United States Patent (19)
`Yamada et al.
`
`US006072450A
`Patent Number:
`11
`(45) Date of Patent:
`
`6,072,450
`Jun. 6, 2000
`
`54) DISPLAY APPARATUS
`
`75 Inventors: Hiroyasu Yamada, Hachioji; Tomoyuki
`Shirasaki, Higashiyamato; Yoshihiro
`Kawamura, Fussa, all of Japan
`
`73 Assignee: Casio Computer Co., Ltd., Tokyo,
`Japan
`
`5,302,966 4/1994 Stewart ................................ 315/169.3
`5,427,858 6/1995 Nakamura et al. ..................... 428/421
`5,640,067 6/1997 Yamauchi et al. ...................... 313/504
`5,684,365 11/1997 Tang et al. ................................ 345/76
`5,828,181 10/1998 Okuda ....................................... 345/76
`5,847,516 12/1998 Kishita et al.
`... 315/169.3
`5,909,081
`6/1999 Eida et al. .............................. 313/504
`
`21 Appl. No.: 08/976,217
`Primary Examiner Bipin Shallwala
`1-1.
`ASSistant Examiner Vincent E. Kovalick
`Nov. 21, 1997
`22 Filed:
`Attorney, Agent, or Firm-Frishauf, Holtz, Goodman,
`30
`Foreign Application Priority Data
`Langer & Chick, P.C.
`Nov. 28, 1996
`JP
`Japan .................................... 8-331388
`57
`ABSTRACT
`Nov. 28, 1996
`JP
`Japan .................................... 8-33.1389
`(51) Int. Cl." ..................................................... H01L 27/12 R tion "STER, td
`52 U.S. Cl. ................................. 345/76; 345/36; 345/45;
`hich i. isible light
`f
`d b
`the Selecti
`313/500; 313/504; 315/169.3 AdR St. Anio" Ove E.
`58 Field of Search .................................. 345/36, 45, 76;
`ith flat insulation films b
`he cathod F.
`313/498–512; 315/169.3
`areas, With flat insulation films between the cathode elec
`s
`trodes and the Selection and drive transistors. An organic EL
`9.
`layer and an anode electrode are Sequentially formed on the
`cathode electrodes.
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,302.468 4/1994 Nimike et al. .......................... 313/500
`
`18 Claims, 21 Drawing Sheets
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 1 of 21
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`FIG.1
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 2 of 21
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`6,072,450
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 3 of 21
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`FIG.3
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 4 of 21
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 5 of 21
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 6 of 21
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`U.S. Patent
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`Jun. 6, 2000
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`6,072,450
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`Jun. 6, 2000
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`U.S. Patent
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`Jun. 6, 2000
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`U.S. Patent
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`Jun. 6, 2000
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`Jun. 6, 2000
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`Jun. 6, 2000
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 14 of 21
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`Jun. 6, 2000
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 16 of 21
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`6,072,450
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 17 of 21
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`6,072,450
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 18 of 21
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`6,072,450
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 19 of 21
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`6,072,450
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 20 of 21
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`U.S. Patent
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`Jun. 6, 2000
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`Sheet 21 of 21
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`6,072,450
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`1
`DISPLAY APPARATUS
`
`6,072,450
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`2
`formed in that part of the passivation film 104 which is
`located on the transparent anode electrode 103. An organic
`EL layer 106, which absorbs the energy generated due to the
`recombination of electrons and holes when a current flows,
`is deposited in the contact hole 105 extending up to the
`transparent anode electrode 103. A cathode electrode 107,
`which reflects visible light and which extends over a plu
`rality of pixels, is laminated on the passivation film 104 and
`the organic EL layer 106. In this EL display apparatus, the
`efficiency of the injection of carriers into the organic EL
`layer 106 depends on the ionization potential of the anode
`electrode 103 and the electron affinity (the work function) of
`the cathode electrode 107. In order to improve the light
`emitting efficiency of the organic EL layer 106 which
`depends on the carrier injection efficiency, the cathode
`electrode 107 is formed using a material whose work
`function is low. Since the cathode electrode 107 is normally
`formed of a metal Such as magnesium whose work function
`is low, the cathode electrode 107 reflects light having a
`wavelength in a range of wavelength of light which the
`organic EL layer 106 emits. Due to this, in the above EL
`display apparatus, the light emitted by the organic EL layer
`106 travels through the anode electrode 103 and the Sub
`strate 101. The organic EL layer 106 is arranged so as not to
`overlap the thin film transistors T1 and T2. The purpose of
`thus arranging the organic EL layer 106 is to prevent the
`light emitted by the organic EL layer 106 from entering the
`thin film transistors T1 and T2. If the emitted light entered
`the thin film transistors T1 and T2, unnecessary photoelec
`tromotive force would be generated in the channel regions of
`the thin film transistors T1 and T2, which entails the
`possibility of the thin film transistors T1 and T2 malfunc
`tioning.
