Abstract:
To increase the proportion of the perfects to the whole lot of final products and to reduce the cost for active matrix EL display devices by checking the operation of a TFT substrate before depositing an EL material. A capacitor for testing is connected to a drain terminal of a driving TFT in a pixel portion to observe charging and discharging of the capacitor. Whether the driving TFT is normal or not is judged by the observation, so that the rejects can be removed before the manufacturing process is completed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an electronic display (electro optical device) formed by fabricating an EL (electro luminescence) on a substrate. In particular, the present invention relates to a display device using a semiconductor element (an element which uses a semiconductor thin film). Further, the present invention relates to an electronic device using an EL display in a display portion and the method of detecting the EL display.  
           [0003]    The EL element herein refers to both an element that utilizes light emission from a singlet exciton (fluorescence) and an element that utilizes light emission from a triplet exciton (phosphorescence).  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, a technique for forming a thin film transistor (hereinafter, referred to as TFT) on a substrate has been remarkably developed, and a development of its application to an active matrix display device has been continuously made. In particular, TFTs using a polysilicon film can operate at high speed, because such TFTs have a higher field effect mobility than TFTs using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally conducted by a driver circuit provided outside a substrate, can be performed by a driver circuit provided on the same substrate on which the pixels are provided.  
           [0006]    Such an active matrix display device includes various circuits and elements formed on the same substrate. With this structure, the active matrix display device provides various advantages such as reduced manufacturing cost, reduced size of a display device, an increased yield, and a reduced throughput.  
           [0007]    Furthermore, an active matrix EL display device including an EL element as a self-luminescent element has been actively studied. The EL display device is also called Organic EL Display (OELD) or Organic Light Emitting Diode (OLED).  
           [0008]    In contrast with the liquid crystal display device, the EL display device is self-luminescent. The EL element has such a structure that an EL layer is sandwiched between a pair of electrodes (anode and cathode). However, the EL layer has normally a lamination structure. As a typical example of the lamination structures, a lamination structure “hole transport layer/light emitting layer/electron transport layer” proposed by Tang et al. of Eastman Kodak Company is cited. This structure has an extremely high light emitting efficiency. For this advantage, most light emitting devices, which are currently under study and development, employ this structure.  
           [0009]    Furthermore, the light emitting device may have such a lamination structure that a hole injection layer, a hole transport layer, a light emitting layer and an electron transport layer are deposited on an anode or a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are deposited on an anode in this order. Moreover, the light emitting layer may be doped with a fluorescent pigment or the like.  
           [0010]    All layers formed between a cathode and an anode are referred to generically as EL layers within this specification. The above stated hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer, and the like are therefore all contained within the EL layer.  
           [0011]    A predetermined voltage is then applied to the EL layer having the above structure by a pair of electrodes, thus recombination of a carrier thus occurs in the light emitting layer, and light is emitted. Note that the emission of light by the EL element is referred to as driving the EL element throughout this specification. Further, an EL element formed by an anode, an EL layer, and a cathode is referred to as an EL element throughout this specification.  
           [0012]    As a method of driving an EL display device, an analog driving method (analog drive) can be given. The analog drive of an EL display device is described with reference to FIGS. 10 and 11.  
           [0013]    [0013]FIG. 10 shows a structure of a pixel portion of an EL display device that is driven in an analog manner. Gate signal lines (G 1  through Gy) to which a gate select signal from a gate signal line driver circuit is input are connected to a gate electrode of a switching TFT  1801  included in each pixel. One of a source region and a drain region of the switching TFT  1801  included in each pixel is connected to source signal lines (also referred to as data signal lines) S 1  to Sx to which an analog video signal is input, whereas the other is connected to a gate electrode of an EL driver TFT  1804  included in each pixel and a capacitor  1808  included in each pixel.  
           [0014]    A source region and a drain region of the driver TFT  1804  included in each pixel are connected to power source supply lines V 1  through Vx and to an EL element  1806 , respectively. An electric potential of the power source supply lines V 1  through Vx is referred to as an power source electric potential. The power source supply lines V 1  through Vx are connected to the capacitors  1808  included in the respective pixels.  
           [0015]    The EL element  1806  includes an anode, a cathode and an EL layer sandwiched between the anode and the cathode. If the anode of the EL element  1806  is connected to the source or the drain region of the driver TFT  1804 , the anode and the cathode of the EL element  1806  become a pixel electrode and an opposing electrode, respectively. On the other hand, if the cathode of the EL element  1806  is connected to the source or the drain region of the driver TFT  1804 , the anode and the cathode of the EL element  1806  become an opposing electrode and a pixel electrode, respectively.  
           [0016]    Note that the electric potential of the opposing electrode is referred to as an opposing electric potential in this specification. Note also that an power source for imparting the opposing electric potential to the opposing electrode is referred to as an opposing electric power supply. The electric potential difference between the electric potential of the pixel electrode and the electric potential of the opposing electrode is an EL driver voltage, and the EL driver voltage is applied to the EL layer.  
