Patent Abstract:
It is an object of the present invention to form an organic transistor including an organic semiconductor having high crystallinity without loosing an interface between an organic semiconductor of a channel where carriers are spread out and a gate insulating layer and deteriorating a yield. A semiconductor device according to the present invention has a stacked structure of organic semiconductor layers, and at least the upper organic semiconductor layer is in a polycrystalline or a single crystalline state and the lower organic semiconductor layer is made of a material serving as a channel. Carrier mobility can be increased owing to the upper organic semiconductor layer having high crystallinity; thus, insufficient contact due to the upper organic semiconductor layer can be compensated by the lower organic semiconductor layer.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor device using an organic semiconductor and a manufacturing method thereof.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, a study of an organic transistor using an organic semiconductor has been actively carried out. It is expected that a thin film is easily formed and that an organic semiconductor device is formed over a flexible substrate such as plastic, a film substrate, or a paper which is lightweight with flexibility by making the most of the characteristic of an organic semiconductor. Further, an organic semiconductor material for forming a channel is extremely inexpensive and low in deposition temperature, and a process without using a vacuum chamber such as an ink-jet method, a printing method, a stamp method, or the like can also be applied. Therefore, it is expected to drastically reduce the manufacturing cost of a general semiconductor device using an organic transistor.  
         [0005]     An organic transistor is composed of a glass substrate, a gate electrode, a gate insulating layer, source and drain electrodes, and an organic semiconductor layer. A structure where source and drain electrodes are provided below an organic semiconductor layer is referred to as a bottom contact structure and a structure where a source and drain electrodes are provided over an organic semiconductor layer is referred to as a top contact structure.  
         [0006]     In the case of an organic transistor, carrier mobility can be increased by employing the top contact structure. However, it is difficult to use a step such as photolithography to perform microfabrication of pattern or the like in this structure; thus, microfabrication may be limited to the bottom contact structure where an organic semiconductor layer is formed after forming source and drain electrodes. Therefore, the structure of an organic transistor needs to be applied in accordance with an advantage and a disadvantage of each structure.  
         [0007]     It is known that carrier mobility of an organic transistor largely depends on morphology (an amorphous, polycrystalline, single crystalline state, or the like) of an organic semiconductor. In particular, an organic transistor using a single crystal of an organic semiconductor layer shows high carrier mobility and has carrier mobility almost the same as that of amorphous silicon. A liquid phase growth method, a vapor-phase transport method, or the like can be given as a method for obtaining a favorable single crystal of an organic material.  
         [0008]     In order to improve characteristics by actively using a single crystal for an organic transistor, a technique for selectively coating a position where a single crystal is made to grow is necessary. In coating a single crystal in such a manner, such a method as wettability of a substrate portion is reformulated to selectively grow a single crystal is typically used.  
         [0009]     As an example where a single crystal of an organic semiconductor layer is used, a structure where an island-shaped projection layer in which island-shaped projections having low surface energy are formed by being dispersed is provided over a surface of a gate insulating layer is suggested in order to suppress a crystalline state of a pentacene deposited film and to provide an organic semiconductor element having high carrier mobility with low voltage driving (for example, see Reference 1: Japanese Patent Application Laid-Open No. 2004-23021).  
         [0010]     In addition, an organic transistor the characteristic of which is to form a fluorine polymer layer over a surface of a gate insulating layer is suggested by limiting a contact angle of deionized water with respect to the surface of a gate insulating layer to 50° C. or more and 120° C. or less (for example, see Reference 2: Japanese Patent Application Laid-Open No. 2001-94107).  
         [0011]     When a single crystal of an organic semiconductor layer is formed as in the above Reference 1, an interface between an organic semiconductor of a channel where carriers are spread out and a gate insulating layergate insulating layer looses its planarity; therefore, it is hard to say that carrier mobility of a single crystal of an organic semiconductor layer is utilized sufficiently.  
         [0012]     In addition, in the above Reference 2, a yield is to deteriorate when a single crystal of an organic semiconductor layer cannot be grown through the entire channel.  
       SUMMARY OF THE INVENTION  
       [0013]     Thus, it is an object of the present invention to efficiently apply a single crystal of an organic semiconductor layer or a film having high crystallinity to an organic transistor.  
         [0014]     In view of the above object, a semiconductor device according to the present invention has a stacked structure of organic semiconductor layers, and at least the upper organic semiconductor layer is in a polycrystalline or a single crystalline state and the lower organic semiconductor layer is made of a material serving as a channel. In this case, the crystal grain size of a single crystal is larger than one crystal grain size in a polycrystal. The lower organic semiconductor layer has crystallinity the same or less than that of the upper organic semiconductor layer. Carrier mobility can be increased owing to the upper organic semiconductor layer having high crystallinity. In addition, even when a crystal of the upper organic semiconductor layer is not grown sufficiently for a channel area in a direction parallel to a substrate, insufficient contact can be compensated because the lower organic semiconductor layer is in contact with source and drain electrodes.  
         [0015]     There is an organic semiconductor layer in a single crystal as an example of an organic semiconductor layer having high crystallinity. It is considered that such an organic semiconductor layer having high crystallinity has insufficient physical contact of the single crystal with source and drain electrodes or with a gate insulating layer depending on a crystal condition or a crystalline state, for example, in a case where a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate. However, even in such a case, the organic semiconductor layer provided in the lower layer can compensate insufficient contact and thus can serve as a channel by being in contact with source and drain electrodes or a gate insulating layer.  
         [0016]     Specifically, a single crystal of an organic semiconductor layer is used in the upper layer of the stacked organic semiconductor layers and a polycrystalline organic semiconductor layer is used in the lower layer in order to improve carrier mobility of the organic semiconductors. Of course, the lower organic semiconductor layer preferably has the same crystalline state as the upper organic semiconductor layer. This is because electrical resistance of carrier mobility can be further reduced. According to the present invention, an amorphous organic semiconductor layer can also be used for the lower organic semiconductor layer. This is because it is possible to accomplish effect that insufficient contact of a single crystal with source and drain electrodes is compensated even in an amorphous state.  
         [0017]     In another mode, a polycrystalline organic semiconductor layer can be used for the upper organic semiconductor layer and an amorphous organic semiconductor layer can be used for the lower organic semiconductor layer. Of course, the lower organic semiconductor layer preferably has the same crystalline state as the upper organic semiconductor layer. This is because electrical resistance of carrier mobility can be further reduced.  