`In the active matrix type EL display apparatus described
`above, the light emitting area of each pixel in which a part
`of the organic EL layer 106 is located is limited to an area
`in which the thin film transistors T1 and T2 are not located,
`and therefore the ratio of the light emitting area to the pixel
`area is Small. If the light emitting area is enlarged and if a
`Voltage applied to the organic EL layer 106 is intensified to
`attain the desired luminance, the organic EL layer 106 will
`be considerably deteriorated. The cathode electrode 107 is
`made of a metal, while the organic EL layer 106 is made of
`an organic material. Hence, it is difficult to join the cathode
`electrode 107 and the organic EL layer 106 together in a
`preferred condition. AS time passes, a gap can easily occur
`between the cathode electrode 107 and the organic EL layer
`106, which entails the possibility that the organic EL layer
`106 may become emit no light. The organic EL layer can
`emit light at the same luminance as that of an inorganic EL
`layer even when the organic EL layer is formed as thin as 40
`nm to 250 nm. The thicker the organic EL layer 106, the
`higher an effective Voltage/current for causing the organic
`EL layer to emit light at the desired luminance. This limits
`the range of value at which the thickness of the organic EL
`layer can be Set Meanwhile, the thickness of the passivation
`film 104, which covers the thin film transistors T1 and T2,
`is Set at Such a value as to prevent the occurrence of a
`parasitic capacitance in the thin film transistors T1 and T2.
`Owing to a difference in thickness between the passivation
`film 104 and the organic EL layer 106, a step is present on
`the upper Surfaces of the passivation film 104 and organic
`EL layer 106. There is the possibility that the cathode
`electrode 107 may break at that step. If the cathode electrode
`107 breaks, the display apparatus cannot perform a display
`operation.
`
`65
`
`SUMMARY OF THE INVENTION
`It is accordingly one object of the present invention to
`provide a display apparatus which has a light emitting area
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a display apparatus, and
`more particularly to an electroluminescent (hereinafter
`referred to as EL) display apparatus with a matrix display
`panel including EL elements.
`2. Description of the Related Art
`An EL display apparatus with organic EL elements, that
`is, display elements which emit light Spontaneously and
`which are arranged in a matrix pattern, is known conven
`tionally. A passive matrix type EL display apparatus is
`available as Such an EL display apparatus. In this type of EL
`15
`display apparatus, parallel cathode lines Serve as common
`lines, while parallel anode lines which are perpendicular to
`the cathode lines and which are made of ITO (indium tin
`oxide) serve as data lines. An organic EL layer is arranged
`between the set of the cathode lines and the set of the anode
`lines. A positive Voltage is applied to the data lines in each
`of cathode Selection periods, thereby driving organic EL
`elements located at the interSections of the common lines
`and the data lines. The display apparatus displays an image
`which corresponds to the Voltage applied to the data lines. In
`25
`the case of the passive matrix type EL display apparatus
`which displays an image by driving Such organic EL
`elements, the larger the number of common lines and/or the
`number of data lines, the shorter the Selection period (duty
`H) per pixel. The period of time over which the organic EL
`layer keeps emitting light even after the application of a
`voltage between the set of the cathode lines and the set of the
`anode lines is short. In consideration of this, according to the
`conventional passive matrix type EL display apparatus, the
`instantaneous luminance of the organic EL layer of each
`pixel during the Selection periods is intensified So that the
`organic EL layer apparently emits light over 1 frame period.
`The organic EL layer can emit light at a high instantaneous
`luminance by applying a high Voltage to the organic EL
`layer. In this case, however, the organic EL layer can easily
`40
`deteriorate.
`In the passive matrix type EL display apparatus, the larger
`the number of common lines and data lines, the more
`possibility of the occurrence of crosstalk. This makes it
`difficult to enable the passive matrix type EL display appa
`ratus to display a highly precise image.