           [0017]    [0017]FIG. 11 shows a timing chart in the case where the EL display device shown in FIG. 10 is driven in an analog manner. The period from the selection of one gate signal line until the selection of a next gate signal line is called one line period (L). The period from the display of one image to another image corresponds to one frame period (F). In the case of the EL display device shown in FIG. 10, since there are y gate signal lines, y line periods (L 1  to Ly) are provided within one frame period.  
           [0018]    With the enhancement in resolution, the number of line periods within one frame period increases. As a result, the driver circuit must be driven at a high frequency.  
           [0019]    An power source electric potential at the power source supply lines (V 1  through Vx) is held constant, and an opposing electric potential at the opposing electrodes is also held constant. The opposing electric potential has a potential difference with the power source electric potential to such a degree that a EL element  1806  emits light.  
           [0020]    The gate signal line G 1  is selected in the first line period L 1  by a gate signal input to the gate signal line G 1  from the gate signal line driver circuit. Then an analog video signal is then input in order to the source signal lines S 1  to Sx. All of the switching TFTs  1801  connected to the gate signal line G 1  are in an ON state, and therefore the analog video signal input to the source signal lines S 1  to Sx is input to gate electrodes of the driver TFTs  1804  through the switching TFTs  1801 .  
           [0021]    The description here takes as an example a timing chart of the case where the switching TFT  1801  and the driving TFT  1804  are both n-channel TFTs. The switching TFT and the driving TFT may instead be p-channel TFTs, or one of them may be an n-channel TFT while the other is a p-channel TFT.  
           [0022]    In this specification, the TFT being turned ON means that the gate voltage of the TFT is changed such that the source-drain thereof is brought into conductive state.  
           [0023]    The amount of a current flowing through a channel formation region of the driver TFT  1804  is controlled by a level of an electric potential (voltage) of a signal input to the gate electrode of the driver TFT  1804 . Accordingly, the electric potential applied to the pixel electrode of the EL element  1806  is determined by the level of the electric potential of the analog video signals input to the gate electrode of the driver TFT  1804 . Then, the EL element  1806  is controlled by the electric potential of the analog video signals to emit light.  
           [0024]    When the above-described operation is repeated to complete the input of analog video signals to the source signal lines (S 1  through Sx), the first line period (L 1 ) terminates. One line period may alternatively be constituted by the period until the completion of input of the analog video signals to the source signal lines (S 1  through Sx) and a horizontal blanking period. Then, a second line period (L 2 ) starts where a gate signal line G 2  is selected by a gate signal. And, as in the first line period (L 1 ), analog video signals are sequentially input to the source signal lines (S 1  through Sx) during the second line period.  
           [0025]    When all gate signal lines (G 1  through Gy) are selected in this manner, all lines periods (L 1  through Ly) are completed. The completion of all the line periods (L 1  through Ly) corresponds to the completion of one frame period. All pixels perform display during one frame period to form an image. One frame period may be alternatively constituted by all line periods (L 1  through Ly) and a vertical blanking period.  
           [0026]    The amount of light emitted by the EL element  1806  is thus controlled in accordance with the analog video signal, and gray scale display is performed by controlling the amount of light emitted. This method is namely a driving method referred to as an analog driving method, gray scale display is performed by changing the electric potential of the analog video signal input to the source signal lines.  
           [0027]    In the conventional EL display device, the drain region of the driving TFT  1804  in the pixel portion is connected only to the EL element  1806  as shown in FIG. 10.  
           [0028]    TFTs are formed on a substrate having an insulating surface in order to constitute pixel TFTs (each formed of a switching TFT and a driving TFT) and driver circuits (including a source signal line driving circuit and a gate signal line driving circuit). An EL material is then deposited and the driving TFT is electrically connected to an EL element. The manufacturing steps prior to the step of depositing the EL material are called herein TFT steps.  
           [0029]    Before the EL material is deposited, the drain region of the driving TFT in the conventional display device is thus in an open state from the design of the circuit. Whether a certain pixel TFT operates normally or not cannot be judged until the EL material is deposited to complete the display device and lighting test is performed on the completed device. Therefore, it is not until after the manufacturing process reaches the final step that a display device incapable of normal display because of a defective pixel TFT can be found out. This is utterly a waste.  
           [0030]    As described above, the conventional EL display device does not allow its pixel TFTs to be checked for their operation during the steps prior to deposition of the EL material, thereby incurring a waste in manufacturing cost.  
         SUMMARY OF THE INVENTION  
         [0031]    The present invention has been made in view of the above problem, and an object of the present invention is therefore to provide an active matrix EL display device that allows its pixel TFTs to be checked for their operation before depositing an EL material.  
           [0032]    In order to attain the object above, the inventors of the present invention have thought of cutting waste along the manufacture line by testing driving TFTs and switching TFTs before depositing an EL material and removing substrates having TFTs that are found to be defective through the test (hereinafter referred to as the rejects) from the subsequent step of depositing the EL material.  
           [0033]    The structure of the EL display device of the present invention is as described below.  
           [0034]    In a first aspect of the present invention, an EL display device is provided which comprises a plurality of source signal lines, a plurality of gate signal lines, a plurality of power supply lines, a plurality of switching thin film transistors and a plurality of driving thin film transistors which are all formed on an insulating substrate, comprising a testing capacitor which is set such that one end thereof is connected to a drain region of each of the driving thin film transistors and the other end thereof is connected to one of the gate signal lines, and characterized in that the power supply lines are led out of the insulating substrate through switches.  
