Abstract:
An organic thin film transistor (TFT) includes: an organic semiconductor film; source and drain electrodes electrically connected to the organic semiconductor film; a gate electrode electrically insulated from the source and drain electrodes and the organic semiconductor film; and an organic acceptor film interposed between the source and drain electrodes and the organic semiconductor film.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled ORGANIC THIN FILM TRANSISTOR COMPRISING ORGANIC ACCEPTOR FILM filed with the Korean Intellectual Property Office on 29 Apr. 2004, and there duly assigned Serial No. 2004-30222. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an organic thin film transistor (TFT) and, more particularly, to an organic TFT which includes an organic acceptor film disposed between source and drain electrodes and an organic semiconductor film to obtain a doping effect. 
     2. Related Art 
     Since the development of polyacetylene, which is a conjugated organic polymer, intensive research for organic semiconductors has progressed in the fields of functional electronic and optical devices. Like other organic materials, organic semiconductors can be variously synthesized and easily molded into fibers or films. Also, organic semiconductors are highly flexible, conductive and economical. Organic thin film transistors (TFTs) of devices using conductive polymers include organic active films, and studies of organic TFTs began in 1980 and continue all over the world. 
     An organic TFT is structurally similar to a silicon TFT, except that a semiconductor active region is formed from an organic material instead of silicon. Compared to the silicon TFT, the organic TFT is simple to manufacture, economical, highly resistant to shock, and suitable for a substrate that is bent or folded. In particular, when an organic TFT is manufactured on a large area, the organic TFT is useful for products that require a low process temperature and need to be bent. 
     It is highly feasible that organic TFTs be used for driving devices of active matrix organic electro-luminescent display devices, smart cards, and plastic chips for smart tags or radio frequency identification (RFID). Thus, organic TFTs are now studied by many manufacturers, laboratories and colleges worldwide. The performance of an organic TFT depends on the capability of injecting carriers into an interface between a source/drain electrode and an organic semiconductor film. 
     Generally, the organic TFT is structurally similar to a silicon transistor. Like a field effect transistor (FET), the organic TFT operates on the principle that, when a voltage is applied to a gate, an electric field is applied to a gate insulating film. A current flowing through the organic TFT is obtained by applying a voltage between source and drain regions. In this case, the source region is grounded and supplies electrons or holes. The semiconductor active film located on the source and drain regions is an organic semiconductor film. 
     When no voltage is applied to the source and drain regions and the gate, charge is uniformly distributed throughout the semiconductor active film. 
     When a voltage lower than the threshold voltage of the TFT is applied to the gate, a current flows between drain and source in proportion on the applied voltage. If a voltage higher than the threshold voltage of the TFT (i.e., a positive voltage) is applied to the gate, positive charges (i.e., holes) are pushed upward due to an electric field caused by the applied voltage. As a result, a depletion film which includes no conductive charges is formed near a gate insulating film. In this case, when a voltage is applied between the source and drain regions, conductive charge carriers are reduced so that a current flows therebetween, and that current is smaller than when no voltage is applied to the gate. 
     On the contrary, if a voltage lower than the threshold voltage (i.e., a negative voltage) is applied to the gate, an electric field is generated in the gate insulating film. The electric field induces conductive charge carriers in the semiconductor active film, and the conductive charge carriers are accumulated between drain and source. The accumulated conductive charge carriers form the current channel between drain and source. 
     Therefore, the current flowing between the source and drain regions can be controlled by continuously applying a voltage therebetween, and by applying a positive or negative voltage to the gate. The ratio of the current when a positive voltage is applied to the current when a negative voltage is applied is referred to as the on/off ratio. The on/off ratio of an organic TFT is, preferably, as high as possible. 
     Various materials for forming the semiconductor active film of the organic TFT have been developed. The semiconductor active film maybe formed of organic semiconductors, such as pentacene, oligo-thiophene, poly(alkyl-thiophene), and poly(thienylenevinylene). Also, the organic semiconductor active film may be formed using vacuum deposition, preferably by thermal evaporation. 
