Patent Publication Number: US-9893094-B2

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/098,473, filed Apr. 14, 2016, now allowed, which is a continuation of U.S. application Ser. No. 14/489,526, filed Sep. 18, 2014, now U.S. Pat. No. 9,343,485, which is a continuation of U.S. application Ser. No. 14/072,843, filed Nov. 6, 2013, now U.S. Pat. No. 8,841,680, which is a continuation of U.S. application Ser. No. 13/483,080, filed May 30, 2012, now U.S. Pat. No. 8,586,991, which is a continuation of U.S. application Ser. No. 12/630,869, filed Dec. 4, 2009, now U.S. Pat. No. 8,212,257, which is a continuation of U.S. application Ser. No. 10/914,081, filed Aug. 10, 2004, now U.S. Pat. No. 7,629,612, which is a continuation of U.S. application Ser. No. 10/198,693, filed Jul. 16, 2002, now U.S. Pat. No. 6,774,419, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2001-243984 on Aug. 10, 2001, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device having thin film transistors (hereinafter, referred to as TFTs) formed on an insulating surface of glass, plastics, or the like. In particular, included in the semiconductor devices are pulse output circuits such as a shift register circuit, latch circuit, buffer circuit, and level shift circuit, and amplification circuits such as an amplifier, each being used as a driver circuit of a display device. 
     2. Description of the Related Art 
     In recent years, a display device having a semiconductor thin film formed on an insulator such as a glass substrate, in particular, an electronic circuit manufactured with TFTs is used in various fields. The electronic circuit is often used in a display device. Active matrix display devices such as an LCD (liquid crystal display) are used in many products and widely spread. In the active matrix display device formed with TFTs, several hundred thousands to several millions of pixels are arranged in a matrix form, and a charge at each pixel is controlled by the TFT disposed at each pixel to thereby display an image. 
     As a further updated technique, a polysilicon TFT technique is being developed in which on the substrate a driver circuit is formed in a peripheral region of a pixel portion by using TFTs simultaneously with the pixel portion including pixel TFTs is which constitute pixels. This greatly contributes to reduction in size and lowering of power consumption of the device, and accordingly such a display device is becoming indispensable to a display unit and the like provided in a mobile information terminal whose application field is notably widened in recent years. 
     Incidentally, in recent years, the display device is adopted in a display unit of various electronic equipments, and its industrial field is steadily expanding. Recently, it has been actively adopted in relatively inexpensive electronic equipments, and further reduction in costs is thus desired. 
     In general, in a semiconductor device, a CMOS circuit is adopted in which both n-channel TFTs and p-channel TFTs are used in combination. A display device has a multilayer structure with manufacturing steps of: film formation; exposure with photomasks; and etching are repeated. The steps are extremely complicated, and manufacturing costs thus increase. In addition, in a case of integrally forming the driver circuit and the pixel portion on the substrate as described above, yield is intensely affected by the steps since the defect of a part leads to the defect of a product as a whole. 
     A method of reducing manufacturing costs comprises reducing the number of steps as much as possible and manufacturing a device in a simple way as well as in a short period of time. Here, a display device is manufactured not with the CMOS structure but with a structure with TFTs of a single polarity in which either n-channel TFTs or p-channel TFTs are used, as a driver circuit structure. Thus, the number of steps of doping an impurity which imparts a conductivity type to semiconductor layers can be mathematically reduced to half, and further, the number of photomasks can also be reduced, which is effective to a great extent. Moreover, the manufacturing steps become simpler with a contribution to an improvement of yield. 
       FIG. 2  shows an example of an inverter formed of two n-channel TFTs. The inverter is of a dual input type in which signals are inputted to gate electrodes of TFTs  201  and  202 , and an inverted signal of an input signal of one TFT is the input of the other TFT. 
     An operation of the inverter shown in  FIG. 2  is now simply explained. It should be noted that in this specification, on explaining a structure and operation of a circuit, different names are appropriately given to three electrodes of a TFT, that is, “gate electrode, input electrode, and output electrode” or “gate electrode, source region, and drain region”. When the operation of the TFT is explained, a gate-source voltage is considered in many cases. However, it is difficult to make a rigid distinction between the source region and the drain region of the TFT due to its structure. If unified names are given thereto, confusion may be caused on contrary. That is the reason why the different names are used here. When the input/output of a signal is explained, the electrodes are referred to as input electrode and output electrode. When the gate-source voltage or the like of the TFT is explained, one of the input electrode and the output electrode is referred to as source region, and the other as drain region. 
     Further, “a TFT is ON” means a state in which the absolute value of the gate-source voltage of the TFT exceeds a threshold voltage with a current flowing between the source and the drain. On the other hand, “a TFT is OFF” means a state in which the absolute value of the gate-source voltage of the TFT does not reach a threshold voltage with no current flowing between the source and the drain. As to the threshold value, for the sake of simple explanation, it is assumed that there is no fluctuation in respective TFTs. Threshold values of n-channel TFTs are uniformly set to VthN, and threshold values of p-channel TFTs are uniformly set to VthP. 
     First, when H level is inputted to an input terminal (In) and L level is inputted to an inverted input terminal (Inb), the TFT  201  is turned OFF and the  202  is turned ON. Then, L level appears at an output terminal (Out) and its voltage becomes VSS. On the other hand, when L level is inputted to the input terminal (In) and H level is inputted to the inverted input terminal (Inb), the TFT  201  is turned ON and the TFT  202  is turned OFF. Then, H level appears at the output terminal (Out). 
     At this time, a potential at the time when the output terminal (Out) becomes H level is considered. 
     In  FIG. 2 , when H level is inputted to the gate electrode of the TFT  201 , L level is inputted to the gate electrode of the TFT  202 . Then, the TFT  201  is turned ON, the TFT  202  is turned OFF, and thus, the potential of the output terminal (Out) begins to increase. When the potential of the output terminal (Out) reaches (VDD−VthN), the gate-source voltage of the TFT  201  becomes equal to the threshold value VthN. That is, at this moment, the TFT  201  is turned OFF so that the potential of the output terminal (Out) cannot increase any further. 