         [0018]     In other words, the lower organic semiconductor layer has lower crystallinity than that of the upper organic semiconductor layer and the lower organic semiconductor layer may be formed with a material that serves as a channel.  
         [0019]     A vapor-phase transport method can be used as a deposition method of a single crystalline organic semiconductor layer. Then, a single crystalline organic semiconductor layer can be selectively formed along the lower organic semiconductor layer which is selectively formed. In other words, it becomes possible to selectively grow a single crystal between channels owing to the lower organic semiconductor layer.  
         [0020]     Specifically, according to one feature of the present invention, a semiconductor device comprises a gate electrode; a first organic semiconductor layer provided over the gate electrode through a gate insulating layer; and a second organic semiconductor layer provided over the first organic semiconductor layer to be in contact therewith, wherein the second organic semiconductor layer has crystallinity higher than that of the first organic semiconductor layer.  
         [0021]     According to another feature of the present invention, a semiconductor device comprises a gate electrode; a first organic semiconductor layer provided over the gate electrode through a gate insulating layer; and a second organic semiconductor layer provided over the first organic semiconductor layer to be in contact therewith, wherein the second organic semiconductor layer has a crystal grain larger than that of the first organic semiconductor layer.  
         [0022]     According to another feature of the present invention, a semiconductor device comprises a gate electrode; a first organic semiconductor layer provided over the gate electrode through a gate insulating layer; and a second organic semiconductor layer provided over the first organic semiconductor layer to be in contact therewith, wherein the first organic semiconductor layer is polycrystalline or amorphous, and wherein the second organic semiconductor layer is single crystalline.  
         [0023]     According to another feature of the present invention, a semiconductor device comprises a gate electrode; a first organic semiconductor layer provided over the gate electrode through a gate insulating layer; and a second organic semiconductor layer provided over the first organic semiconductor layer to be in contact therewith, wherein the first organic semiconductor layer is amorphous, and wherein the second organic semiconductor layer is polycrystalline.  
         [0024]     According to another feature of the present invention, a semiconductor device comprises a gate electrode; a first organic semiconductor layer provided over the gate electrode through a gate insulating layer; and a second organic semiconductor layer provided over the first organic semiconductor layer to be in contact therewith, wherein the first organic semiconductor layer is single crystalline, and wherein the second organic semiconductor layer is single crystalline including a material different from the first organic semiconductor layer.  
         [0025]     In addition, a specific method for manufacturing a semiconductor device according to the present invention will be shown.  
         [0026]     According to one feature of the present invention, a method for manufacturing a semiconductor device comprises the steps of forming a gate electrode and a gate insulating layer over a substrate; forming a first organic semiconductor layer over the gate electrode through the gate insulating layer, and forming a second organic semiconductor layer in contact with the first organic semiconductor layer film, wherein the second organic semiconductor layer is formed to have a crystal grain larger than that of the first organic semiconductor layer.  
         [0027]     According to another feature of the present invention, a method for manufacturing a semiconductor device comprises the steps of forming a gate electrode and a gate insulating layer over a substrate; forming a first organic semiconductor layer over the gate electrode through the gate insulating layer, and forming a second organic semiconductor layer in contact with the first organic semiconductor layer, wherein the second organic semiconductor layer is formed to have crystallinity higher than that of the first organic semiconductor layer.  
         [0028]     According to another feature of the present invention, a method for manufacturing a semiconductor device comprises the steps of forming a gate electrode and a gate insulating layer over a substrate; forming a first organic semiconductor layer over the gate electrode through the gate insulating layer, and forming a second organic semiconductor layer in contact with the first organic semiconductor layer, wherein the first organic semiconductor layer is formed to be polycrystalline or amorphous, and wherein the second organic semiconductor layer is formed to be single crystalline.  
         [0029]     According to another feature of the present invention, a method for manufacturing a semiconductor device comprises the steps of forming a gate electrode and a gate insulating layer over a substrate; forming a first organic semiconductor layer over the gate electrode through the gate insulating layer, and forming a second organic semiconductor layer in contact with the first organic semiconductor layer, wherein the first organic semiconductor layer is formed to be amorphous, and wherein the second organic semiconductor layer is formed to be polycrystalline.  
         [0030]     According to the present invention, a single crystal of an organic semiconductor layer can be formed efficiently and selectively. The single crystal can be used as a channel forming region; thus, carrier mobility can be increased.  
         [0031]     In addition, even when a single crystal of an organic semiconductor layer is not grown sufficiently for a channel area in a direction parallel to a substrate, by applying the present invention where a thin film of an organic semiconductor is formed in advance, the thin film of an organic semiconductor can serve as a channel that compensates insufficient growth of a single crystal when a second organic semiconductor layer is not formed sufficiently. Consequently, it is possible to provide an organic transistor with high carrier mobility in a preferable yield by preventing incomplete channel formation due to insufficient growth of a single crystal in an organic semiconductor layer.  
         [0032]     Moreover, off current of an organic transistor can be reduced by improving adhesion between a single crystal of an organic semiconductor layer (or an organic semiconductor layer having high crystallinity) and a gate insulating layer by providing an organic semiconductor layer. In addition, as a result of improving contact between a single crystal of an organic semiconductor layer (or an organic semiconductor layer having high crystallinity) and source and drain electrodes in an organic semiconductor layer, it is possible to reduce an injection barrier of a carrier, increase on current, and reduce a shift in threshold voltage.  
         [0033]     Further, since a single crystal is selectively grown according to the present invention, it is possible to reduce loss of an organic semiconductor material. It is a feature of the present invention to provide an organic transistor the operation of which is stabilized with a long lifetime and the manufacturing method of which is simple and easy.  
         [0034]     These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description along with the accompanied drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     In the accompanying drawings:  
         [0036]      FIGS. 1A  to  1 C are views each showing a manufacturing process of an organic transistor according to the present invention and a cross section thereof;  
         [0037]      FIGS. 2A  to  2 C are views each showing a manufacturing process of an organic transistor according to the present invention and a cross section thereof;  
         [0038]      FIG. 3  is a cross-sectional view of a display element substrate according to the present invention;  
         [0039]      FIG. 4  is a cross-sectional view of an organic semiconductor device according to the present invention;  
         [0040]      FIG. 5  is a cross-sectional view of a display device according to the present invention;  
         [0041]      FIGS. 6A and 6B  are views each showing selectivity of a growth position in a single crystal of an organic transistor according to the present invention;  
         [0042]      FIG. 7  is a diagram showing a transistor characteristic of an organic transistor according to the present invention;  
         [0043]      FIGS. 8A  to  8 D are views each showing an electronic device on which an organic transistor according to the present invention is mounted;  
         [0044]      FIGS. 9A  to  9 B are views each showing a cross section of an organic transistor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]     Hereinafter, one mode of the present invention will be explained. However, it is to be easily understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the invention, they should be construed as being included therein.  