     An organic TFT has the disadvantage of low charge mobility. After an FET including a pentacene thin film was developed by Brown et al. of Philips in 1995, Jackson et al. from Pennsylvania State University facilitated crystallization, and invented a transistor having a charge mobility of 1.5 cm 2 /Vs and an on/off ratio of about 10 8 , which are equivalent to the characteristics of an a-Si:H FET. Pentacene, which consists of five benzene rings, is considered to be the most useful material that meets the requisite performance for a TFT. 
     A pentacene organic TFT exhibits the highest mobility among p-type semiconductors and realizes the same performance as an a-Si transistor. However, it is known that pentacene reacts with oxygen in the air, thus generating pentacenequinone. Once an organic semiconductor active film is oxidized, its coupling structure is broken, lowering charge mobility and causing lattice distortion of the interior of crystals. As a result, charge traps are formed so as to provoke charge scattering, which further deteriorates charge mobility. 
     Meanwhile, a method of doping a pentacene active film to enhance charge mobility was disclosed by Brown et al. of Philips. However, this method reveals a problem in that, although charge mobility increases with an increase in doping, the conductivity of an active film increases more than the charge mobility, thus reducing the on/off ratio. Accordingly, the foregoing doping method produces adverse effects, that is, an increase in the conductivity of the active film and a reduction in the on/off ratio. 
     Meanwhile, an organic light emitting device (OLED), which includes an organic film formed by depositing both an F 4 -TCNQ acceptor film and an amorphous TDATA, was proposed by Xiang Zhou et al. in Applied Physics Letters, Vol. 78, No. 4, 22 Jan. 2001. According to this paper, experimental results show that, as the OLED had a double or multiple layered structure including an amorphous TDATA and an F 4 -TCNQ acceptor film, current density increased, turn-on voltage decreased, and brightness improved. 
     SUMMARY OF THE INVENTION 
     The present invention provides an organic thin film transistor (TFT) which improves the charge mobility of an organic semiconductor film. 
     The present invention also provides an organic TFT which interposes an organic acceptor film between source and drain electrodes and an organic semiconductor film. Thus, contact resistance between the source and drain electrodes and the organic semiconductor film is reduced, and charge mobility is improved. 
     The present invention further provides an organic TFT which includes a surface processing film for crowding pentacene molecules and reinforcing interfacial adhesion, thus improving charge mobility. 
     According to an aspect of the present invention, there is provided an organic thin film transistor comprising: an organic semiconductor film; source and drain electrodes electrically connected to the organic semiconductor film; a gate electrode electrically insulated from the source and drain electrodes and the organic semiconductor film; and an organic acceptor film interposed between the source and drain electrodes and the organic semiconductor film. 
     The organic semiconductor film is formed of pentacene. 
     The transistor further includes a gate insulating film which electrically insulates the source and drain electrodes from the gate electrode, and the organic acceptor film contacts the source and drain electrodes and the gate insulating film. In this case, the transistor includes a surface processing film which reinforces adhesion between the gate insulating film and the organic acceptor film. 
     In another case, the transistor further includes a gate insulating film which electrically insulates the source and drain electrodes from the gate electrode, and the organic acceptor film contacts the source and drain electrodes. In this case, the transistor further includes a surface processing film which reinforces adhesion between the gate insulating film and the organic semiconductor film. 
     The organic acceptor film is formed of at least one electron withdrawing material selected from the group consisting of aromatic compounds, olefine compounds, aromatic-olefine compounds, aromatic-aromatic conjugated compounds, fused aromatic compounds, and hetero cyclic compounds, which contain at least one selected from the group consisting of nitro group (NO 2 ), cyano group (CN), sulfonyl group (SO 2 ), sulfoxide group (SO), carbonyl group (CO), carboxyl group (CO 2 ), ester group (COO), anhydride, imide, imine, halogen group, fluoroalkyl group, and fluoroaromatic group. 