     A case is considered in which inverters are connected in a plurality of stages, as shown in  FIG. 12A . Among the inverters of  FIG. 12A , only an initial inverter (InvA) is of such a single input and single output type as shown in  FIG. 12B . Each of subsequent inverters (InvB) is of such a dual input and single output type as shown in  FIG. 12C  in order to suppress a shoot-through current at the time of the circuit operation as much as possible. It should be noted here that a gate electrode of a TFT  1201  is connected to a high potential side power supply VDD and remains in an ON state as long as the gate-source voltage of the TFT  1201  becomes lower than the threshold value. Therefore, even when a TFT  1202  is turned ON, it is possible to obtain L level output by setting a current ability of the TFT  1202  larger than that of the TFT  1201 , though the output does not become completely equal to VSS. 
     In such a case, even when an amplitude of the input signal is in a range of VDD to VSS, the amplitude may be attenuated after passing through the stages of inverters one after another due to an influence of the threshold values of the TFTs  1201  and  1211 , as shown in  FIG. 12D . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above, and an object of the present invention is therefore to provide a circuit which is formed of TFTs of a single polarity and which is capable of operating without causing such amplitude attenuation of an output signal as described above. 
     To solve the above problems, the present invention employs the following measures. 
     In an inverter shown in  FIG. 2 , the cause of generation of output amplitude attenuation is as follows. That is, when L level is inputted to an input terminal (In) and H level is inputted to an inverted input terminal (Inb), a potential applied to a gate electrode of a TFT  201  is equal to a potential on the input electrode side of the TFT  201 , that is, a high potential side power supply VDD. Therefore, a potential of an output terminal (Out) is only allowed to increase up to (VDD−VthN). 
     In other words, when H level appears at the output terminal (Out), in order to obtain a state in which its potential is equal to VDD, the potential applied to the gate electrode of the TFT  201  needs to be higher than VDD, or to be precise, higher than (VDD+VthN). 
     Therefore, in the present invention, to solve the above problems, a capacitor means is employed to store a charge equivalent to a threshold voltage of the TFT  201  in advance. When a signal is inputted thereto, the charge thus stored is added to the input signal, whereby the potential applied to the gate electrode of the TFT  201  is raised to (VDD+VthN). 
     According to the present invention, there is provided a semiconductor device comprising first to fourth transistors and a capacitor means, characterized in that: 
     the first to fourth transistors each have the same conductivity type; 
     a first electrode of the capacitor means is electrically connected to a first signal input terminal, and a second electrode of the capacitor means is electrically connected to a gate electrode of the first transistor; 
     a gate electrode of the second transistor is electrically connected to a second signal input terminal; 
     an input electrode of the first transistor is electrically connected to a first power supply, and an output electrode of the first transistor is electrically connected to a signal output terminal; 
     an input electrode of the second transistor is electrically connected to a second power supply, and an output electrode of the second transistor is electrically connected to the signal output terminal; 
     a gate electrode and an output electrode of the third transistor each are electrically connected to the signal output terminal, and an input electrode of the third transistor is electrically connected to the second electrode of the capacitor means; and 
     a gate electrode and an output electrode of the fourth transistor each are electrically connected to the second electrode of the capacitor means, and an input electrode of the fourth transistor is electrically connected to the first electrode of the capacitor means. 
     In addition, according to the present invention, there is provided a semiconductor device comprising first to fourth transistors and a capacitor means, characterized in that: 
     the first to fourth transistors each have the same conductivity type; 
     a first electrode of the capacitor means is electrically connected to a first signal input terminal, and a second electrode of the capacitor means is electrically connected to a gate electrode of the first transistor; 
     a gate electrode of the second transistor is electrically connected to a second signal input terminal; 
     an input electrode of the first transistor is electrically connected to a first power supply, and an output electrode of the first transistor is electrically connected to a signal output terminal; 
     an input electrode of the second transistor is electrically connected to a second power supply, and an output electrode of the second transistor is electrically connected to the signal output terminal; 
     a gate electrode and an output electrode of the third transistor each are electrically connected to the signal output terminal, and an input electrode of the third transistor is electrically connected to the second electrode of the capacitor means; and 
     a gate electrode of the fourth transistor is electrically connected to the second electrode of the capacitor means, an input electrode of the fourth transistor is electrically connected to the first electrode of the capacitor means, and an output electrode of the fourth transistor is electrically connected to the signal output terminal. 
     According to the present invention, the capacitor means is a capacitor means storing a threshold voltage of the fourth transistor, and it is characterized in that the stored voltage is added to a potential of a signal inputted from the first signal input terminal, and the thus obtained potential is applied to the gate electrode of the first transistor. With this structure, a gate-source voltage of the first transistor is at least the threshold value all the time, making it possible to obtain the output without causing the amplitude attenuation. 
     Further, according to the present invention, it is characterized in that the semiconductor device is consist of transistors of a single polarity, i.e., consist of only n-channel transistors or only p-channel transistors. With this structure, it is possible to simplify manufacturing steps of a display device. 
     In a display device of the present invention, the capacitor means may be formed of a capacitance between the gate electrode and the input electrode of the fourth transistor, or formed of two materials selected from the group consisting of an active layer material, a material for forming a gate electrode, and a wiring material, and an insulating layer between the two materials. 
     In the display device of the present invention, it is characterized in that a signal inputted to the second signal input terminal is obtained by inverting the polarity of a signal inputted to the first signal input terminal. With this structure, when a signal appearing at the output terminal is either H level or L level, no shoot-through current is generated between a power supply VDD and a power supply VSS in a circuit, making it possible to reduce the consumption current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are diagrams for explaining a circuit structure and operation of an embodiment mode of the present invention; 
         FIG. 2  is a diagram for explaining an inverter formed of TFTs of a single polarity and an operation thereof; 
         FIGS. 3A and 3B  are diagrams for explaining a potential at each node at the time of the circuit operation in the embodiment mode of the present invention; 
         FIG. 4  is a diagram showing an embodiment of the present invention with a different structure from that of the embodiment mode; 
         FIGS. 5A and 5B  are diagrams for explaining sectional structures of a bottom gate type TFT and a dual gate type TFT; 
         FIGS. 6A to 6G  are diagrams showing examples of electronic devices to which the present invention can be applied; 
         FIGS. 7A to 7C  are diagrams showing an example of manufacturing steps of a liquid crystal display device; 
         FIGS. 8A to 8C  are diagrams showing an example of manufacturing steps of a liquid crystal display device; 
         FIGS. 9A to 9C  are diagrams showing an example of manufacturing steps of an active matrix substrate including a circuit formed of p-channel TFTs; 
         FIGS. 10A and 10B  are diagrams showing an example of manufacturing steps of a light emitting device; 
         FIGS. 11A and 11  are diagrams showing an example of manufacturing steps of a light emitting device; 
         FIGS. 12A to 12D  are diagrams for explaining a structure in which inverters consisting of TFTs of a single polarity are connected in a plurality of stages and an operation thereof; and 
         FIGS. 13A and 13B  are diagrams showing an example of a driver circuit of the present invention, which is consisting of p-channel TFTs. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment Mode 
     A fundamental circuit structure of the present invention is shown in  FIG. 1A . The circuit operates in the same manner as an inverter shown in  FIG. 2 , and is of a dual input and single output type. A signal, which is obtained by inverting the polarity of a signal inputted to an input terminal (In), appears at an output terminal (Out). 