       Embodiment Mode 1  
       [0046]     One mode of an organic transistor according to the present invention and a manufacturing method thereof will be explained with reference to  FIGS. 1A  to  1 C.  
         [0047]     A gate electrode  102  is formed over a substrate  101 . The gate electrode  102  may be formed by patterning a deposited conductive layer into a desired shape by a photolithography method or by discharging a droplet containing a conductive material by an ink-jet method or the like. Note that the method for manufacturing the gate electrode  102  according to the present invention is not limited thereto. As a material for forming the gate electrode  102 , for example, aluminum, copper, gold, silver, or the like can be used. Note that the material used for the gate electrode  102  according to the present invention is not limited thereto and any material is acceptable as long as the material has conductivity. In addition, as the substrate  101 , a flexible substrate such as plastic or polycarbonate can be used in addition to glass, quartz, or the like. Note that the substrate used in the present invention is not limited thereto. In addition, since an organic transistor does not need high-temperature processing, a flexible substrate such as plastic or polycarbonate can be used; thus, weight saving and a thin shape of a semiconductor device can be achieved and flexibility thereof can be improved.  
         [0048]     Next, a gate insulating layer  103  is formed to cover the gate electrode  102 . The gate insulating layer  103  may be formed by depositing an insulator such as silicon oxide or silicon nitride by a CVD method or the like or may be formed by oxidizing the surface of the gate electrode with the use of an anodic oxidation method, for example. Alternatively, an organic material such as polyimide, polyamic acid, or polyvinylphenol may be coated by a method such as a cast method, a spinner method, a printing method, or an ink-jet method to form the gate insulating layer  103 . Note that the gate insulating layer according to the present invention is not limited thereto.  
         [0049]     Then, an organic transistor forming region  106  is manufactured by forming source and drain electrodes  104  and  105  over the gate insulating layer  103  ( FIG. 1A ). The source and drain electrodes  104  and  105  may be formed using an organic conductive material or the like containing poly (ethylene dioxythiophene)/poly (styrenesulfonic acid) mixture (PEDOT/PSS) or the like as well as an inorganic conductive material such as gold, silver, or tungsten. Note that the material of the source and the drain electrodes according to the present invention is not limited thereto. In addition, the source and drain electrodes  104  and  105  may be formed by patterning into a desired shape a conductive layer formed with a film forming apparatus such as a sputtering apparatus or a vapor deposition apparatus or may be formed by discharging a droplet containing a conductive material by an inkjet method or the like. Note that the method for manufacturing the source and drain electrodes according to the present invention is not limited thereto.  
         [0050]     An organic semiconductor layer  107  is formed in a region for forming a single crystal of an organic semiconductor layer over the organic transistor forming region  106 . The organic semiconductor layer  107  is formed at least between the source and drain electrodes. As a result of providing the organic semiconductor layer  107 , a single crystal of an organic semiconductor layer can be formed efficiently thereover; thus, a single crystal can be grown from the region.  
         [0051]     When a single crystal of an organic semiconductor layer is formed in this manner to increase carrier mobility of an organic transistor, it is considered that crystals are not formed over an entire channel area and thus physical contact with source and drain electrodes or a gate insulating layer become insufficient when a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate, depending on a crystal condition or a crystalline state of a shape or the like of a crystal grain. Thus, according to the present invention, the organic semiconductor layer  107  compensates incomplete connection and serves as a channel by being in contact with the source and drain electrodes or the gate insulating layer. Therefore, any material of the organic semiconductor layer  107  is acceptable as long as the material serves as a channel. Such an organic semiconductor  107  is formed to be 2 nm or more and 10 nm or less in thickness.  
         [0052]     For example, a phthalocyanine-based material such as phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), titanyl phthalocyanine (TiOPc), or vanadyl phthalocyanine (VoPc), an acene-based material such as anthracene, tetracene, or pentacene, a thiophene-oligomer-based material such as sexithiophene (α-6T) or quarterthiophene (α-4T), or other material such as fullerene (C60) or perylene can be used to form the organic semiconductor layer  107 . However, it is desirable to use an organic semiconductor having high carrier mobility such as anthracene, tetracene, or pentacene. The organic semiconductor having high mobility such as anthracene, tetracene, or pentacene can subsequently form a single crystal of an organic semiconductor layer. Accordingly, the organic semiconductor layer  107  is preferable in terms of adhesion or a manufacturing process when being formed with the same material as a single crystal of an organic semiconductor layer that will be subsequently formed.  
         [0053]     However, the organic semiconductor layer  107  according to the present invention is not limited thereto, and an organic compound such as an aromatic amine compound typified by 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), or 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD) can be used.  
         [0054]     Such an organic semiconductor layer  107  can be formed using a vapor deposition method, an ink-jet method, a printing method, a stamp method, or the like. In a case of using a vapor deposition method or the like, the organic semiconductor layer  107  can be selectively formed using a mask. Note that the method for manufacturing the organic semiconductor layer according to the present invention is not limited thereto.  
         [0055]     The organic semiconductor layer  107  can bring about an effect of the present invention when an organic semiconductor layer having the same crystal state as that of a single crystal of an organic semiconductor layer that will be subsequently formed is used. For example, in a case of using a single crystal of an organic semiconductor layer as an organic semiconductor layer that will be subsequently formed, it is preferable to use an organic semiconductor layer having such high crystallinity as that in a single crystalline or a polycrystalline state for the organic semiconductor layer  107 . In addition, in a case of forming a polycrystalline organic semiconductor layer, it is preferable to use an organic semiconductor layer in a polycrystalline state for the organic semiconductor layer  107 .  
         [0056]     Next, a single crystal  108  of an organic semiconductor layer is formed over the organic semiconductor layer  107  as an organic semiconductor layer having high crystallinity to form an organic transistor  109  ( FIG. 1B ). Since the organic semiconductor layer  107  is formed in advance, the single crystal  108  of an organic semiconductor layer can be formed selectively and efficiently; thus, crystal growth can be realized. The single crystal can be used as a channel forming region and can increase carrier mobility. In addition, the organic semiconductor layer  107  that is formed in advance can be used as a nucleus of the single crystal.  