           [0035]    In a second aspect of the present invention, an EL display device is provided characterized in that, the switches are provided for the plural power supply lines on one on one basis, and a driver circuit for driving the switches successively is placed on the insulating substrate.  
           [0036]    In a third aspect of the present invention, an EL display device is provided characterized in that the driver circuit for driving the switches successively has some components shared with a source signal line driving circuit.  
           [0037]    In a fourth aspect of the present invention, a method of testing an EL display device is provided, which comprises a plurality of source signal lines, a plurality of gate signal lines, a plurality of power supply lines, a plurality of switching thin film transistors and a plurality of driving thin film transistors which are all formed on an insulating substrate, the driving thin film transistors each having a drain region that is connected to a testing capacitor, the method comprising the steps of: operating the driving thin film transistors to charge the testing capacitor until it reaches and keeps a certain level of electric potential; turning the driving thin film transistors OFF and then setting the electric potential of the power supply lines to a level different from the electric potential of the testing capacitor; and leading the electric charges charged in the testing capacitor out for each pixel through its associated power supply line, so that a change in electric potential can be detected.  
           [0038]    In a fifth aspect of the present invention, a computer, a video camera, a head mount display, an image reproducing device, a portable information terminal characterized by using an EL display device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    In the accompanying drawings:  
         [0040]    [0040]FIG. 1 is a diagram showing the circuit structure of a pixel portion of a display device according to the present invention;  
         [0041]    [0041]FIG. 2 is a block diagram of a display device according to the present invention;  
         [0042]    [0042]FIG. 3 is a timing chart illustrating a method of driving a display device according to the present invention;  
         [0043]    [0043]FIG. 4 is a diagram showing an embodiment of a testing driver circuit in a display device according to the present invention;  
         [0044]    [0044]FIG. 5 is a diagram showing an embodiment of an external testing circuit for a display device according to the present invention;  
         [0045]    [0045]FIGS. 6A and 6B are a top view of a display device according to the present invention and a sectional view thereof, respectively;  
         [0046]    [0046]FIG. 7 is a circuit diagram showing a source signal line driving circuit of a display device according to the present invention;  
         [0047]    [0047]FIG. 8 is a top view of a latch of a display device according to the present invention:  
         [0048]    [0048]FIGS. 9A to  9 E show electronic equipments employing a display device of the present invention;  
         [0049]    [0049]FIG. 10 is a circuit diagram of a pixel portion of a conventional display device;  
         [0050]    [0050]FIG. 11 is a timing chart illustrating an analog driving method for a display device;  
         [0051]    [0051]FIG. 12 is a diagram showing a cellular phone that employs the present invention; and  
         [0052]    [0052]FIG. 13 is a diagram illustrating how to use the cellular phone that employs the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]    The structure of an EL display device of the present invention and a method of testing the same will be described below.  
         [0054]    [0054]FIG. 1 shows Embodiment Mode 1 of the present invention. Shown in FIG. 1 is a structural diagram of a pixel portion in the display device of the present invention.  
         [0055]    Each pixel in the pixel portion comprises a power supply line (one of V 1  to Vx), a source signal line (one of S 1  to Sx), a gate signal line (one of G 1  to Gy), a switching TFT  9102 , a driving TFT  9106 , a capacitor storage  9107 , an EL element  9105  and a testing capacitor  9108 .  
         [0056]    The switching TFT  9102  shown in FIG. 1 has a double gate structure. However, the switching TFT in each pixel of the display device according to the present invention is not limited to a double gate structure. It may take a single gate structure or a multi-gate structure having three or more gates.  
         [0057]    The driving TFT  9106  shown in FIG. 1 has a single gate structure. However, the driving TFT in each pixel of the display device according to the present invention may take a double gate structure or a multi-gate structure.  
         [0058]    A drain region of the driving TFT  9106  is connected not only to the EL element  9105  but also to the testing capacitor  9108 . The testing capacitor  9108  is interposed between the drain region of the driving TFT  9106  and the gate signal line in this example. However, the structure of the present invention is not limited thereto and the testing capacitor may be placed between the drain region and another individual wiring.  
         [0059]    [0059]FIG. 2 is a block diagram showing the display device of the present invention.  
         [0060]    The display device comprises a source signal line driving circuit  9201 , gate signal line driving circuits  9202  and  9203 , a testing driver circuit  9204 , switches  9205  and  9206 , source signal lines  9207  to  9209 , gate signal lines  9210  to  9212 , pixel portion power supply lines  9213  and  9214 , a power supply line lead out terminal  9215  and an external testing circuit  9216 .  
         [0061]    [0061]FIG. 2 shows some of the source signal lines, the gate signal lines, the power supply lines and the switches as representatives of the rest. The number of these lines actually provided corresponding to the number of pixels that constitute the display device.  