     The circuit is structured by TFTs  101  to  104  and a capacitor means  105 . 
     An operation of the circuit is explained. In  FIGS. 3A and 3B , a potential at each node at the time of the circuit operation is shown. First, when L level is inputted to a first input terminal (In 1 ) and H level is inputted to a second input terminal (In 2 ), the TFT  102  is turned ON, and a potential of the output terminal (Out) begins to decrease toward VSS. At this point, the potential of the output terminal (Out) has not lowered yet to reach L level, and the TFT  103  thus remains in an ON state. Then, a current flows from the output terminal (Out) to the capacitor means  105 , and a potential applied to a gate electrode of the TFT  104  increases. Therefore, the TFT  104  is also turned ON. As the potential of the output terminal (Out) further decreases, a gate-source voltage of the TFT  103  becomes equal to VthN, with the TFT  103  being turned OFF. At this point, even when the TFT  104  is still in an ON state, a charge accumulated in the capacitor means  105  is discharged through the TFT  104 . Then, a gate-source voltage of the TFT  104  continuously decreases so that the TFT  104  will be turned OFF before long. 
     With this structure, a threshold voltage VthN of the TFT  104  is stored in the capacitor means  105 . At the first input terminal (In 1 ), L level appears and its potential is VSS. Thus the potential applied to the gate electrode of the TFT  101  is higher than VSS by a voltage stored in the capacitor means  105 . That is, the potential at this time applied to the gate electrode of the TFT  101  is (VSS+VthN). Since L level appears at the output terminal (Out) and its potential is VSS, a gate-source voltage of the TFT  101  is VthN, and the TFT  101  is turned OFF ( FIG. 3A ). 
     Further, an operation of the circuit is explained when H level is inputted to the first input terminal (In 1 ) and L level is inputted to the second input terminal (In 2 ). First, at the second input terminal (In 2 ), H level is switched to L level, and the TFT  102  is turned OFF. On the other hand, at the first input terminal (In 1 ), L level is switched to H level. At this time, the TFT  103  remains in an OFF state so that no transfer of the charge stored in the capacitor means  105  occurs. As to the TFT  104 , a potential of a source region thereof increases, whereas a gate-source voltage is VthN as it stands, the TFT  104  remaining in an OFF state. Therefore, even when L level is switched to H level at the first input terminal (In 1 ), the voltage between both electrodes of the capacitor means  105  is still stored. Accordingly, since the potential of the first input terminal (In 1 ) increases to VDD from VSS, the potential applied to the gate electrode of the TFT  101  increases to (VDD+VthN) from (VSS+VthN). Therefore, H level appears at the output terminal (Out) with its potential being equal to VDD ( FIG. 3 ). 
     In accordance with the operation descried above, it is possible to normally obtain from the signal input having an amplitude in a range of VDD to VSS, the output having the amplitude in the same range without the amplitude attenuation. As a result, it is possible to manufacture the semiconductor device formed of the TFTs of a single polarity with employing the methods described above. This contributes to the reduction in the number of manufacturing steps as well as in manufacturing costs. 
     Embodiments 
     Embodiments of the present invention are described below. 
     Embodiment 1 
       FIG. 4  shows a circuit structure in which connections of the circuit shown in  FIG. 1A  are partially modified. In  FIG. 1A , the output electrode of the TFT  104  is connected to the gate electrode of the TFT  101 , whereas it is connected to the output terminal (Out) in  FIG. 4 . 
     An operation of the circuit is the same as described in the embodiment mode so that no explanation thereof is given here. The gate electrode of the TFT  101  is now considered in the circuit structure. In the circuit shown in  FIG. 1A , even after the TFT  103  is turned OFF, the charge can be transferred through the TFT  104  to some extent. However, in the circuit shown in  FIG. 4 , no transfer passage exists for the charge accumulated in the gate electrode of the TFT  101  when the TFT  103  is turned OFF. If a fluctuation is supposed to be generated in threshold values of the TFTs which form the circuit, there is a possibility that the gate-source voltage of the TFT  101  does not sufficiently decrease to the level equal to the threshold value of the TFT  101 . In consideration of the above, by setting a current ability of the TFT  102  larger than that of the TFT  101 , it is possible to obtain the normal L level output even when the TFT  101  is not completely turned OFF. 
     Embodiment 2 
     In the following, a method of simultaneously manufacturing TFTs of driving circuit portions provided in the pixel portion and the periphery thereof on the same substrate is described. Although the step of manufacturing a liquid crystal display device is shown in this embodiment, the present invention is not limited to the liquid crystal display device as mentioned above. 
     First, as shown in  FIG. 7A , a base film  5002  made of an insulating film such as a silicon oxide film, a silicon nitride film, or a silicon nitride oxide film is formed on a substrate  5001  made of glass such as barium borosilicate glass or alumino borosilicate glass, typified by #7059 glass or #1737 glass of Corning Inc. For example, not shown in figures particularly, a silicon nitride oxide film fabricated from SiH 4 , NH 3  and N 2 O by a plasma CVD method is formed with a thickness of 10 to 200 nm (preferably 50 to 100 nm), and a hydrogenated silicon nitride oxide film similarly fabricated from SiH 4  and N 2 O is formed with a thickness of 50 to 200 nm (preferably 100 to 150 nm) to form a lamination. 
     Island-like semiconductor layers  5003  to  5005  are formed of a crystalline semiconductor film manufactured by using a laser crystallization method on a semiconductor film having an amorphous structure, or by using a known thermal crystallization method. The thickness of the island-like semiconductor films  5003  to  5005  is set from 25 to 80 nm (preferably between 30 and 60 nm). There is no limitation on the crystalline semiconductor film material, but it is preferable to form the film from silicon or a silicon germanium (SiGe) alloy. 