`Proposed as a display apparatus free from the above
`described problems is an active matrix type display appa
`ratus which includes, as shown in FIG. 22, pairs of thin film
`transistors which confer a Voltage Storing capability on the
`pixels. Each of the pairs of thin film transistors consists of
`a selection transistor T1 and a drive transistor T2. The
`Selection transistor T1 is connected to a data line DL for
`Supplying a data Signal and a gate line GL for Supplying a
`gate Signal. The gate electrode of the drive transistor T2 is
`connected to the selection transistor T1. The Source of the
`drive transistor T2 is connected to a constant Voltage line
`VL. In this display apparatus, as shown in FIG. 23, the thin
`film transistors T1 and T2 are formed in a pixel area on a
`glass Substrate 101, and the gates of the thin film transistors
`are covered with a gate insulation film 102. In an area
`adjacent to the thin film transistors T1 and T2, a transparent
`anode electrode 103 is provided on the gate insulation film
`102. The transparent anode electrode 103 is connected to the
`drain of the drive transistor T2. A passivation film 104
`covers the thin film transistors T1 and T2. A contact hole
`extending up to the transparent anode electrode 103 is
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`enlarged So as to emit light at a Satisfactorily high lumines
`cence even though a Voltage applied to an EL layer is low,
`and which has a long luminance life.
`It is another object of the present invention to provide a
`display apparatus which prevents light from entering active
`elements Such as transistors, to thereby avoid the malfunc
`tion of the active elements.
`In order to achieve the above objects, a display apparatus
`according to one aspect of the present invention comprises:
`a Substrate;
`active elements which are formed over the Substrate and
`which are driven by an externally Supplied Signal;
`an insulation film formed over the Substrate So as to cover
`the active elements and having at least one contact
`hole;
`at least one first electrode formed on the at least one
`insulation film So as to cover the active elements, and
`connected to the active elements through the at least
`one contact hole, the at least one first electrode being
`made of a material which shields visible light;
`an electroluminescent layer formed on the at least one first
`electrode and including at least one layer which emits
`light in accordance with a Voltage applied to the at least
`one layer, and
`at least one Second electrode formed on the electrolumi
`neScent layer.
`In this display apparatus, the at least one first electrode is
`formed So as to cover the active elements, and the electrolu
`minescent layer and the at least one Second electrode are
`laminated Sequentially on the at least one first electrode.
`Under those conditions, the area occupied by an electrolu
`minescent element, which is formed of the at least one first
`electrode, the electroluminescent layer and the at least one
`Second electrode, is not limited by the active elements, and
`a light emitting area can be enlarged accordingly. This
`enables the electroluminescent layer to emit light at the same
`luminescence as that of a conventional display apparatus,
`even though a Voltage applied to the electroluminescent
`layer is low. In this case, the load on the electroluminescent
`layer is Small, which ensures a long life to the display
`apparatus. Since the at least one first electrode is made of a
`material which shields visible light, the light emitted by the
`electroluminescent layer does not enter the active elements,
`and therefore the active elements do not malfunction due to
`the light.
`In order to achieve the above-described objects, a display
`apparatus according to the Second aspect of the present
`invention comprises:
`a Substrate;
`Selection transistors formed over the Substrate and
`arranged in a matrix pattern;
`drive transistors formed over the Substrate and arranged in
`a matrix pattern, each of the drive transistors being
`connected to one of the Selection transistors,
`address lines connected to the Selection transistors and
`through which a Signal for turning on the Selection
`transistorS is Supplied;
`data lines connected to the Selection transistors, a signal
`which corresponds to image data being Supplied to the
`drive transistors through the data lines and the Selection
`transistors while the Selection transistorS is on;
`an insulation film formed over the Substrate So as to cover
`the drive transistors, the address lines and the data
`lines, the insulation film having contact holes formed in
`correspondence with the drive transistors,
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`15
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`6,072,450
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`4
`first electrodes made of a material which shields visible
`light, and formed on the insulation film So as to cover
`the election transistors and the drive transistors, the first
`electrodes being arranged in a matrix pattern in areas
`Surrounded by the address lines and the data lines, and
`being connected to the drive transistors through the
`contact holes,
`an electroluminescent layer formed on the first electrodes
`and including at least one layer which emits light in
`accordance with an applied Voltage;
`a Second electrode formed on the electroluminescent
`layer, a first driver circuit for Selectively Supplying the
`address Signal to the address lines in Sequence; and
`a Second driver circuit for Supplying the image data to the
`data lines.