         [0062]    Unlike the prior art, the testing driver circuit  9204  is added to the display device of the present invention. Also added are the switches  9205  and  9206  that are controlled by the testing driver circuit  9204  and interposed between the power supply line lead out terminal  9215  and the pixel portion power supply line  9213  and between the power supply line lead out terminal  9215  and the pixel portion power supply line  9214 , respectively. The power supply line lead out terminal is connected to the external testing circuit  9216 .  
         [0063]    The testing driver circuit  9204  is arranged independently in FIG. 2. However, the source signal line driving circuit may also serve as the testing driver circuit if the signal line driving circuit is of analog type. (Not shown)  
         [0064]    Next, a testing method used in the present invention will be described.  
         [0065]    The description will be given with reference to FIGS. 1 and 2.  
         [0066]    Assumed here is a test of a substrate in a stage after the TFT steps but before deposition of an EL material. The test may instead be conducted at other stages as long as wire connection has already been established between the TFTs, and between the capacitors (including the capacitor storage, the testing capacitor, etc.) and resistors or other components which constitute the display device.  
         [0067]    Note that, although the EL element  9105  is found in FIG. 1, the EL element is not deposited and hence the EL element  9105  is not formed vet at the time of the test described below.  
         [0068]    The first step is to apply a voltage corresponding to a signal “Hi”. e.g. 10 V, to the power supply lines. Then successively running the driving circuits (the source signal line driving circuit  9201  and the gate signal line driving circuits  9202  and  9203 ), the driving TFT  9106  in each pixel is turned ON to write the 10 V voltage of the power supply lines V 1  to Vx in each testing capacitor  9108 . Capacitance of the testing capacitor is set from 0.05 pF to 1 pF.  
         [0069]    The second step is as follows. The source signal line driving circuit  9201  and the gate signal line driving circuits  9202  and  9203  are operated to turn every driving TFT  9106  in the pixel portion OFF. Then the electric potential of the power supply lines V 1  to Vx is set to a voltage corresponding to a signal “Lo”. e.g. 0 V. The switches  9205  and  9206  at this point are remained ON.  
         [0070]    The power supply lines V 1  to Vx in the pixel portion thus come to have 0V.  
         [0071]    The third step includes the following operations.  
         [0072]    The driving TFT  9106  in each pixel is put in motion in accordance with a timing illustrated in FIG. 3.  
         [0073]    The timing chart of FIG. 3 shows the case where the switching TFT is an n-channel TFT and the driving TFT is a p-channel TFT. However, the switching TFT may either be an n-channel TFT or a p-channel TFT and the same applies to the driving TFT.  
         [0074]    The source signal lines S 1  to Sx are sequentially operated. In the illustration of FIG. 3, two source signal lines S 1  and S 2  represent the rest of the source signal lines and merely the operation for S 1  and S 2  will be described here. However this operation is performed on all of the source signal lines S 1  to Sx.  
         [0075]    In a pixel whose source signal line has received input of a signal “Lo”, the driving TFT  9107  is turned ON when a signal “Hi” is inputted to the gate signal lines G 1  to Gy successively.  
         [0076]    If all of the power supply lines V 1  to Vx are connected, wiring capacitor is too large to detect the voltage. Therefore, in order to check the operation of the pixel TFTs, the switches to be connected to the power supply lines have to be arranged such that one switch is allocated to one column of pixels.  
         [0077]    One column of pixels here is of pixels having switching TFTs connected to the same source signal line.  
         [0078]    During the source signal line S 1  is selected, a switch connected to the power supply line V 1  for supplying power to pixels whose switching TFTs have source regions connected to the source signal line S 1  is turned ON. Meanwhile, the switches connected to the power supply lines V 2  to Vx that are associated with the other pixels are all turned OFF.  
         [0079]    The source signal line S 2  is selected next, during which a switch connected to the power supply line V 2  for supplying power to pixels whose switching TFTs have source regions connected to the source signal line S 2  is turned ON. Meanwhile, the switches connected to the power supply lines V 1  and V 3  to Vx that are associated with the other pixels are all turned OFF.  
         [0080]    In FIG. 3, denoted by T1 and T2 are signals for turning ON or OFF the switch connected to the power supply line V 1  and the switch connected to the power supply line V 2 , respectively.  
         [0081]    In this embodiment, if T1 and T2 are signals “Hi” and inputted to power supply lines, the switches connected to those power supply lines are turned ON. On the other hand, the switches are turned OFF if T1 and T2 are signals “Lo”.  
         [0082]    When the driving TFT  9106  is turned ON in each pixel, electric charges held in the testing capacitor  9108  are discharged to the power supply lines V 1  to Vx. The discharge gives a voltage to the power supply lines V 1  to Vx.  
         [0083]    This voltage is given as follows. The voltage generated is expressed as V out  and is obtained from the equation 1:  
           V   out =10 ×C 3/( C 1+ C 2+ C 3)  
         [0084]    where C1 is a wiring capacitance value of the power supply lines in the pixel portion. C2 is a capacitance up through the power supply line lead out terminal  9215 , and C3 is a capacitance of the testing capacitor.  
         [0085]    If C1=C2=10 pF and C3=0.1 pF, the voltage V out  is 0.05 V.  