     A laser such as a pulse oscillation type or continuous emission type excimer laser, a YAG laser, or a YVO 4  laser is used for manufacturing the crystalline semiconductor film in the laser crystallization method. A method of condensing laser light emitted from a laser oscillator into a linear shape by an optical system and then irradiating the light to the semiconductor film may be employed when these types of lasers are used. The crystallization conditions may be suitably selected by the operator, but the pulse oscillation frequency is set to 30 Hz, and the laser energy density is set from 100 to 400 mJ/cm 2  (typically between 200 and 300 mJ/cm 2 ) when using the excimer laser. Further, the second harmonic is utilized when using the YAG laser, the pulse oscillation frequency is set from 1 to 10 kHz, and the laser energy density may be set from 300 to 600 mJ/cm 2  (typically between 350 and 500 mJ/cm 2 ). The laser light which has been condensed into a linear shape with a width of 100 to 1000 μm, for example 400 μm, is then irradiated over the entire surface of the substrate. This is performed with an overlap ratio of 80 to 98%. 
     Next, a gate insulating film  5006  is formed covering the island-like semiconductor layers  5003  to  5005 . The gate insulating film  5006  is formed of an insulating film containing silicon with a thickness of 40 to 150 nm by a plasma CVD method or a sputtering method. A 120 nm thick silicon nitride oxide film is formed in this embodiment. The gate insulating film is not limited to such a silicon nitride oxide film, of course, and other insulating films containing silicon may also be used, in a single layer or in a lamination structure. For example, when using a silicon oxide film, it can be formed by the plasma CVD method with a mixture of TEOS (tetraethyl orthosilicate) and O 2 , at a reaction pressure of 40 Pa, with the substrate temperature set from 300 to 400° C., and by discharging at a high frequency (13.56 MHz) with electric power density of 0.5 to 0.8 W/cm 2 . Good characteristics of the silicon oxide film thus manufactured as a gate insulating film can be obtained by subsequently performing thermal annealing at 400 to 500° C. 
     A first conductive film  5007  and a second conductive film  5008  are then formed on the gate insulating film  5006  in order to form gate electrodes. In this Embodiment, the first conductive film  5007  is formed from tantalum (Ta) with a thickness of 50 to 100 nm, and the second conductive film  5008  is formed from tungsten (W) with a thickness of 100 to 300 nm ( FIG. 7A ). 
     The Ta film is formed by sputtering, which a Ta target is sputtered by using Ar. If an appropriate amount of Xe or Kr is added to the Ar during sputtering, the internal stress of the Ta film will be relaxed, and film peeling can be prevented. The resistivity of an α phase Ta film is on the order of 20 μΩcm, and the α phase Ta film can be used for the gate electrode, but the resistivity of β phase Ta film is on the order of 180 μΩcm and the β phase Ta film is unsuitable for the gate electrode. The α phase Ta film can easily be obtained if a tantalum nitride (TaN) film, which possesses a crystal structure near that of α phase Ta, is formed with a thickness of 10 to 50 nm as a base for Ta in order to form the α phase Ta film. 
     The W film is formed by sputtering with W as a target. The W film can also be formed by a thermal CVD method using tungsten hexafluoride (WF 6 ). Whichever is used, it is necessary to make the film low resistant in order to use it as the gate electrode, and it is preferable that the resistivity of the W film be set 20 μΩcm or less. The resistivity can be lowered by enlarging the crystal grain of the W film, but for cases where there are many impurity elements such as oxygen within the W film, crystallization is inhibited, and the film becomes high resistant. Therefore, a W target having a purity of 99.9999% is thus used in sputtering. In addition, by forming the W film while taking sufficient care such that no impurities from the inside of the gas phase are introduced at the time of film formation, a resistivity of 9 to 20 μΩcm can be achieved. 
     Note that although the first conductive film  5007  and the second conductive film  5008  are formed from Ta and W, respectively, in this embodiment, the conductive films are not limited to these. Both the first conductive film  5007  and the second conductive film  5008  may also be formed from an element selected from the group consisting of Ta, W, Mo, Al, and Cu, or from an alloy material or a chemical compound material having one of these elements as its main constituent. Further, a semiconductor film, typically a polysilicon film, into which an impurity element such as phosphorous is doped, may also be used. Examples of preferable combinations other than that in this embodiment include: the first conductive film formed from TaN and the second conductive film formed from W; the first conductive film formed from TaN and the second conductive film formed from Al; and the first conductive film formed from TaN and the second conductive film formed from Cu. 
     Next, a mask  5009  is formed from resist, and a first etching process is performed in order to form electrodes and wirings. An ICP (inductively coupled plasma) etching method is used in this embodiment. A gas mixture of CF 4  and Cl 2  is used as an etching gas, and a plasma is generated by applying a 500 W RF electric power (13.56 MHz) to a coil shape electrode at 1 Pa. A 100 W RF electric power is also applied to the substrate side (test sample stage), effectively applying a negative self-bias voltage thereto. The W film and the Ta film are both etched on the same order when CF 4  and Cl 2  are mixed as the etching gas. 
     Edge portions of the first conductive layer and the second conductive layer are made into a tapered shape by using a suitable resist mask shape and the effect of the bias voltage applied to the substrate side with the above etching conditions. The angle of the tapered portions is from 15 to 45°. The etching time may be increased by approximately 10 to 20% in order to perform etching without leaving any residue on the gate insulating film. The selectivity of a silicon nitride oxide film with respect to a W film is from 2 to 4 (typically 3), and therefore approximately 20 to 50 nm of the exposed surface of the silicon nitride oxide film is etched by this over-etching process. First shape conductive layers  5010  to  5013  are thus formed of the first conductive layers  5010   a  to  5013   a  and the second conductive layers  5010   b  to  5013   b  by the first etching process. At this point, regions of the gate insulating film  5006  not covered by the first shape conductive layers  5010  to  5013  are made thinner by approximately 20 to 50 nm by etching ( FIG. 7B ). 
     Then, a first doping process is performed to add an impurity element for imparting an n-type conductivity ( FIG. 7B ). Doping may be carried out by an ion doping method or an ion injecting method. The condition of the ion doping method is that a dosage is 1×10 13  to 5×10 14  atoms/cm 2 , and an acceleration voltage is 60 to 100 keV. As the impurity element for imparting the n-type conductivity, an element belonging to group 15, typically, phosphorus (P) or arsenic (As) is used, but phosphorus is used here. In this case, the conductive layers  5010  to  5013  become masks to the impurity element to impart the n-type conductivity, and first impurity regions  5014  to  5016  are formed in a self-aligning manner. The impurity element to impart the n-type conductivity in the concentration range of 1×10 20  to 1×10 21  atoms/cm 3  is added to the first impurity regions  5014  to  5016 . 