         [0057]     In addition, a phthalocyanine-based material such as phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), titanyl phthalocyanine (TiOPc), or vanadyl phthalocyanine (VoPc), an acene-based material such as anthracene, tetracene, or pentacene, a thiophene-oligomer-based material such as sexithiophene (α-6T) or quarterthiophene (α-4T), or other material such as fullerene (C60) or perylene can be used to form the single crystal  108  of an organic semiconductor layer by an OMBE (Organic Molecular Beam Epitaxy), an HWE (Hot Wall Epitaxy) method, or a Vapor-phase Transport method such as a PVT (Physical Vapor Transport) method.  
         [0058]     Accordingly, the upper organic semiconductor layer has a feature of crystallinity higher than that of the lower organic semiconductor layer. High crystallinity also means that the grain size is large. Then, according to the organic semiconductor layer having high crystallinity, carrier mobility can be increased and, when the organic semiconductor layer has insufficient contact with the source and drain electrodes or the gate insulating layer, the lower organic semiconductor layer can compensate the insufficient contact by being in contact with the source and drain electrodes or the gate insulating layer.  
         [0059]     Note that a large number of carriers flow on the side of the gate insulating layer of the organic semiconductor layer in the organic transistor. Therefore, a region where carriers easily flow is separated from the single crystal which is low in resistance depending on a film thickness of the organic semiconductor layer  107  provided for compensating the insufficient contact; thus, there is a fear that carriers do not efficiently flow on the single crystal side. Thus, the film thickness of the organic semiconductor layer  107  is preferably 2 nm or more and 10 nm or less.  
         [0060]     Further, even in a case where a plurality of the organic transistor  109  is included over the substrate  101 , an organic semiconductor device  110  ( FIG. 1C ) can be formed through the same process.  
         [0061]     As mentioned above, since the organic semiconductor layer  107  is formed in advance in the organic transistor  109  manufactured according to the present invention, the single crystal  108  of an organic semiconductor layer can be selectively formed and thereafter crystal growth can be realized. The single crystal can be used as a channel forming region and carrier mobility can be increased.  
         [0062]     Furthermore, there is a case where a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate only with the single crystal  108  of an organic semiconductor layer. In such a case, the organic semiconductor layer  107  formed below the single crystal  108  of an organic semiconductor layer serves as a channel by being in contact with the source and drain electrodes or the gate insulating layer; thus, a yield can be improved.  
         [0063]     In addition, adhesion between the single crystal  108  of an organic semiconductor layer and the gate insulating layer  103  can be improved by providing the organic semiconductor layer  107 . Consequently, off current of the organic transistor  109  can be decreased. Moreover, contact of the single crystal  108  of an organic semiconductor layer with the source and drain electrodes  104  and  105  can be improved by the organic semiconductor layer  107 . Consequently, it is possible to reduce an injection barrier of a carrier, increase on current, and reduce a shift in threshold voltage. In this embodiment mode, a bottom-gate organic transistor is explained. However, the present invention also can be applied to a top-gate organic transistor. A top-gate organic transistor  908  according to this embodiment mode and a manufacturing method thereof will be explained with reference  FIG. 9A . Firstly, a source electrode  902  and a drain electrode  903  are formed over the substrate  901  by using a well-known method. Secondly, a first organic semiconductor layer  904  is formed over the source electrode  902  and the drain electrode  903  by using a vapor deposition method, an ink-jet method, a printing method, a stamp method, or the like. Thirdly, a second organic semiconductor layer  905  is formed over the first organic semiconductor layer  904  by using an OMBE method, an HWE method, or a vapor-phased transport method such as a PVT. Then, a gate electrode  907  is formed over the second organic semiconductor layer  905  through a gate insulating layer  906  by using a well-known method.  
       Embodiment Mode 2  
       [0064]     One mode of an organic transistor according to the present invention and a manufacturing method thereof will be explained with reference to  FIGS. 21A  to  2 C.  
         [0065]     A gate electrode  202  is formed over a substrate  201 . The gate electrode  202  may be formed by patterning a deposited conductive layer into a desired shape by a photolithography method or by discharging a droplet containing a conductive material by an ink-jet method or the like. However, the method for manufacturing the gate electrode  202  according to the present invention is not limited thereto. As a material for forming the gate electrode  202 , for example, aluminum, copper, gold, silver, or the like can be used. However, the material used for the gate electrode  202  according to the present invention is not limited thereto. In addition, as the substrate  201 , a flexible substrate such as plastic or polycarbonate can be used in addition to glass, quartz, or the like. Note that the organic transistor does not need high-temperature processing at 600° C. or more; therefore, a flexible substrate such as plastic or polycarbonate can be used; thus, weight saving and a thin shape of a semiconductor device can be achieved and flexibility thereof can be improved.  
         [0066]     Next, a gate insulating layer  203  is formed to cover the gate electrode  202 . The gate insulating layer  203  may be formed by depositing an insulator such as silicon oxide or silicon nitride by a CVD method or the like or may be formed by oxidizing the surface of the gate electrode with the use of an anodic oxidation method, for example. Alternatively, an organic material such as polyimide, polyamic acid, or polyvinylphenol may be coated by a method such as a cast method, a spinner method, a printing method, or an ink-jet method to form the gate insulating layer  203 . Accordingly, an organic transistor forming region  204  is manufactured ( FIG. 2A ).  
         [0067]     An organic semiconductor layer  205  is formed in a region for forming a single crystal of an organic semiconductor layer over the organic transistor forming region  204 . The organic semiconductor layer  205  is formed at least between the source and drain electrodes. As a result of providing the organic semiconductor layer  205 , a single crystal of an organic semiconductor layer can be formed efficiently thereover; thus, a single crystal can be grown from the region.  
         [0068]     When a single crystal of an organic semiconductor layer is formed in this manner to increase carrier mobility of an organic transistor, it is considered that contact with source and drain electrodes become insufficient when a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate, depending on a crystal condition or a crystalline state of a shape or the like of a crystal grain. Thus, the organic semiconductor layer  205  according to the present invention compensates incomplete connection and serve as a channel. Therefore, any material of the organic semiconductor layer  205  is acceptable as long as the material serves as a channel. Such an organic semiconductor  205  is formed to be 2 nm or more and 10 nm or less in thickness.  