         [0086]    The voltage V out  is small and hence the detection thereof requires the external testing circuit  9216  connected to the power supply line lead out terminal  9215 .  
         [0087]    The voltage V out  is not generated if the pixel TFT tested is defective because charge or discharge cannot be made successfully.  
         [0088]    In the timing chart of FIG. 3, the voltage V out  is generated in the power supply line lead out terminal  9215  every time a pixel is selected if the pixels are normal. If the pixel TFT is defective, it can be found out as a lack in voltage signal as indicated by  9301 .  
         [0089]    In this way, the pixel TFTs can be tested by selecting all pixels one by one.  
         [0090]    Embodiments of the present invention will be descried below.  
         [0091]    [Embodiment 1] 
         [0092]    Embodiment 1 shows an example of the structure of a testing driver circuit in a display device of the present invention.  
         [0093]    In FIG. 4, the testing driver circuit comprises a shift register  9402 . NAND circuits  9403 ,  9404  and  9405 , and buffer circuits  9406 ,  9407  and  9408 . The shift register is comprised of a DFF  9401 . The buffer circuits are comprised of inverters.  
         [0094]    Although FIG. 4 shows merely a portion of the testing driver circuit which corresponds to three power supply lines, the actual testing driver circuit has all the circuits that correspond to all of the power supply lines.  
         [0095]    The buffer circuits  9406 ,  9407  and  9408  have outputs  9409 ,  9410  and  9411 , respectively. The outputs are connected to switches  9205 ,  9206 , . . . shown in FIG. 2 and further connected to pixel portion power supply lines and a power supply line lead out terminal.  
         [0096]    When a voltage “Hi” is inputted to an input terminal  9400  of the shift register  9402 , the terminals  9409  to  9411  all outputs signals corresponding to “Hi”, whereby all the switches are turned ON.  
         [0097]    [Embodiment 2] 
         [0098]    Embodiment 2 shows an example of the structure of an external testing circuit for a display device of the present invention.  
         [0099]    In FIG. 5, an external testing circuit  9501  comprises a switch  9502  for switching connections, an amplifier  9505  for detecting a signal, a voltage source  9503 , a resistor  9504 , etc.  
         [0100]    The switch  9502  selects one connection out of connections with three input terminals consisting of the voltage source  9503 , a voltage source  9508  and the amplifier  9505 . The voltage source  9503  is for a voltage corresponding to a signal “Hi”, i.e., 10V. The voltage source  9508  is for a voltage corresponding to a signal “Lo”, i.e., 0V. The amplifier  9505  amplifies a signal.  
         [0101]    Note that the voltages of the voltage sources  9503  and  9508  are not limited to the values above but may be set to optimal values if necessary.  
         [0102]    The power supply line lead out terminal of the substrate of the display device to be tested is connected to an input  9507 , and the test is conducted following the steps described in Embodiment Mode 1. In the test, judgement is made by monitoring an output  9506  of the amplifier  9505 .  
         [0103]    The amplifier  9505  used here has a voltage gain of about 10 to 1000 folds, and amplifies a detection signal generated in the power supply line to sense the signal. A desirable gain of the amplifier is on the order of 100 folds.  
         [0104]    This embodiment can be carried out in combination with Embodiment 1 without restriction.  
         [0105]    [Embodiment 3] 
         [0106]    A driving TFT  108  in the present invention may be an n-channel TFT or a p-channel TFT. However, if an anode of an EL element  110  serves as a pixel electrode and a cathode thereof serves as an opposite electrode, a p-channel TFT is preferably used for the driving TFT  108 . On the other hand, when the anode of the EL element  110  serves as the opposite electrode and the cathode thereof serves as the pixel electrode, the driving TFT  108  is preferably an n-channel TFT.  
         [0107]    This embodiment can be carried out in combination with Embodiment 1 or 2 without restriction.  
         [0108]    [Embodiment 4] 
         [0109]    Embodiment 4 shows an example of manufacturing an EL display device in accordance with the present invention. FIG. 6A is a top surface diagram of an EL display device using the present invention. In FIG. 6A, reference numeral  4010  denotes a substrate,  4011  denotes a pixel portion.  4012  denotes a source signal line driver circuit, and  4013   a  and  4013   b  denote gate signal line driver circuits. The respective driver circuits are connected to an external equipment via wirings  4014   a .  4014   b .  4015  and  4016  leading to an FPC  4017 .  
         [0110]    Note that in this embodiment, an example where the source signal line driver circuit  4012  functions as a testing driver circuit is described, however, the present invention is not limited to this structure. The testing driver circuit may be provided apart from the source signal line driver circuit.  
         [0111]    A cover material  6000 , a sealing material (also referred to as a housing material)  7000 , an airtight material (a second sealing material)  7001  are provided at this time so as to surround at least the pixel portion, and preferably the driver circuit and the pixel portion.  
         [0112]    Further, FIG. 6B is a cross sectional structure of an EL display device of this embodiment, and a driver circuit TFT (note that a CMOS circuit in which an n-channel TFT and a p-channel TFT are combined is shown in the figures here)  4022  and a pixel portion TFT  4023  (note that only a driver TFT for controlling the electric current to the EL element is shown in the figures here) are formed on a base film  4021  on the substrate  4010 . Known structures (top gate structures or bottom gate structures) may be used for these TFTs.  