     Next, as shown in  FIG. 7C , a second etching process is performed. 
     The ICP etching method is similarly used, so that CF 4 , Cl 2  and O 2  are mixed with an etching gas, and RF electric power of 500 W is supplied to the coil shape electrode at a pressure of 1 Pa to generate plasma. RF electric power of 50 W is supplied to the substrate side (test sample stage), and a lower self-bias voltage in comparison with the self-bias voltage in the first etching process is applied to thereon. Anisotropic etching of a W film as the second conductive layer is performed under such a condition, and anisotropic etching of the Ta film as the first conductive layer is performed at an etching speed slower than that of the anisotropic etching of the W film so that a second shape conductive layers  5017  to  5020  (first conductive layers  5017   a  to  5020   a  and second conductive layers  5017   b  to  5020   b ) are formed. A region of the gate insulating film  5006  which is not covered with the second shape conductive layers  5017  to  5020  is further etched by about 20 to 50 [nm] so that a thinned region is formed. 
     An etching reaction of the W film or the Ta film by the mixture gas of CF 4  and Cl 2  can be guessed from a generated radical or ion species and the vapor pressure of a reaction product. When the vapor pressures of fluoride and chloride of W and Ta are compared with each other, the vapor pressure of WF 6  being fluoride of W is extremely high, and other WCl 5 , TaF 5 , and TaCl 5  have almost equal vapor pressures. Thus, in the mixture gas of CF 4  and Cl 2 , both the W film and the Ta film are etched. However, when a suitable amount of O 2  is added to this mixture gas, CF 4  and O 2  react with each other to form CO and F, and a large number of F radicals or F ions are generated. As a result, an etching rate of the W film having the high vapor pressure of fluoride is increased. On the other hand, with respect to Ta, even if F is increased, an increase of the etching rate is relatively small. Besides, since Ta is easily oxidized as compared with W, the surface of Ta is oxidized by addition of O 2 . Since the oxide of Ta does not react with fluorine or chlorine, the etching rate of the Ta film is further decreased. Accordingly, it becomes possible to make a difference between the etching rates of the W film and the Ta film, and it becomes possible to make the etching rate of the W film higher than that of the Ta film. 
     Then, as shown in  FIG. 7C , a second doping process is performed. In this case, a dosage is made lower than that of the first doping process and under the condition of a high acceleration voltage, an impurity element for imparting the n-type conductivity is doped. For example, the process is carried out with an acceleration voltage set to 70 to 120 keV and at a dosage of 1×10 13  atoms/cm 2 , so that new impurity regions are formed inside of the first impurity regions formed into the island-like semiconductor layers in  FIG. 7B . Doping is carried out such that the second conductive layers  5017   b  to  5020   b  are used as masks to the impurity element and the impurity element is added also to the regions under the first conductive layers  5017   a  to  5020   a . In this way, second impurity regions  5021  to  5023  overlapping with the first conductive layers are formed. 
     As shown in  FIG. 8A , a third etching process is performed. Cl 2  is used as an etching gas here, and the third etching process is performed by ICP etching device. In this embodiment, the etching is performed for 70 seconds under the condition that a gas flow rate of Cl 2  is set to 60 sccm and RF electric power of 350 W is supplied to the coil shape electrode at a pressure of 1 Pa to generate plasma. RF electric power is also supplied to the substrate side (test sample stage), substantially the negative self-bias voltage is applied thereof. According to the third etching process, the first conductive layers are reduced whereby forming third shape conductive layers  5024  to  5027  (first conductive layers  5024   a  to  5027   a  and second conductive layers  5024   b  to  5027   b ). At this point, a part of the second impurity regions  5021  to  5023  is third impurity regions  5028  to  5030 , which are not overlapping with the first conductive layers. 
     Impurity regions are formed into the respective island-like semiconductor layers by the above mentioned step. The third shape conductive layers  5024  to  5027  overlapping with the island-like semiconductor films function as a gate electrode of TFTs. 
     A step of activating the impurity elements added in the respective island-like semiconductor layers for the purpose of controlling the conductivity type is conducted. This step is carried out by a thermal annealing method using a furnace annealing oven. In addition, a laser annealing method or a rapid thermal annealing method (RTA method) can be applied. The thermal annealing method is performed in a nitrogen atmosphere having an oxygen concentration of 1 ppm or less, preferably 0.1 ppm or less and at 400 to 700° C., typically 500 to 600° C. In this embodiment, a heat treatment is conducted at 500° C. for four hours. However, in the case where a wiring material used for the third shape conductive layers  5024  to  5027  is weak to heat, it is preferable that the activation is performed after an interlayer insulating film (containing silicon as its main constituent) is formed to protect the wiring line or the like. 
     Further, a heat treatment at 300 to 450° C. for 1 to 12 hours is conducted in an atmosphere containing hydrogen of 3 to 100%, and a step of hydrogenating the island-like semiconductor layers is conducted. This step is a step of terminating dangling bonds in the semiconductor layer by thermally excited hydrogen. As another means for thermal hydrogenation, plasma hydrogenation (using hydrogen excited by plasma) may be carried out. 
     Next, as shown in  FIG. 8B , a first interlayer insulating film  5031  made of a silicon nitride oxide film having a thickness of 100 to 200 nm is formed. A second interlayer insulating film  5032  made of an organic insulating material formed thereon. Contact holes are then formed with respect to the first interlayer insulating film  5031 , the second interlayer insulating film  5032 , and the gate insulating film  5006 , films are formed by wiring material, respective wirings  5033  to  5036  are formed by patterning, and then, a pixel electrode  5037  is formed by patterning. 
     Next, the film made from organic resin is used for the second interlayer insulating film  5032 . As the organic resin, polyimide, polyamide, acryl, BCB (benzocyclobutene) or the like can be used. Especially, since the second interlayer insulating film  5032  has rather the meaning of flattening, acryl excellent in flatness is desirable. In this embodiment, an acryl film is formed to such a thickness that stepped portions formed by the TFTs can be adequately flattened. The thickness is preferably made 1 to 5 μm (more preferably 2 to 4 μm). 