`In this display apparatus, the first electrodes are arranged
`in the areas Surrounded by the address lines and the data
`lines. Under this condition, electroluminescent elements,
`each being formed of one of the first electrodes and the
`electroluminescent layer and the Second electrode, do not
`overlap the address lines or the data lines. Consequently, no
`parasitic capacitance occurs between the address/data lines
`and the electrodes of the electroluminescent elements, thus
`preventing Signal transmission from being slowed down.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a plan view of an display apparatus according to
`one embodiment of the present invention;
`FIG. 2 is a croSS Section taken along the line 2-2 show
`in FIG. 1;
`FIG. 3 is an equivalent circuit diagram showing an EL
`display circuit corresponding to one pixel,
`FIG. 4 is an equivalent circuit diagram which specifically
`shows the Structure of the EL display circuit;
`FIG. 5 is a graph showing the electric characteristic of a
`drive transistor Q2;
`FIG. 6 is a graph showing the luminance of an organic EL
`element;
`FIG. 7 is a diagram illustrating driver circuits used in the
`display apparatus depicted in FIG. 1;
`FIG. 8 is a diagram showing waveforms for driving the
`display apparatus,
`FIG. 9 is a sectional view of a display apparatus which
`includes cathode electrodes, an organic EL layer, and a
`dielectric film between the organic EL layer and the cathode
`electrodes,
`FIG. 10 is a Sectional view of a display apparatus having
`R, G and B wavelength range conversion layers,
`FIG. 11 is a Sectional view of a display apparatus having
`R and G wavelength range conversion layers,
`FIG. 12 is a Sectional view of a display apparatus having
`R, G and B wavelength range conversion layerS and color
`filter layers;
`FIG. 13 is a Sectional view of a display apparatus having
`color filter layers,
`FIG. 14 is a Sectional view of a display apparatus having
`R and G wavelength range conversion layerS and color filter
`layers,
`FIG. 15 is a Sectional view of a display apparatus having
`concave R, G and B wavelength range conversion layerS and
`color filters;
`FIG. 16 is a diagram showing driver circuits used in a
`display apparatus which includes drive transistorS Q2 and
`anode electrodes connected to the drive transistorS Q2;
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`FIG. 17 is a plan view of a display apparatus according
`another embodiment of the present invention;
`FIG. 18 is a cross section taken along the line 18-18
`shown in FIG. 17;
`FIG. 19 is a cross section taken along the line 19-19
`shown in FIG. 17;
`FIG. 20 is a diagram showing driver circuits used in the
`display apparatus illustrated in FIG. 17;
`FIG. 21 is a Sectional view of a display apparatus includ
`ing a cathode electrode having a rough Surface;
`FIG.22 is a plan view of a display apparatus according to
`the related art; and
`FIG. 23 is a sectional view of the display apparatus
`illustrated in FIG. 21.
`
`15
`
`6
`located on the drain side of the semiconductor layer 7B,
`while the ohmic layer 9D is located on the source side of the
`semiconductor layer 7B. In the selection transistor Q1, a
`data line 10A is laminated on and connected to the ohmic
`layer 9A located on the drain Side, and a Source electrode
`10B is laminated on and connected to the ohmic layer 9B
`located on the Source side. The Source electrode 10B is
`connected to a contact hole 11 formed in the gate insulation
`film 6 of the drive transistor O2. In the drive transistor O2,
`a constant Voltage line 12 which is Set at a ground potential
`is laminated on and connected to the ohmic layer 9C located
`on the drain Side, and a Source electrode 13 having two ends
`is laminated on the ohmic layer 9D located on the source
`side. One end of the Source electrode 13 is connected to the
`ohmic layer 9D, while the other end of the source electrode
`13 is connected to one of cathode electrodes 15 of organic
`EL elements 3. The gate electrode 4B, the constant voltage
`line 12 and the gate insulation film 6 therebetween form a
`capacitor Cp.
`The structures of the organic EL elements 3 will now be
`described. In the entire display area of the display apparatus,
`a flat interlayer insulation film 14 is deposited to a thickness
`of about 400 nm to 1200 nm on selection transistors O1,
`drive transistorS Q2 and the gate insulation film 6. Contact
`holes 14A are formed in those parts of the interlayer insu
`lation film 14 which are located at end portions of the source
`electrodes 13 of the drive transistors O2. Each of the contact
`holes 14A and a corresponding one of the aforementioned
`end portions of the Source electrodes 13 are located almost
`in the center of one pixel area. The cathode electrodes 15
`which are made of Mgn or the like are patterned on the
`interlayer insulation film 14. Each cathode electrode 15 has
`an area and a shape (almost Square in this embodiment)
`which are enough to cover the most part of one pixel area
`Surrounded by the adjacent data lines 10A and the adjacent
`address lines 4. The selection transistors Q1 and the drive
`transistors O2 are formed under the cathode electrodes 15.