         [0113]    Note that a testing capacitor connected to a drain electrode of the driver TFT is not shown in FIG. 6B.  
         [0114]    After completing the driver circuit TFT  4022  and the pixel portion TFT  4023 , a pixel electrode  4027  made from a transparent conductive film for electrically connecting to a drain of the pixel portion TFT  4023  is formed on an interlayer insulating film (leveling film)  4026  made from a resin material. A compound of indium oxide and tin oxide (referred to as ITO) and a compound of indium oxide and zinc oxide can be used as the transparent conductive film. An insulating film  4028  is formed once the pixel electrode  4027  is formed, and an open portion is formed on the pixel electrode  4027 .  
         [0115]    Then, an EL layer  4029  is formed. A lamination structure of a known EL material (hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer), or a single layer structure, may be used for the EL layer  4029 . The structure is formed using a known technique. Further, there are low molecular weight materials and high molecular weight materials (polymer materials) for the EL material. An evaporation method is used when a low molecular weight material is used, but it is possible to use a simple method such as spin coating, printing or ink-jet printing when a high molecular weight material is used.  
         [0116]    The EL layer  4029  is formed by evaporation using a shadow mask in this embodiment. Color display becomes possible by forming light emitting layers (a red color light emitting layer, a green color light emitting layer, and a blue color light emitting layer) capable of emitting light at different wavelength for each pixel using the shadow mask. In addition, a method of combining a color changing layer (CCM) and a color filter, and a method of combining a white color light emitting layer and a color filter are available, and both may be used. Of course, a single color light emitting EL display device can also be made.  
         [0117]    After forming the EL layer  4029 , a cathode  4030  is formed on the EL layer. It is preferable to remove as much moisture and oxygen as possible from the interface between the cathode  4030  and the EL layer  4029 . Therefore, a method in which the EL layer  4029  and the cathode  4030  are formed in succession within a vacuum, or in which the EL layer  4029  is formed in an inert atmosphere and the cathode  4030  is then formed without exposure to the atmosphere is necessary. In this embodiment, the above film formation can be performed by using a multi-chamber method (cluster tool method) film formation apparatus.  
         [0118]    Note that a lamination structure of a LiF (lithium fluoride) film and an Al (aluminum) film is used as the cathode  4030  in this embodiment. Specifically, a 1 nm thick LiF (lithium fluoride) film is formed by evaporation on the EL layer  4029 , and a 300 nm thick aluminum film is formed on the LiF film. An MgAg electrode, which is a known cathode material, may of course also be used. Then, the cathode  4030  is connected to the wiring  4016  in a region denoted by reference numeral  4031 . The wiring  4016  is an electric power source supply line for applying a predetermined voltage to the cathode  4030 , and is connected to the FPC  4017  through a conductive paste material  4032 .  
         [0119]    The cathode  4030  and the wiring  4016  are electrically connected in the region shown by reference numeral  4031 , and therefore it is necessary to form contact holes in the interlayer insulating film  4026  and the insulating film  4028 . These contact holes is may be formed during etching of the interlayer insulating film  4026  (when the pixel electrode contact hole is formed) and during etching of the insulating film  4028  (when forming the open portion before forming the EL layer). Further, etching may also be performed together through to the interlayer insulating film  4026  when etching the insulating film  4028 . A contact hole having a good shape can be formed in this case provided that the interlayer insulating film  4026  and the insulating film  4028  are formed by the same resin material.  
         [0120]    A passivation film  6003 , a filling material  6004 , and the cover material  6000  are formed covering the surface of the EL element thus formed.  
         [0121]    In addition, the sealing material  7000  is formed on the inside of the cover material  6000  and the substrate  4010  so as to surround the EL element portion. The airtight material (the second sealing material)  7001  is formed on the outside of the sealing material  7000 .  
         [0122]    The filling material  6004  functions as an adhesive for bonding the cover material  6000 . PVC (polyvinyl chloride), epoxy resin, silicone resin, PVB (polyvinyl butyral) and EVA (ethylene vinyl acetate) can be used as the filling material  6004 . A moisture absorption effect can be maintained if a drying agent is formed on the inside of the filling material  6004 , and therefore it is preferable to do so.  
         [0123]    Furthermore, spacers may be included within the filling material  6004 . The spacers may be made from a powdered substance comprising a material such as BaO, giving the spacers themselves moisture absorbency.  
         [0124]    In the case of providing the spacers, the passivation film  6003  can relieve the spacer pressure. Further, a film such as a resin film, separate from the passivation film  6003 , may also be formed for relieving the spacer pressure.  
         [0125]    Further, a glass plate, an aluminum plate, a stainless steel plate, an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film, a mylar film, a polyester film, and an acrylic film can be used as the cover material  6000 . Note that when using PVB or EVA as the filling material  6004 , it is preferable to use a sheet having a structure in which several  10  of μm of aluminum foil is sandwiched by a PVF film or a mylar film.  