         [0069]     For example, a phthalocyanine-based material such as phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), titanyl phthalocyanine (TiOPc), or vanadyl phthalocyanine (VoPc), an acene-based material such as anthracene, tetracene, or pentacene, a thiophene-oligomer-based material such as sexithiophene (α-6T) or quarterthiophene (α-4T), or other material such as fullerene (C60) or perylene can be used to form the organic semiconductor layer  205 . However, it is desirable to use an organic semiconductor having high carrier mobility such as anthracene, tetracene, or pentacene. The organic semiconductor having high mobility such as anthracene, tetracene, or pentacene can subsequently form a single crystal of an organic semiconductor layer. Accordingly, the organic semiconductor layer  205  is preferable in terms of adhesion or a manufacturing process when being formed with the same material as a single crystal of an organic semiconductor layer that will be subsequently formed.  
         [0070]     However, the organic semiconductor layer  205  according to the present invention is not limited thereto, and an organic compound such as an aromatic amine compound typified by 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), or 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD) can be used.  
         [0071]     Such an organic semiconductor layer  205  can be formed using a vapor deposition method, an ink-jet method, a printing method, a stamp method, or the like. In a case of using a vapor deposition method, the organic semiconductor layer  205  can be selectively formed using a mask. Note that the method for manufacturing the organic semiconductor layer according to the present invention is not limited thereto.  
         [0072]     The organic semiconductor layer  205  can bring about an effect of the present invention when an organic semiconductor layer having the same crystal state as that of a single crystal of an organic semiconductor layer that will be subsequently formed is used. For example, in a case of using a single crystal of an organic semiconductor layer as an organic semiconductor layer that will be subsequently formed, it is preferable to use for the organic semiconductor layer  205  an organic semiconductor layer having such high crystallinity as that in a single crystalline or a polycrystalline state. In addition, in a case of forming a polycrystalline organic semiconductor layer, it is preferable to use an organic semiconductor layer in a polycrystalline state for the organic semiconductor layer  205 .  
         [0073]     Next, a single crystal  206  of an organic semiconductor layer is formed over the organic semiconductor layer  205  as an organic semiconductor layer having high crystallinity. Since the organic semiconductor layer  205  is formed in advance, the single crystal  206  of an organic semiconductor layer can be formed selectively and efficiently; thus, crystal growth can be realized. The single crystal can be used as a channel forming region and can increase carrier mobility. In addition, the organic semiconductor layer  205  formed in advance can be used as a nucleus of the single crystal.  
         [0074]     In addition, a phthalocyanine-based material such as phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), titanyl phthalocyanine (TiOPc), or vanadyl phthalocyanine (VoPc), an acene-based material such as anthracene, tetracene, or pentacene, a thiophene-oligomer-based material such as sexithiophene (α-6T) or quarterthiophene (α-4T), or other material such as fullerene (C60) or perylene can be used to form the single crystal  206  of an organic semiconductor layer by a vapor-phase transport method.  
         [0075]     Accordingly, the upper organic semiconductor layer has a feature of crystallinity higher than that of the lower organic semiconductor layer. High crystallinity also means that the grain size is large. Then, according to the organic semiconductor layer having high crystallinity, carrier mobility can be increased and, when the organic semiconductor layer has insufficient contact with the source and drain electrodes or the gate insulating layer, the lower organic semiconductor layer can compensate the insufficient contact.  
         [0076]     A large number of carriers flow on the side of the gate insulating layer of the organic semiconductor layer in the organic transistor. Therefore, a region where carriers easily flow is separated from the single crystal which is low in resistance depending on a film thickness of the organic semiconductor layer  205  provided for compensating the insufficient contact and thus there is a fear that carriers do not efficiently flow on the single crystal side. Thus, the film thickness of the organic semiconductor layer  107  is preferably 2 nm or more and 10 nm or less.  
         [0077]     Next, an organic transistor  209  is completed by forming source and drain electrodes  207  and  208  over the single crystal  206  of an organic semiconductor layer ( FIG. 2B ). The source and drain electrodes  207  and  208  may be formed using an organic conductive material or the like containing poly (ethylene dioxythiophene)/poly (styrenesulfonic acid) mixture (PEDOT/PSS) or the like as well as an inorganic conductive material such as gold, silver, or tungsten. However, the material of the source and drain electrodes according to the present invention is not limited thereto. In addition, the source and drain electrodes  207  and  208  may be formed by patterning into a desired shape a conductive layer formed with a film forming apparatus such as a sputtering apparatus or a vapor deposition apparatus or may be formed by discharging a droplet containing a conductive material by an ink-jet method or the like. However, the method for manufacturing the source and drain electrodes is not limited thereto.  
         [0078]     Further, in a case where a plurality of the organic transistor  209  is included over the substrate  201 , an organic semiconductor device  210  ( FIG. 2C ) can be formed through the same process.  
         [0079]     As mentioned above, since the organic semiconductor layer  205  is formed in advance in the organic transistor  209  manufactured according to the present invention, the single crystal  206  of an organic semiconductor layer can be selectively formed and crystal growth can be realized thereafter. The single crystal can be used as a channel forming region and can increase carrier mobility.  
         [0080]     Furthermore, there is a case where a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate only with the single crystal  206  of an organic semiconductor layer. In such a case, the organic semiconductor layer  205  formed below the single crystal  206  of an organic semiconductor layer serves as a channel by being in contact with the source and drain electrodes or the gate insulating layer; thus, a yield can be improved.  
         [0081]     In addition, adhesion between the single crystal  206  of an organic semiconductor layer and the gate insulating layer  203  can be improved by providing the organic semiconductor layer  205 . Consequently, off current of the organic transistor  209  can be decreased. Moreover, contact of the single crystal  206  of an organic semiconductor layer with the source and drain electrodes  207  and  208  can be improved by the organic semiconductor layer  205 . Consequently, it is possible to reduce an injection barrier of a carrier, increase on current, and reduce a shift in threshold voltage. In this embodiment mode, a bottom-gate organic transistor is explained. However, the present invention also can be applied to a top-gate organic transistor. A top-gate organic transistor  918  according to this embodiment mode and a manufacturing method thereof will be explained with reference  FIG. 9B . Firstly, a first semiconductor layer  912  is formed over a substrate  911  by using a vapor deposition method, an ink-jet method, a printing method, a stamp method, or the like. Secondly, a second semiconductor layer  913  is formed over the first semiconductor layer  912  by using an OMBE method, an HWE method, or a vapor-phased transport method such as a PVT. Thirdly, a source electrode  914  and a drain electrode  915  are formed over the second semiconductor layer  913  by using a well-known method. Then, a gate electrode  917  is formed over the source electrode  914  and the drain electrode  915  through a gate insulating layer  916  by using a well-known method.  