`In the entire display area, an organic EL layer 16 is formed
`on the cathode electrodes 15 and the interlayer insulation
`film 14, and a transparent anode electrode 17 which is made
`of ITO (indium tin oxide) or IZnO (indium zinc oxide) is
`formed on the organic EL layer 16. A driving power Source
`(not shown) is connected to the anode electrode 17.
`The organic EL layer 16 includes an electron carrying
`layer, a luminous layer and a hole carrying layer. Of those
`layers included in the organic EL layer 16, the electron
`carrying layer is closest to the cathode electrodes 15, and the
`hole carrying layer is farthest from the cathode electrodes
`15. The electron carrying layer is made of aluminum-tris
`(8-hydroxyquinolinate) (hereinafter referred to as Alq3).
`The luminous layer is made of 96 wt % 4,4'-bis(2,2-
`diphenylvinylene)biphenyl (hereinafter referred to as
`DPVBi) and 4 wt % 4,4'-bis(2-carbazole)vinylene)biphenyl
`(hereinafter referred to as BCzVBi). The hole carrying layer
`is made of N,N'-di(C.-naphthyl)-N,N'-diphenyl-1,1'-
`biphenyl-4,4'-diamine (hereinafter referred to as C-NPD).
`The thickness of the organic EL layer 16 is on the order of
`40 nm to 250 nm.
`The constitutional formulas of Alq3, DPVBi, BCzVBi
`and O-NPD are shown below:
`
`25
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`Display apparatuses according to embodiments of the
`present invention will now be described in detail, with
`reference to the accompanying drawings.
`The Structure of the display apparatus according to one
`embodiment of the present invention will now be described
`with reference to FIGS. 1 and 2. FIG. 1 is a plan view of that
`part of the display apparatus of this embodiment which
`corresponds to one pixel. FIG. 2 is a croSS Section taken
`along the line 2-2 shown in FIG. 1. In those drawings, a
`reference numeral 1 denotes the display apparatus. That part
`of the display apparatus 1 which is illustrated in FIGS. 1 and
`2 includes a Substrate 2, an n-channel transistor Q1, an
`n-channel transistor Q2, an organic EL element 3, etc. which
`are formed over the Substrate 2. The Substrate 2 is made of
`glass or Synthetic resin, and make visible light pass through.
`The n-channel transistor Q1 Serves as a Selection transistor,
`while the n-channel transistor Q2 Serves as a drive transistor.
`The structure of the display apparatus 1 will be more
`Specifically described. Parallel address lines 4 extending in
`a predetermined direction are formed at equal intervals on
`the Substrate 2 by patterning a gate metal film which is made
`of aluminum (Al) or the like. The selection transistor Q1 has
`a gate electrode 4A formed in integration with one address
`line 4. The drive transistor Q2 has a gate electrode 4B.
`Anodic oxidation films 5 are formed on the gate electrodes
`4A and 4B and the address lines 4. A gate insulation film 6
`which is made of silicon nitride is formed so as to cover the
`address lines 4, the gate electrodes 4A and 4B and the
`substrate 2. Semiconductor layers 7A and 7B, which are
`made of amorphous Silicon (a-Si) or polycrystalline Silicon
`(p-Si), are patterned on the gate insulation film 6 covering
`the gate electrodes 4A and 4.B. Blocking layers 8A and 8B
`are formed on the middle portions of the Semiconductor
`layers 7A and 7B, respectively, and extend in a channel
`widthwise direction. Ohmic layers 9A and 9B are formed on
`the Semiconductor layer 7A, and are isolated from each other
`at the blocking layer 8A. The ohmic layer 9A is located on
`that side (the drain side) of the semiconductor layer 7A
`which is dose to a drain, while the ohmic layer 9B is located
`on that side (the source side) of the semiconductor layer 7A
`which is close to a source. Ohmic layers 9C and 9D are
`60
`formed on the semiconductor layer 7B, and are isolated from
`each other at the blocking layer 8B. The ohmic layer 9C is
`
`45
`
`50
`
`55
`
`35
`
`40
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`8
`
`Formula 1
`
`Formula 2
`
`Formula 3
`
`CHCH
`
`Formula 4
`
`The organic EL layer 16 thus formed emits blue light upon
`the application of a predetermined Voltage.