         [0126]    Note that, depending upon the direction of light emitted from the EL elements (light emission direction), it may be necessary for the cover material  6000  to have light transmitting characteristics.  
         [0127]    Further, the wiring  4016  is electrically connected to the FPC  4017  through a gap between the sealing material  7000  and the airtight material  7001 , and the substrate  4010 . Note that, although the wiring  4016  is explained here, the other wirings  4014   a .  4014   b , and  4015  are also electrically connected to the FPC  4017  through a gap between the sealing material  7000  and the airtight material  7001 , and the substrate  4010 .  
         [0128]    Note that the cover material  6000  is bonded after forming the filling material  6004  in Embodiment 5, and that the sealing material  7000  is attached so as to the side surface (exposed surface) of the filling material  6004 , but the filling material  6004  may also be formed after attaching the cover material  6000  and the sealing material  7000 . A filling material injection port passing through the gap formed by the substrate  4010 , the cover material  6000  and the sealing material  7000  is formed in this case. The gap is then placed in a vacuum state (equal to or less than 10 −2  Torr), and after immersing the injection port in a tank containing the filling material, the pressure on the outside of the gap is made higher than the pressure within the gap, and the filling material fills the space.  
         [0129]    This embodiment can be carried out in combination with Embodiments 1 to 3 without restriction.  
         [0130]    [Embodiment 5] 
         [0131]    Embodiment 5 describes the structure of a source signal line driving circuit in the case of employing a digital time gray scale driving method instead of the analog gray scale driving method that has been described in the example of the prior art.  
         [0132]    [0132]FIG. 7 shows as a circuit diagram an example of the source signal line driving circuit used in this embodiment.  
         [0133]    The present invention can adopt any one of the analog gray scale driving method, the digital time gray scale driving method and the digital area ratio gray scale driving method. The invention may also take a driving method that uses these gray scale methods in combination.  
         [0134]    The source signal line driving circuit has a shift register  801 , latches (A)  802  and latches (B)  803  which are arranged as shown in FIG. 7.  
         [0135]    In this embodiment, a pair of latches (A)  802  and a pair of latches (B)  803  are associated with outputs to four source signal lines S_a to S_d. Therefore, four input lines VD are provided for digital image signals that are inputted from the external. The four input lines VD respectively receive signals to be inputted to the source signal lines S_a to S_d.  
         [0136]    A level shifter for changing the amplitude of the voltage of the signal is not provided in this embodiment. However, the level shifter may be provided at discretion.  
         [0137]    A clock signal CLK, an inverted clock signal CLKB obtained by inverting the polarity of the CLK, a start pulse signal SP, and a drive direction switching signal SL/R are inputted to the shift register  801  respectively from the wirings shown in FIG. 7. A digital data signal VD is inputted to the latches (A)  802  from the wirings shown in FIG. 7. A latch signal S_LAT and an inverted signal S_LATb obtained by inverting the polarity of the S_LAT are inputted to the latches (B)  803  respectively from the wirings shown in FIG. 7.  
         [0138]    Details of the structure of the latches (A)  802  will be described taking as an example a portion  804  that is a part of the latches (A)  802  associated with the source signal line S_a. The portion  804  that is a part of the latches (A)  802  has two clocked inverters and two inverters.  
         [0139]    [0139]FIG. 8 shows a top view of the portion  804  that is a part of the latches (A)  802 . Denoted by  831   a  and  831   b  are active layers of TFTs that constitute one of the inverters of the portion  804  that is a part of the latches (A)  802 . Reference symbol  836  denotes a common gate electrode of the TFTs constituting the one inverter. The other inverter of the portion  804  that is a part of the latches (A)  802  comprises TFTs whose active layers are denoted by  832   a  and  832   b . On the active layers  832   a  and  832   b , gate electrodes  837   a  and  837   b  are provided. The gate electrodes  837   a  and  837   b  are electrically connected to each other.  
         [0140]    Denoted by  833   a  and  833   b  are active layers of TFTs that constitute one of the clocked inverters of the portion  804  that is a part of the latches (A)  802 . On the active layer  833   a , gate electrodes  838   a  and  838   b  are formed to provide a double gate structure. On the active layer  833   b , the gate electrode  838   b  and a gate electrode  839  are formed to provide a double gate structure.  
         [0141]    Denoted by  834   a  and  834   b  are active layers of TFTs that constitute the other clocked inverter of the portion  804  that is a part of the latches (A)  802 . On the active layer  834   a , the gate electrode  839  and a gate electrode  840  are formed to provide a double gate structure. On the active layer  834   b , the gate electrode  840  and a gate electrode  841  are formed to provide a double gate structure.  
         [0142]    This embodiment can be carried out in combination with any one of Embodiments 1 through 4 without restriction.  
         [0143]    [Embodiment 6] 
         [0144]    According to an EL display device of the present invention, a material used for an EL layer of an EL element is not limited to an organic EL material but may be an inorganic EL material. However, every inorganic EL material at present is very high in drive voltage and hence a TFT to be used has to have a withstand voltage characteristic that allows the TFT to withstand such a high drive voltage.  
         [0145]    If an inorganic EL material of lower drive voltage is to be developed in future, it may also be used in the present invention.  