     In the formation of the contact holes, dry etching or wet etching is used, and contact holes reaching the n-type impurity regions  5014  to  5016 , a contact hole reaching a source signal line (not illustrated), a contact hole reaching a gate signal line (not illustrated); a contact hole reaching a power supply line, and contact hole reaching the gate electrodes  5024  to  5026  (not illustrated) are formed, respectively. 
     Further, the wirings  5033  to  5036  are formed by patterning into a desired shape a film consisting of a three layer laminate in which a 100 nm thick Ti film, a 300 nm thick Al film containing Ti, and a 150 nm thick Ti film are continuously formed by sputtering. Other conductive materials may of course be used. The pixel electrode  5037  is formed of a material having a high reflectivity when a display device is of a reflection type. In this case, it may be formed simultaneously with the wirings. On the other hand, in a case of a transmission type display device, the pixel electrode  5037  is formed of a transparent conductive material such as indium tin oxide (ITO). In this specification, the substrate which has reached the state shown in  FIG. 8B  through conducting steps is referred to as active matrix substrate. 
     Then, an opposing substrate  5038  is prepared. A light shielding film  5039  is formed on the opposing substrate  5038 . The light shielding film is made of chrome (Cr) or the like with a thickness of 100 to 200 nm. 
     On the other hand, an opposing electrode  5040  is formed in the pixel portion. The opposing electrode is made of a transparent conductive material such as ITO. Further, a film thickness of the opposing electrode is desirably 100 to 120 nm to keep the transmission of visible light high. 
     On the active matrix substrate and the opposing substrate, orientation films  5041  and  5042  are formed. It is preferred that the orientation films  5041  and  5042  have a thickness of 30 to 80 nm. As the orientation film, SE7792 manufactured by Nissan Chemical Industries, Ltd. may be used for example. By using an orientation film with a high pretilt angle, it is possible to suppress the generation of disclination at the time of driving the liquid crystal display device driven with the active matrix method. 
     Then, the orientation films  5041  and  5042  are rubbed. Preferably, the direction of rubbing shows the counterclockwise TN (twisted nematic) orientation when the liquid crystal display device is completed. 
     Though not particularly shown in Embodiment 2, spacers may be formed in pixels by dispersion or patterning, thereby improving cell gap uniformity. In Embodiment 2, a photosensitive resin film is formed and is then subjected to patterning to form the spacers with a height of 4.0 μm. 
     Further, the active matrix substrate and the opposing substrate are bonded to each other with a sealant  5043 . As the sealant, there is used XN-21S manufactured by Mitsui Chemicals which is of a thermosetting type. In the sealant, a filler is mixed. The height of the filler is 4.0 μm. After the sealant is cured, the active matrix substrate and the opposing substrate are simultaneously cut into a desired size. 
     Subsequently, liquid crystal  5044  is injected. As the liquid crystal material, one having the low viscosity is preferred in consideration of high speed responsibility and the like. In Embodiment 2, nematic liquid crystal is used with which the orientation control is easily performed. Needless to say, high speed responsive ferroelectric liquid crystal or antiferroelectric liquid crystal may be used. 
     After the injection of liquid crystal, an injection inlet is sealed with UV setting resin or the like. Then, a polarizing plate is attached employing a known method. Finally, a connector (flexible printed circuit: FPC) is mounted with which terminals drawn from elements or circuits formed on the substrate and external signal terminals are connected, thereby completing the product ( FIG. 8C ). In this specification, the product in such a state that it is ready for shipment as described above is referred to as liquid crystal display device: 
     In accordance with the steps shown in Embodiment 2, only four photomasks are required to form the active matrix substrate (that is, island-like semiconductor layer pattern, first wiring pattern (gate wiring, island-like source wiring, and capacitor wiring), contact hole pattern, and second wiring pattern (which includes pixel electrode and connection electrode). As a result, the number of steps can be reduced, contributing to the reduction in manufacturing costs and improvement of yield. 
     In Embodiment 2, the top gate type TFT is explained as an example. In addition, the embodiment can be implemented by using a bottom gate type TFT including a gate electrode formed below an active layer, as shown in  FIG. 5A , or a dual gate type TFT containing gate electrodes vertically located so as to sandwich an active layer therebetween, as shown in  FIG. 5B . 
     Embodiment 3 
     The steps shown in Embodiment 2 are explained as an example of a case in which pixels and peripheral driver circuits are formed of n-channel TFTs. However, it is possible to implement the present invention by using p-channel TFTs. 
     In the case of n-channel TFTs, an impurity region, which is called overlap region, is provided in a region overlapping a gate electrode to inhibit the hot carrier degradation etc. On the other hand, in the case of p-channel TFTs, an influence due to the hot carrier degradation is small so that there is no need to particularly provide the overlap region. In this case, the pixels and peripheral driver circuits can be manufactured through simpler steps. 
     As shown in  FIG. 9A , according to Embodiment 2, a base film  6002  is formed on an insulating substrate  6001  made of glass or the like. Then, island-like semiconductor layers  6003  to  6005 , a gate insulating film  6006 , and conductive layers  6007  and  6008  are formed thereon. Although the conductive layers  6007  and  6008  are laminated here, they may be a composed of a single layer. 
     Then, as shown in  FIG. 9B , resist masks  6009  are formed to conduct first etching processing. In Embodiment 2, anisotropic etching is performed with utilization of a selection ratio between materials of the laminated conductive layers. However, since there is no need to provide a region functioning as an overlap region in this example, normal etching may be alternatively performed. At this time, in the gate insulating film  6006  there is formed a region which is thinned by about 20 to 50 nm due to etching in comparison with other regions. 
     Subsequently, first doping processing is conducted to dope an impurity element which imparts p-type conductivity to the island-like semiconductor layers. Conductive layers  6010  to  6013  are used as masks against the impurity element, and impurity regions are formed in a self-aligning manner. Boron (B) or the like is representative of the impurity element imparting p-type conductivity. In this example, the impurity regions are formed by ion doping with diborane (B 2 H 6 ) such that the semiconductor layers have the impurity concentration of 2×10 20  to 2×10 21  atoms/cm 3 . 
     The resist masks are removed to obtain the state shown in  FIG. 9C . From this stage, the pixels and peripheral driver circuits are manufactured in accordance with the step shown in  FIG. 8B  of Embodiment 2 and the subsequent steps. Thus, the present invention can be implemented by using the p-channel TFTs. 