       Embodiment Mode 3  
       [0082]     One mode of an organic semiconductor device according to the present invention and a manufacturing method thereof will be explained with reference to FIGS.  3  to  5 .  
         [0083]     A gate electrode  302  is formed over a substrate  301 . The gate electrode  302  may be formed by patterning a deposited conductive layer into a desired shape by a photolithography method or by discharging a droplet containing a conductive material by an ink jet method or the like in the same manner as the above embodiments. As a material for forming the gate electrode  302 , for example, aluminum, copper, gold, silver, or the like can be used. In addition, as the substrate  301 , a flexible substrate such as plastic or polycarbonate can be used in addition to glass, quartz, or the like in the same manner as the above embodiments.  
         [0084]     Next, a gate insulating layer  303  is formed to cover the gate electrode  302 . The gate insulating layer  303  may be formed by depositing an insulator such as silicon oxide or silicon nitride by a CVD method or the like or may be formed by oxidizing the surface of the gate electrode with the use of an anodic oxidation method, for example. Alternatively, an organic material such as polyimide, polyamic acid, or polyvinylphenol may be coated by a method such as a cast method, a spinner method, a printing method, or an ink-jet method to form the gate insulating layer  303 .  
         [0085]     Then, an organic semiconductor device forming region  306  is manufactured by forming a source electrode  304  and an electrode  305  which serves as a drain electrode and an anode of a light-emitting element over the gate insulating layer  303  ( FIG. 3 ). The source and drain electrodes  304  and  305  may be formed using an organic conductive material or the like containing poly (ethylene dioxythiophene)/poly (styrenesulfonic acid) mixture (PEDOT/PSS) or the like as well as an inorganic conductive material such as gold, silver, or tungsten in the same manner as the above embodiments. In addition, the anode  305  of a light-emitting element may also be formed using an organic conductive material or the like containing poly (ethylene dioxythiophene)/poly (styrenesulfonic acid) mixture (PEDOT/PSS) or the like as well as an inorganic conductive material such as gold, silver, or tungsten.  
         [0086]     Moreover, when the anode is light transmitting, the above materials may be made extremely thin or a transparent conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), zinc oxide added with gallium (GZO), or indium tin oxide containing silicon oxide (ITSO) can be used to form the anode. Note that the material for the anode according to the present invention is not limited thereto. In addition, as a method for forming such a source electrode  304  and an electrode  305  which serves as a drain electrode and an anode of a light-emitting element, the electrodes may be formed by patterning into a desired shape a conductive layer formed with the use of a film forming apparatus such as a sputtering apparatus or a vapor deposition apparatus or may be formed by discharging a droplet containing a conductive material by an ink-jet method or the like. However, the manufacturing method is not limited thereto. In this embodiment mode, the drain electrode and anode  305  has a combined structure.  
         [0087]     Subsequently, a hole-transporting organic semiconductor layer  307  is formed over the electrode  305  which serves as a drain electrode and an anode of a light-emitting element as shown in  FIG. 4 . As a method for forming the hole-transporting organic semiconductor layer  307 , for example, a vapor deposition method, a spin-coating method, a printing method, an ink-jet method, or the like may be used. However, the manufacturing method is not limited thereto.  
         [0088]     Next, a light-emitting layer and an electron-transporting organic semiconductor layer  308  are formed over the hole-transporting organic semiconductor layer  307 . The hole-transporting organic semiconductor layer  307  includes a hole-transporting substance and a substance that shows electron acceptability against the substance. The hole-transporting substance is a substance having higher transportability of holes than that of electrons. For example, an aromatic amine compound such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), or 4,4′-bis[N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino]biphenyl (abbreviation: DNTPD), or an organic compound such as a phthalocyanine-based material typified by phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), or vanadyl phthalocyanine (VoPc) can be used. Metal oxide such as molybdenum oxide, vanadium oxide, or rhenium oxide can be used for the substance that shows electron acceptability against the hole-transportable substance, for example.  
         [0089]     As for a method for forming the hole-transporting organic semiconductor layer  307 , for example, a vapor deposition method, a spin coating method, a printing method, an ink-jet method, or the like may be used. However, the material and manufacturing method of the hole-transporting layer are not limited thereto.  
         [0090]     The light-emitting layer is preferable to have a layer in which a light-emitting substance is included to disperse in a layer formed of a substance having an energy gap larger than that of a light-emitting substance. Note that the light-emitting substance is a substance with preferable luminous efficiency, which can emit light with a desired emission wavelength. Note that the energy gap indicates an energy gap between the LUMO level and the HOMO level.  
         [0091]     In order to obtain red light emission, for example, the following can be used for the light-emitting layer: 4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTI), 4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran (abbreviation: DCJT), 4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyljulolidin-9-yl)ethenyl]-4H-pyran (abbreviation: DCJTB), periflanthen, 2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene, bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(acetylacetonato) (abbreviation: Ir[Fdpq] 2 (acac)), or the like. However, the present invention is not limited to these materials and a substance which can emit light with a peak of emission spectrum in 600 nm or more and 680 nm or less can be used.  
         [0092]     In order to obtain green light emission, N,N′-dimethylquinacridon (abbreviation: DMQd), coumarin 6, coumarin 545T, tris(8-quinolinolato)aluminum (abbreviation: Alq 3 ), or the like can be used. However, the present invention is not limited to these materials, and a substance which can emit light with a peak of emission spectrum in 500 nm or more and 550 nm or less can be used for the light-emitting layer.  
         [0093]     In order to obtain blue light emission, the following can be used for the light-emitting layer: 9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation: t-BuDNA), 9,9′-bianthryl, 9,10-diphenylanthracene (abbreviation: DPA), 9,10-bis(2-naphthyl)anthracene (abbreviation: DNA), bis(2-methyl-8-quinolinolato)-4-phenylphenolate-gallium (abbreviation: BGaq), bis(2-methyl-8-quinolinolato)-4-phenylphenolate-aluminum (BAlq), or the like. However, the present invention is not limited to these materials, and a substance which can emit light with a peak of emission spectrum in 420 nm or more and 500 nm or less can be used.  
         [0094]     As for a method for forming the light-emitting layer, for example, a vapor deposition method, a spin coating method, a printing method, an inkjet method, or the like may be used. However, the material and manufacturing method of the light-emitting layer are not limited thereto.  