         [0146]    This embodiment can be carried out in combination with any one of Embodiments 1 through 5 without restriction.  
         [0147]    [Embodiment 7] 
         [0148]    An electronic display formed by implementing the present invention, in particular, an EL display device is utilized for various electronic equipments. Electronic equipments incorporating an electronic display formed in accordance with this invention as a display medium will be explained as follows.  
         [0149]    The following can be given as examples of such electronic equipments: a video camera; a television receiving machine; a digital camera; a head mounted display (a goggle type display); a game machine; a telephone; a car navigation system; a personal computer; an image reproducing device; a portable information terminal (such as a mobile computer, a mobile telephone, or an electronic book). Examples of those electronic equipments are shown in FIG. 9.  
         [0150]    [0150]FIG. 9A illustrates a personal computer, which includes a main body  2001 , a frame  2002 , a display portion  2003 , a key board  2004 , or the like. The EL display device of the present invention can be applied to the display portion  2003  of the personal computer.  
         [0151]    [0151]FIG. 9B illustrates a video camera, which includes a main body  2101 , a display portion  2102 , an audio input portion  2103 , operation switches  2104 , a battery  2105 , an image receiving portion  2106 , or the like. The EL display device of the present invention can be applied to the display portion  2102  of the video camera.  
         [0152]    [0152]FIG. 9C illustrates a portion (the right-half piece) of a head mounted display, which includes a main body  2301 , signal cables  2302 , a head mount band  2303 , a display monitor  2304 , an optical system  2305 , a display portion  2306 , or the like. The EL display device of the present invention can be applied to the display portion  2306  of the head mounted display.  
         [0153]    [0153]FIG. 9D shows an image reproducing device provided with a recording medium (specifically, a DVD reproducing device). The image reproducing device comprises a main body  2401 , a recording medium (CD, LD, DVD, or the like)  2402 , operation switches  2403 , a display portion (a)  2404 , a display portion (b)  2405 , etc. The display portion (a) mainly displays image information whereas the display portion (b) mainly displays character information. The EL display device of the present invention can be used for the display portions (a) and (b) of the image reproducing device provided with a recording medium. Note that the present invention also can be applied to other image reproducing machines provided with a recording medium, such as CD players and game machines.  
         [0154]    [0154]FIG. 9E shows a portable (mobile) computer, which includes a main body  2501 , a camera portion  2502 , image receiving portion  2503 , operation switches  2504 , a display portion  2505 , or the like. The EL display device of the present invention can be applied to the display portion  2505  of the portable (mobile) computer.  
         [0155]    If the luminance of light emitted from EL materials is improved in future, the present invention can be used in front or rear type projectors.  
         [0156]    The electronic equipments in this embodiment can be carried out in combination with any one of Embodiments 1 through 6 without restriction.  
         [0157]    [Embodiment 8] 
         [0158]    [0158]FIG. 12 shows an example in which an EL display device is used in a cellular phone.  
         [0159]    The cellular phone comprises a casing A  1201 , a casing B  1202  and an antenna  1205 . A display portion  1200  and a microphone  1209  are formed on a surface A  1203  of the casing A  1201 . Formed on a surface B  1204  of the casing B  1204  are a speaker  1206 , operation keys  1207 , a power switch  1208 , etc.  
         [0160]    An EL display device of the present invention can be used as the display portion  1200  of the cellular phone.  
         [0161]    The arrangement of the speaker  1206 , the operation keys  1207 , the display portion  1200 , the microphone  1209  and the power switch  1208  is not limited to the one described above. The components can be formed on any part of the casing A  1201  and the casing B  1202 .  
         [0162]    In FIG. 12, the cellular phone has two casing portions (the casing A  1201  and the casing B  1202 ) connected by a hinge (not shown) at one side. When the hinge is closed, the surface A  1203  of the casing A  1201  can be laid on top of the surface B  1204  of the casing B  1202 . To lay the surface A  1203  of the casing A  1201  on top of the surface B  1204  of the casing B  1202  is referred to as folding the cellular phone in half.  
         [0163]    In this example, the cellular phone can be used in a way illustrated in FIG. 13. Since the speaker  1206  and the microphone  1209  are set in different casings, it is possible to place the speaker  1206  close to a ear  1211  and the microphone  1209  to a mouth  1212  with an adjustment of the angle between the surface A  1203  and the surface B  1204  on which they are set. This structure has an advantage of blocking the view of the mouth  1212  from the others while the user is speaking on the phone. Moreover, the microphone  1209  being in proximity to the mouth  1212  reduces the influence of noise and thereby improves the phone communication quality. Thus it is effective in reducing the number of noise filters in the cellular phone. With more operation keys  1207 , the cellular phone can also serve as a portable information terminal.  
         [0164]    As described above, the application range of the present invention is so wide that the invention can be applied to every field of electronic equipment. The electronic equipment of this embodiment can be obtained using any combination of structures of Embodiments 1 to 6.  
         [0165]    With the structure above, a display device that allows its pixel TFTs to be tested before forming an EL layer can be provided. This makes it possible to remove the rejects before depositing an EL material, leading to a reduction in manufacturing cost.