     The circuit structure is similar to the structure with the n-channel TFTs as shown in  FIG. 1A . However, the power supply has different connections from those of  FIG. 1A  in which the high potential side power supply VDD and the low potential side power supply VSS are switched. 
     Embodiment 4 
     In Embodiment 4, manufacturing steps of a light emitting device which uses in a pixel portion a light emitting element such as an electroluminescent (EL) element. 
     In accordance with the manufacturing steps shown in Embodiment 2, a state in which first and second interlayer insulating films are formed so far is obtained, as shown in  FIGS. 8A and 8B . 
     Then, contact holes are opened, as shown in  FIG. 10A . The contact holes are formed by dry etching or wet etching so as to reach the impurity region, source signal line, gate signal line, current supply line, and gate electrode, respectively. 
     An anode  7001  of the EL element is formed by depositing a transparent conductive film represented by an ITO film and patterning it into a desired shape. A laminate film consisting of Ti layer, Al layer containing Ti, and Ti layer is formed, and it is patterned into a desired shape, forming wiring electrodes  7002  to  7005  and a pixel electrode  7006 . The thickness of the respective layers may be the same as in Embodiment 2. The pixel electrode  7006  is formed so as to overlap the anode  7001  formed in the earlier stage, thereby establishing a contact therebetween. 
     Next, a third interlayer insulating film  7007  is formed by preparing an insulating film made of, for example, an organic resin material such as acrylic and forming an opening portion at a position corresponding to the anode  7001  of the EL element. It is preferred that the opening portion is formed to have gently tapered side walls. If the tapered shape of the side walls of the opening portion is not sufficiently gentle, degradation and cut step of the EL layer due to the existence of steps become serious problems so that attentions should be given thereto. 
     After forming an EL layer  7008 , a cathode  7009  of the EL element is formed of cesium (Cs) with a thickness of 2 nm or less and silver (Ag) with a thickness of 10 nm or less. By making the film of the cathode  7009  of the EL element extremely thin, light generated at the EL layer is transmitted through the cathode  7009  to be emitted. 
     Subsequently, a protective film  7010  is formed for the protection of the EL element. Thereafter, attachment of an FPC and other operations are conducted, thus completing the light emitting device. 
       FIG. 10B  shows a detailed structure of the EL element in the light emitting device shown in  FIG. 10A  according to Embodiment 4. An anode  7101  of the EL element is made of a transparent conductive film represented by an ITO film. Reference numeral  7102  denotes an EL layer containing a light emitting layer. A cathode of the EL element is made of a Cs film  7103  and an Ag film  7104 , each of which is formed extremely thin. Denoted by reference numeral  7105  is a protective film. 
     By making a region of the EL element on the cathode side extremely thin, light generated at the EL layer  7102  is transmitted through the Cs film  7103  and the Ag film  7104  forming the cathode to be emitted upward. That is, the region where TFTs are formed does not overwhelm the area of the light emitting surface so that the aperture ratio can be set to almost 100%. 
     In this example, the emission direction of light faces the side where the cathode is formed. If it is not desired that the light transmission is made toward the side of the anode made of ITO, it is preferred that a second interlayer insulating film  7000  is formed of an opaque film colored, in block or the like. 
     In the above steps, there is described the structure in which the cathode is formed just above the EL layer and the anode is formed just under the EL layer. If the pixel electrode under the EL layer is made of TiN etc., and the electrode above the EL layer is made of ITO etc., it is possible to arrange the anode just above the EL layer and the cathode just under the EL layer. 
     The following structure may also be adopted, though the aperture ratio is slightly lowered. The anode is arranged just under the EL layer, the cathode is arranged just above the EL layer, the electrode under the EL layer is made of ITO etc., and the electrode above the EL layer is made of MaAg etc., which is different form Embodiment 4, whereby light generated at the EL layer is emitted toward the substrate side where the TFTs are formed, or downward. 
     Embodiment 5 
     In Embodiment 5, steps of manufacturing a light emitting device in a different manner from that of Embodiment 4 are described. 
     In accordance with the manufacturing steps shown in Embodiment 2, the state in which the first and second interlayer insulating films are formed so far is obtained, as shown in  FIGS. 8A and 8B . 
     Then, contact holes are opened, as shown in  FIG. 11A . The contact holes are formed by dry etching or wet etching so as to reach the n-type impurity region, source signal line, gate signal line, current supply line, and gate electrode, respectively. 
     Next, wirings  7201  to  7204  are formed, and a pixel electrode  7205 , which serves as the anode of the EL element, is formed as a laminate film which consists of Ti film, Al film containing Ti, Ti film, and transparent conductive film. 
     Then, a third interlayer insulating film  7206  is formed by preparing an insulating film made of, for example, an organic resin material such as acrylic and forming an opening portion at a position corresponding to the anode  7205  of the EL element. It is preferred that the opening portion is formed to have gently tapered side walls. If the tapered shape of the side walls of the opening portion is not sufficiently gentle, degradation and cut step of the EL layer due to the existence of steps become serious problems so that attentions should be given thereto. 
     After forming an EL layer  7207 , a cathode  7208  of the EL element is formed of cesium (Cs) with a thickness of 2 nm or less and silver (Ag) with a thickness of 10 nm or less. By making the film thickness of the cathode  7208  of the EL element extremely thin, light generated at the EL layer is transmitted through the cathode  7208  to be emitted. 
     Subsequently, a protective film  7209  is formed for protection of the EL element. Thereafter, attachment of an FPC and other operations are conducted, thus completing the light emitting device. 
       FIG. 11B  shows a detailed structure of the EL element in the light emitting device shown in  FIG. 11A  according to Embodiment 5. An anode of the EL element is made of a metal film  7301  made of the laminate of Ti, Al, and Ti films, and a transparent conductive film  7302  represented by is an ITO film. Reference numeral  7303  denotes an EL layer containing a light emitting layer. A cathode of the EL layer is made of a Cs film  7304  and an Ag film  7305 , each of which is formed extremely thin. Denoted by reference numeral  7306  is a protective film. 
     In the light emitting device manufactured in accordance with Embodiment 5, the aperture ratio can be advantageously set to nearly 100%, as in the display device of Embodiment 4 described below. Further, when forming wiring electrodes and pixel electrodes, it is possible to perform patterning on the metal film made of the laminate including Ti film, Al film, and Ti film, and on the transparent conductive film with a common photomask. Thus, it is possible to reduce the number of photomasks and simplify the manufacturing steps. 