`When the organic EL layer 16 between the anode elec
`trode 17 and the cathode electrodes 15 includes a luminous
`layer which can carry electrons and which is made of
`berylliumbis(10-hydroxybenzoh) quinolinate) (hereinafter
`referred to as Bebd2), and a hole carrying layer made of
`O-NPD, the organic EL layer 16 can emit green light.
`The constitutional formula of Bebd2 is shown below:
`
`45
`
`50
`
`|Formula 5
`
`ss
`
`21
`
`N
`“, -O
`t Be
`o1 '.
`A.
`
`21
`
`In the display apparatus 1 of this embodiment, each
`cathode electrode 15 covers one pixel area Surrounded by
`
`60
`
`65
`
`the adjacent data lines 10A and the adjacent address lines 4,
`and therefore each EL element 3 emits light over the entirety
`of one pixel area. This remarkably improves the aperture
`ratio per pixel in the display apparatus 1 of this embodiment
`over that of the conventional active matrix type EL display
`apparatus. The cathode electrodes 15 are formed of Mgn
`which reflects light. Therefore, the light emitted by the
`organic EL layer 16 when a Voltage is applied between the
`anode electrode 17 and the cathode electrodes 15 comes out
`through the anode electrode 17 without leaking downward
`(toward the substrate 2). Thus, the light does not enter the
`Selection transistorS Q1 and the drive transistorS Q2, and
`hence the malfunction of the transistors O1 and O2 due to
`the photoelectromotive force is avoided. The light emitted
`by the organic EL layer 16 goes out of the display apparatus
`through the transparent anode electrode 17, without the light
`being absorbed by the Substrate 2, etc., and therefore bright
`display is realized.
`The area of the interface between the organic EL layer 16
`and the cathode electrodes 15 is large. This permits the
`cathode electrodes 15 and the organic EL layer 16 to be
`joined together in a preferred condition, and ensures to the
`display apparatus 1 of this embodiment a luminous life
`improved over that of the conventional active matrix type
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`9
`EL display apparatus. The cathode electrodes 15 are formed
`on a flat layer having no steps, and therefore are free from
`the possibility of the cathode electrodes 15 breaking at steps.
`The cathode electrodes 15 are arranged So as not to overlap
`the address lines 4 or the data lines 10A. Consequently, the
`Slowing down of Signal transmission, caused by a parasitic
`capacitance which would occur if the cathode electrodes 15
`were arranged So as to overlap the address lines 4 or the data
`lines 10A, is prevented.
`When a layer like the organic EL layer 16 is subjected to
`a temperature higher than a glass-transition temperature for
`an organic EL material, its light emitting characteristic
`deteriorates considerably. In consideration of this, according
`to the display apparatus of this embodiment, the organic EL
`layer 16 is formed after the selection transistors Q1 and the
`15
`drive transistorS Q2 are manufactured by a heat treatment
`under a temperature of Several-hundred degrees. The organic
`EL layer 16 is not Subjected to a temperature higher than the
`glass-transition temperature, and therefore the deterioration
`of the light emitting characteristic does not occur.
`The organic EL layer 16 is thinner than an inorganic EL
`layer. Moreover, the thickness to which the organic EL layer
`is formed through vapor deposition using the organic EL
`material can be very easily controlled during the process of
`forming the organic EL layer. When the organic EL layer 16
`is formed to the thickness corresponding to the wavelength
`25
`at the luminance peak of the light emitted by the organic EL
`layer 16 (in other words, the wavelength of the most intense
`component of the light emitted by the organic EL layer 16),
`the resonance effect which permits light to easily come out
`from the organic EL layer 16 can be achieved. For example,
`in the case of an organic EL element which emits blue light,
`the resonance effect can be attained when it is formed to the
`thickness of 40 nm to 50 nm. In the case of an organic EL
`element which emits green light, the resonance effect can be
`attained when it is formed to the thickness of 50 nm to 60
`
`35
`
`10
`The electric characteristic of the organic EL element 3
`will now be described with reference to FIGS. 5 and 6. AS
`shown in FIG. 5, the Source-drain current Ids of the drive
`transistor Q2 is shifted in accordance with a gate Voltage Vg
`applied to the gate electrode of the drive transistor Q2. The
`Source-drain current Ids of the drive transistor O2 becomes
`Saturated when a Source-drain Voltage Vsd applied between
`the Source and drain of the drive transistor Q2 exceeds
`approximately 5V. As shown in FIG. 6, the organic EL
`element 3 has a luminance characteristic according to an
`anode-cathode Voltage Vac (a forward bias is positive). In
`this embodiment, the luminance (gradation) of the organic
`EL