         [0095]     The electron-transporting organic semiconductor layer  308  includes a layer containing an electron-transporting substance and a substance that shows electron-donating properties against the substance. Note that the electron-transporting substance is a substance having higher transportability of electrons than that of holes. For example, a metal complex such as tris(8-quinolinolato)aluminum (abbreviation: Alq 3 ), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)-4-phenylphenolate-aluminum (abbreviation: BAlq), bis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviation: Zn(BOX) 2 ), or bis[2-(2-hydroxyphenyl)benzothiazolate]zinc (abbreviation: Zn(BTZ) 2 ) can be used. In addition, the following substance can be used as the electron-transporting substance: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen); bathocuproin (abbreviation: BCP), 4,4′-bis(5-methyl-benzoxazol-2-yl)stilbene (abbreviation: BzOS), or the like. However, the electron-transporting substance is not limited thereto.  
         [0096]     As for a method for forming the electron-transporting organic semiconductor layer  308 , for example, a vapor deposition method, a spin coating method, a printing method, an ink-jet method, or the like may be used. However, the material and manufacturing method of the electron-transporting layer are not limited thereto.  
         [0097]     Next, a cathode  309  of a light-emitting element is formed over the electron-transporting organic semiconductor layer  308 . The cathode  309  of the light-emitting element may be formed using an inorganic conductive material such as aluminum, magnesium, or calcium. When the cathode is light transmitting, the above materials may be made extremely thin or a transparent conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), zinc oxide added with gallium (GZO), or indium tin oxide containing silicon oxide (ITSO) can be used to form the anode. Note that the cathode material according to the present invention is not limited thereto.  
         [0098]     A portion composed of the anode  305  of a light-emitting element, the hole-transporting organic semiconductor layer  307 , the light-emitting layer, the electron-transporting organic semiconductor layer  308 , the cathode  309  of a light-emitting element is referred to as a light-emitting element  310 .  
         [0099]     Then, an organic semiconductor layer  311  is formed in a place for forming a channel over the organic semiconductor device forming region  306 . In other words, the organic semiconductor layer  311  is selectively formed in a region where a single crystal of an organic semiconductor layer is formed. As a result of providing the organic semiconductor layer  311 , a single crystal of an organic semiconductor layer can be formed efficiently thereover; thus, a single crystal can be grown from the region.  
         [0100]     The organic semiconductor layer  311  may be formed of a material serving as a channel, specifically; the material can be selected from the materials shown in the above embodiment modes. Such an organic semiconductor layer  311  is formed to be 2 nm or more and 10 nm or less in thickness. The organic semiconductor layer  311  may be formed using a vapor deposition method, an ink-jet method, a printing method, a stamp method, or the like. However, the method for manufacturing the organic semiconductor layer  311  according to the present invention is not limited thereto.  
         [0101]     The organic semiconductor layer  311  can bring about an effect of the present invention when an organic semiconductor layer having the same crystal state as that of a single crystal of an organic semiconductor layer that will be subsequently formed is used. For example, in a case of using a single crystal of an organic semiconductor layer as an organic semiconductor layer that will be subsequently formed, it is preferable to use for the organic semiconductor layer  311  an organic semiconductor layer having such high crystallinity as that in a single crystalline or a polycrystalline state. In addition, in a case of forming a polycrystalline organic semiconductor layer, it is preferable to use an organic semiconductor layer in a polycrystalline state for the organic semiconductor layer  311 .  
         [0102]     Next, a single crystal  312  of an organic semiconductor layer is formed over the organic semiconductor layer  311  as an organic semiconductor layer having high crystallinity to complete an organic transistor  313 . The film forming method of the single crystal  312  of an organic semiconductor layer is the same as the above embodiments. For example, a phthalocyanine-based material such as phthalocyanine (H 2 Pc), phthalocyanine copper (CuPc), titanyl phthalocyanine (TiOPc), or vanadyl phthalocyanine (VoPc), an acene-based material such as anthracene, tetracene, or pentacene, a thiophene-oligomer-based material such as sexithiophene (α-6T) or quarterthiophene (α-4T), or other material such as fullerene (C60) or perylene can be used to form the single crystal  312  of an organic semiconductor by a vapor-phase transport method. Since the organic semiconductor layer  311  is formed in advance according to the present invention, the single crystal  312  of an organic semiconductor layer can be formed selectively and efficiently; thus, crystal growth can be realized.  
         [0103]     Such a light-emitting element  310  and an organic transistor  313  are used to constitute an organic semiconductor device  314 .  
         [0104]     Further, in a case where a plurality of the organic semiconductor device  314  is included over the substrate  301 , a display device  315  can be formed through the same process ( FIG. 5 ).  
         [0105]     As mentioned above, since the organic semiconductor layer  311  is formed in advance in the organic semiconductor device  314  manufactured according to the present invention, the single crystal  312  of an organic semiconductor layer can be selectively formed and crystal growth can be realized thereafter. The single crystal can be used as a channel forming region and can increase carrier mobility.  
         [0106]     Furthermore, there is a case where a crystal is not grown sufficiently for a channel area in a direction parallel to a substrate only with the single crystal  312  of an organic semiconductor layer. In such a case, the organic semiconductor layer  311  formed below the single crystal  312  of an organic semiconductor layer serves as a channel; thus, a yield can be improved.  
         [0107]     In addition, adhesion between the single crystal  312  of an organic semiconductor layer and the gate insulating layer  303  can be improved by providing the organic semiconductor layer  311 . Consequently, off current of the organic transistor  313  can be decreased. Moreover, contact of the single crystal  312  of an organic semiconductor layer with the source and drain electrodes  304  and  305  can be improved by the organic semiconductor layer  311 . Consequently, it is possible to reduce an injection barrier of a carrier, increase on current, and reduce a shift in threshold voltage. This embodiment mode can be freely combined with the above-mentioned embodiment mode.  
       Embodiment Mode 4  
       [0108]     An organic transistor according to the present invention can be mounted on a television device (also referred to as a television or a television receiver), a camera such as a digital camera or a digital video camera, a cellular phone device (also referred to as a cellular phone handset or a cellular phone), a portable information terminal such as a PDA, a portable game machine, a computer monitor, a computer, an audio reproducing device such as a car audio, an image reproducing device provided with a recording medium such as a home game machine, or the like. In particular, the organic transistor according to the present invention can be applied as a switching transistor of a pixel portion. Specific examples of such electronic devices will be explained with reference to  FIGS. 8A  to  8 D.  
         [0109]     A cellular phone handset shown in  FIG. 8A  includes a main body  9101 , a display portion  9102 , and the like. The organic transistor according to the present invention can be mounted as a switching transistor of the display portion  9102 . Consequently, an organic transistor having high carrier mobility is mounted; thus, it is possible to provide a cellular phone handset manufactured by a low-temperature process.  