     In the above steps, there is described the structure in which the cathode is formed just above the EL layer, the anode is formed just under the EL layer. If the pixel electrode under the EL layer is made of TiN etc., and the electrode above the EL layer is made of ITO etc., it is possible to arrange the anode just above the EL layer and the cathode just under the EL layer. 
     The following structure may also be adopted, though the aperture ratio is slightly lowered. The anode is arranged just under the EL layer, the cathode is arranged just above the EL layer, the electrode under the EL layer is made of ITO etc., and the electrode above the EL layer is made of MaAg etc., which is different form Embodiment 5, whereby light generated at the EL layer is emitted toward the substrate side where the TFTs are formed, or downward. 
     Embodiment 6 
     The present invention can be implemented by using p-channel TFTs. In Embodiment 6, the structure and operation are explained. 
       FIG. 13A  shows a structure of a circuit. The circuit is a dual input and single output type inverter structured by TFTs  1301  to  1304  and a capacitor means  1305 . A signal obtained by inverting the polarity of a signal inputted to an input terminal (In) appears at an output terminal (Out). 
     An operation of the circuit is explained. First, when H level is inputted to a first input terminal (In 1 ) and L level is inputted to a second input terminal (In 2 ), then the TFT  1302  is turned ON, and the potential of the output terminal (Out) begins to increase toward VDD. At this time, the potential of the output terminal (Out) does not reach as high as H level, the TFT  1303  thus remains in an ON state. A current flows from the capacitor means  1305  toward the output terminal (Out), a potential applied to a gate electrode of the TFT  1304  decreases, and the TFT  1304  is also turned ON. As the potential of the output terminal (Out) further increases, a gate-source voltage of the TFT  1303  becomes equal to VthP, with the TFT  1303  being turned OFF. At this point, even when the TFT  1304  is still in an ON state, a charge accumulated in the capacitor means  1305  is discharged through the TFT  1304 . Then, a gate-source voltage of the TFT  1304  continuously decreases so that the TFT  1304  will be turned OFF before long. 
     With this structure, a threshold voltage VthP of the TFT  1304  is stored in the capacitor means  1305 . At the first input terminal (In 1 ), H level appears and its potential is VDD. Thus the potential applied to the gate electrode of the TFT  1301  is lower than VDD by a voltage stored in the capacitor means  1305 . That is, the potential applied to the gate electrode of the TFT  1301  is (VDD−VthP) at this time. Since H level appears at the output terminal (Out) and its potential is VDD, the gate-source voltage of the TFT  1301  is VthP, and the TFT  1301  is turned OFF. 
     Further, an operation of the circuit is explained when L level is inputted to the first input terminal (In 1 ) and H level is inputted to the second input terminal (In 2 ). First, at the second input terminal (In 2 ), L level is switched to H level, and the TFT  1302  is turned OFF. On the other hand, at the first input terminal (In 1 ), H level is switched to L level. At this time, the TFT  1303  remains in an OFF state so that no transfer of the charge stored in the capacitor means  1305  occurs. As to the TFT  1304 , a potential of a source region thereof decreases, whereas a gate-source voltage is VthP as it stands, the TFT  1304  remaining in an OFF state. Therefore, even when H level is switched to L level at the first input terminal (In 1 ), the voltage between both electrodes of the capacitor means  1305  is still stored. Accordingly, since the potential applied to the first input terminal (In 1 ) decreases to VSS from VDD, the potential applied to the gate electrode of the TFT  1301  decreases to (VSS−VthP) from (VDD−VthP). Thus, L level appears at the output terminal (Out) with its potential being equal to VSS. 
     In accordance with the operation descried above, even when the circuit is structured by p-channel TFTs, it is possible to normally obtain, from the signal input having an amplitude in a range of VDD to VSS, the output having the amplitude in the same range without the amplitude attenuation. 
     Embodiment 7 
     The present invention can be applied to manufacturing the display device of various electric equipments. As such electronic equipments, there are pointed out a portable information terminal (electronic book, mobile computer, cellular phone of the like), a video camera, digital camera, a personal computer, a television, a cellular phone and the like. Example of these are shown in  FIGS. 6A to 6G . 
       FIG. 6A  illustrates a liquid crystal display or OLED display constituted by a casing  3001 , a support stand  3002 , a display portion  3003  or the like. The present invention can be applied to the display portion  3003 . 
       FIG. 6B  illustrates a video camera constituted by a main body  3011 , a display portion  3012 , a audio input portion  3013 , operation switches  3014 , a battery  3015 , an image receiving portion  3016  or the like. The present invention can be applied to the display portion  3012 . 
       FIG. 6C  illustrates a notebook personal computer constituted by a main body  3021 , a casing  3022 , a display portion  3023 , a keyboard  3024  or the like. The present invention can be applied to the display portion  3023 . 
       FIG. 6D  illustrates a portable information terminal, constituted by a main body  3031 , a stylus  3032 , a display portion  3033 , operation switches  3034 , a external interface  3035  or the like. The present invention can be applied to the display portion  3033 . 
       FIG. 6E  illustrates an audio reproduction device, more specifically an audio device mounted in a motor vehicle and constituted by a main body  3041 , a display portion  3042 , operation switches  3043  and  3044  or the like. The present invention can be applied to the display portion  3042 . The invention may be applied to any of portable or home audio devices other than the above-described audio device mounted in a motor vehicle. 
       FIG. 6F  illustrates a digital camera constituted by a main body  3051 , a display portion (A)  3052 , an ocular portion  3053 , operation switches  3054 , a display portion (B)  3055 , a battery  3056  or the like. The present invention can be applied to each of the display portion (A)  3052  and the display portion (B)  3055 . 
       FIG. 6G  illustrates a cellular phone constituted by a main body  3061 , an audio output portion  3062 , an audio input portion  3063 , a display portion  3064 , operating switches  3065 , an antenna  3066  or the like. The present invention can be applied to the display portion  3064 . 
     Note that the above-described devices of this embodiment are only examples and that the invention is not exclusively applied to them. 
     With the circuit of the present invention, it is possible to normally obtain, from the signal input having an amplitude in a range of VDD to VSS, the output having the amplitude in the same range without the amplitude attenuation. Therefore, it is possible to manufacture the driver circuit of the display device with the TFTs of a single polarity in accordance with the methods described above. This contributes to the reduction in the number of the manufacturing steps and the lowering of the manufacturing costs.