gate type field effect transistor is formed in id the vicinity
`of the corresponding conversion use insulating gate type
`field effect transistor inside the pixel and has an equivalent
`threshold Voltage to that of the conversion use insulating
`gate type field effect transistor. The drive use insulating gate
`type field effect transistor operates in the Saturated region
`and passes a drive current in accordance with a difference
`between the level of the Voltage applied to the gate thereof
`and the threshold Voltage through the light emitting element.
`In another embodiment, the drive part shares the conver
`Sion use insulating gate type field effect transistor together
`with the converting part in a time division manner. The drive
`part Separates the conversion use insulating gate type field
`effect transistor from the receiving part and uses the same for
`driving after the conversion of the Signal current is com
`pleted and passes the drive current to the light emitting
`element through the channel in a State where the held voltage
`level is applied to the gate of the conversion use insulating
`gate type field effect transistor. The drive part has a con
`trolling means for cutting off an unnecessary current flowing
`to the light emitting element via the conversion use insu
`lating gate type field effect transistor at times other than the
`time of drive. The controlling means cuts off the unneces
`Sary current by controlling a Voltage between terminals of a
`two terminal type light emitting element having a rectifica
`tion function. Alternatively, the controlling means comprises
`a control use insulating gate type field effect transistor
`inserted between the conversion use insulating gate type
`field effect transistor and the light emitting element, and the
`control use insulating gate type field effect transistor
`becomes nonconductive in State and Separates the conver
`Sion use insulating gate type field effect transistor and the
`light emitting element when the light emitting element is not
`driven and Switches to the conductive State when the light
`emitting element is driven. In addition, the controlling
`means controls a ratio between a time for cutting off the
`drive current when the light emitting element is not to be
`driven and placing the light emitting element in the non-light
`emitting State and a time of passing the drive current when
`the light emitting element is to be driven and placing the
`light emitting element in the light emitting and thereby to
`enable the control of the brightness of the pixel. According
`to a certain case, the drive part has a potential fixing means
`for fixing the potential of the drain with reference to the
`Source of the conversion use insulating gate type field effect
`transistor in order to stabilize the current level of the drive
`current flowing to the light emitting element through the
`conversion use insulating gate type field effect transistor.
`In a further developed form of the present invention, the
`receiving part, the converting part, and the drive part con
`figure a current circuit combining a plurality of insulating
`gate type field effect transistors, and one or two or more
`insulating gate type field effect transistors have a double gate
`Structure for Suppressing current leakage in the current
`circuit. Further, the drive part includes the insulating gate
`type field effect transistor provided with the gate, drain, and
`the Source and passes the drive current passing between the
`drain and the Source to the light emitting element in accor
`dance with the level of the Voltage applied to the gate, the
`light emitting element is a two terminal type having an
`anode and a cathode, and the cathode is connected to the
`drain. Alternatively, the drive part includes an insulating
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`of a Voltage value, in contrast, the remarkable characterizing
`feature of the display device of the present invention is that
`the brightness information is given in the form of a current
`value, that is, of a current written type.
`AS already mentioned, an object of the present invention
`is to accurately pass the intended current through the OLEDS
`without being affected by variations in the characteristics of
`the TFTs. The reason why the present object can be achieved
`by the first through fourth characterizing features will be
`explained below. Note that hereinafter the conversion use
`insulating gate type field effect transistor will be described
`as the TFT1, the drive use insulating gate type field effect
`transistor will be described as the TFT2, the fetch use
`insulating gate type field effect transistor will be described
`as the TFT3, and the Switch use insulating gate type field
`effect transistor will be described as the TFT4. Note that the
`present invention is not limited to TFTs (thin film
`transistors). Insulating gate type field effect transistors can
`be widely employed as the active elements, for example,
`Single crystalline Silicon transistors formed on a Single
`crystalline silicon Substrate or SOI substrate. The signal
`current passing through the TFT1 at the time of writing of
`the brightness information is defined as Iw, and the Voltage
`between the gate and source created in the TFT1 as a result
`of this is defined as Vgs. At the time of writing, due to the
`TFT4, the gate and the drain of the TFT1 are short-circuited,
`So the TFT1 operates in the Saturated region. Accordingly,
`Iw is given by the following equation.
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`Here, the meanings of the parameters are similar to the
`case of equation (1). Next, when defining the current flowing
`through an OLED as IdrV, IdrV is controlled in its current
`level by the TFT2 connected to the OLED in series. In the
`present invention, the Voltage between the gate and Source
`thereof coincides with Vgs in equation (3). Therefore, when
`assuming that the TFT2 operates in the Saturated region, the
`following equation Stands:
`(4)
`Idry-u2. Cox 2: W2/L2/2(Vgs-Vih2)?
`The meanings of the parameters are Similar to the case of
`equation (1). Note that, the condition for the operation of the
`insulating gate type field effect transistor in the Saturated
`region is generally given by the following equation while
`defining Vds as the Voltage between the drain and Source.
`Vdse Vgs-Vth
`(5)
`Here, TFT1 and TFT2 are formed close inside a small
`pixel, So it can be considered that de facto u1 = u2, Cox1 =
`Cox2, and Vth 1=Vth2. Then, at this time, the following
`equation is easily derived from equation (3) and equation
`(4):
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`gate type field effect transistor provided with a gate, a drain,
`and a Source and passes a drive current passing between the
`drain and the Source to the light emitting element in accor
`dance with the level of the Voltage applied to the gate, the
`light emitting element is a two terminal type having an
`anode and a cathode, and the anode is connected to the
`Source. Further, it includes an adjusting means for down
`Wardly adjusting the Voltage level held by the converting
`part and Supply