         [0110]     A laptop computer shown in  FIG. 8B  includes a main body  9401 , a display portion  9402 , and the like. The organic transistor according to the present invention can be mounted as a switching transistor of the display portion  9402 . Consequently, an organic transistor having high carrier mobility is mounted; thus, it is possible to provide a laptop computer manufactured according to a low-temperature process.  
         [0111]     A television device shown in  FIG. 8C  includes a main body  9501 , a display portion  9502 , and the like. The organic transistor according to the present invention can be mounted as a switching transistor of the display portion  9502 . Consequently, an organic transistor having high carrier mobility is mounted; thus, it is possible to provide a television set manufactured according to a low-temperature process.  
         [0112]     A card shown in  FIG. 8D  includes a supporting body  9541 , a display portion  9542 , an integrated circuit chip  9543  such as a memory incorporated in the supporting body  9541 , and the like. The organic transistor according to the present invention can be mounted as a switching transistor of the display portion  9542 . Consequently, an organic transistor having high carrier mobility is mounted; thus, it is possible to provide a card manufactured according to a low-temperature process.  
         [0113]     According to the present invention, an organic transistor having high carrier mobility can be applied to a switching transistor used for a pixel portion of an electronic device. Then, power consumption and a cost of an electronic device may be reduced. This embodiment mode can be freely combined with the above-mentioned embodiment mode.  
       Embodiment  
       [0114]     This embodiment will explain that much preferable organic transistor properties can be obtained by implementing the present invention.  
         [0115]     Tungsten is formed over a quartz substrate  101  in 100 nm thick by a sputtering method. This tungsten is patterned by a photolithography method to form a gate electrode  102 . A gate insulating layer  103  of SiON is formed over the gate electrode  102  by a CVD method.  
         [0116]     Tungsten is formed over the gate insulating layer  103  in 100 nm thick by a sputtering method and patterned by a photolithography method to form source and drain electrodes  104  and  105 . A channel length and a channel width between the source and drain electrodes are 5 μm and 8 mm, respectively.  
         [0117]     An organic semiconductor, pentacene, is formed in 2.5 nm thick over the gate insulating layer  103 , and the source and drain electrodes  104  and  105  by a vapor deposition method to serve as an organic semiconductor layer  107 .  
         [0118]     Next, a single crystal  108  of an organic semiconductor layer is formed over the organic semiconductor layer  107  with the use of a vapor-phase transport method. In Embodiment 1, a single crystal of pentacene is formed by a vapor-phase transport method. Physical properties of an organic semiconductor are significantly affected by slight impurities of several ppm; therefore, purification has to be adequately performed in order to realize single crystal growth of an organic semiconductor layer. It is important to perform optimum purification by utilizing the difference of chemical properties between individual compound and impurities. In this embodiment, pentacene to be a single crystal is used by being purified for 6 times or more by a sublimation purification method. It is desirable to set the purity of the organic semiconductor layer to 99.9% or more as an indicator of mixture of definitive impurities.  
         [0119]     A single crystal of pentacene is grown by a vapor-phase transport method with the use of this sample. A vapor-phase transport method refers to a method for performing crystal growth by flowing carrier gas slowly to a glass tube where there is a temperature gradient and transporting the sample sublimated at a high-temperature portion with the carrier gas. In this embodiment, the glass tube is vacuum evacuated and the carrier gas of Ar flows in 10 ml/min so that the degree of vacuum in the glass tube becomes approximately 25 Pa and then pentacene that is the sample is heated up to 220° C. A single crystal can be selectively obtained over pentacene that is the organic semiconductor layer  107  after the sublimated pentacene is transported by the carrier gas of Ar.  
         [0120]      FIGS. 6A and 6B  each show a state of an organic transistor  109 . In  FIG. 6A , the left side of a dotted line portion is a region where pentacene that is the organic semiconductor layer  107  is not formed, and the right side is a region where pentacene that is the organic semiconductor layer  107  is formed to be 2.5 nm in thickness. In  FIG. 6B , a state where the single crystal  108  of an organic semiconductor layer is formed is shown. It is found that the single crystal  108  of an organic semiconductor layer is not formed in the region where pentacene that is the organic semiconductor layer  107  is not formed but formed in the region where pentacene that is the organic semiconductor layer  107  is formed. It is found that the single crystal  108  of an organic semiconductor layer is formed efficiently and selectively by providing pentacene that is the organic semiconductor layer  107 .  
         [0121]     Therefore, in order to obtain a single crystal of a preferable organic semiconductor layer, a larger and preferable single crystal can be obtained with efficiency by providing such a condition that optimum carrier gas is selected and a flow rate thereof is suppressed, distribution of the glass tube in temperature gradient is adjusted, or a degree of supersaturation (degree of difference between equilibrium vapor pressure and actual vapor pressure) is suppressed. The organic transistor  109  is manufactured by forming the single crystal  108  of an organic semiconductor layer over pentacene that is the organic semiconductor layer  107 .  
         [0122]     The organic transistor  109  is measured under vacuum at room temperature. The organic transistor  109  is measured by being set in a cryostat, which is vacuumed up to 1.0×10 0  Pa or less by a rotary pump. The measured Id-Vg characteristic is shown in  FIG. 7 . This is a result of the measurement by applying a gate voltage of −30V or more and 30 V or less with the fixture of a drain voltage at −10 V.  
         [0123]     According to the present invention, adhesion between the gate insulating layer and a single crystal of pentacene is improved, and off current is decreased up to 10 −11  A by inserting the organic semiconductor layer of pentacene. In addition, as a result of improving contact between the source and drain electrodes and the single crystal of pentacene, it is possible to reduce an injection barrier of a carrier, increase on current, and reduce a shift in threshold voltage. Moreover, since the organic semiconductor layer  107  of pentacene is formed in advance over a formation region of the single crystal, incomplete channel formation only with the single crystal  108  of pentacene can be prevented, the organic semiconductor layer  107  of pentacene can operate as a channel that compensates a portion where a channel is not formed, and thus the organic transistor  109  can be obtained in a preferable yield. Further, it is possible to reduce loss of an organic semiconductor material (pentacene) by selectively growing the single crystal of pentacene.  
         [0124]     The present application is based on Japanese Patent Application serial No. 2005-087133 filed on Mar. 24, 2005 in Japanese Patent Office, the entire contents of which are hereby incorporated by reference.

Technology Classification (CPC): 7