     In particular, the organic acceptor film is formed of at least one electron withdrawing material selected from the group consisting of 2,4,7-trinitrofluorenone, 4-nitroaniline, 2,4-dinitroaniline, 5-nitroanthranilonitrile, 2,4-dinitrophenylamine, 1,5-dinitronaphthalene, 4-nitrobiphenyl, 9,10-dicyanoanthracene, 3,5-dinitrobenzonitrile, and N,N′-bis(di-t-buytlphenyl)-3,4,9,10-perylenedicarboxyimide. 
     The organic acceptor film has a thickness of 1 to 100 Å. 
     The surface processing film is formed of any one selected from the group consisting of trichlorosilyl-(—SiCl 3 ), trimethoxysilyl-(—Si(OMe) 3 ), and mercapto-(—SH) moieties. 
     Also, the organic acceptor film is formed by co-depositing the material which forms the organic semiconductor film and the material which forms the organic acceptor film around the interface between the organic semiconductor film and the source and drain electrodes, and is formed of 0.1 to 10% by weight of the material forming the organic acceptor film. 
     According to another aspect of the present invention, there is provided an organic thin film transistor comprising: a gate electrode disposed on a substrate; a gate insulating film covering the substrate and the gate electrode; source and drain electrodes disposed on the gate insulating film; an organic semiconductor film disposed on the source and drain electrodes; and an organic acceptor film interposed between the source and drain electrodes and the organic semiconductor film. 
     According to yet another aspect of the present invention, there is provided an organic thin film transistor comprising: a gate electrode disposed on a substrate; a gate insulating film covering the substrate and the gate electrode; an organic semiconductor film disposed on the gate insulating film; source and drain electrodes disposed on the organic semiconductor film; and an organic acceptor film interposed between the source and drain electrodes and the organic semiconductor film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIGS. 1A  thru  1 C are conceptual diagrams showing the structure of an organic thin film transistor (TFT) and the principle on which it operates; 
         FIGS. 2A and 2B  are cross-sectional views of an organic TFT according to an embodiment of the present invention; 
         FIGS. 3A and 3B  are cross-sectional views of an organic TFT according to another embodiment of the present invention; 
         FIG. 4A  shows the variation of the dipole moment of a molecule with respect to an electric field; 
         FIG. 4B  shows the formation of a charge transfer complex where electrons are donated and withdrawn from an electron donating material and an electron withdrawing material; 
         FIG. 5  is a graph of current density versus voltage of an organic TFT according to the present invention; 
         FIGS. 6A and 6B  are cross-sectional views of an organic TFT wherein a surface processing film is formed to reinforce interfacial adhesion; 
         FIGS. 7A and 7B  are cross-sectional views of another organic TFT wherein a surface processing film is formed to reinforce interfacial adhesion; and 
         FIG. 8  shows the coupling structure of the surface of an oxide film processed with OTS. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an organic thin film transistor (TFT) according to exemplary embodiments of the present invention will be described with reference to the attached drawings. 
       FIGS. 1A  thru  1 C are conceptual diagrams illustrating the principle on which an organic TFT operates. Generally, the organic TFT is structurally similar to a silicon transistor. Like a field effect transistor (FET), the organic TFT operates on the principle that, when a voltage is applied to a gate  110 , an electric field is applied to a gate insulating film  120 . A current flowing through the organic TFT is obtained by applying a voltage between a source region  130   a  and a drain region  130   b . In this case, the source region  130   a  is grounded and supplies electrons or holes. The semiconductor active film  140  disposed on the source region  130   a  and the drain region  130   b  is an organic semiconductor film. 
     The principle of the organic TFT will now be described in connection with a p-type semiconductor film. 
     Referring to  FIG. 1A , when no voltage is applied to the source region  130   a  and drain region  130   b  and to the gate  110 , charge is uniformly distributed throughout the semiconductor active film  140 . 
     When a voltage lower than the threshold voltage of the TFT is applied to the gate  110 , a current flows between drain region  130   b  and source region  130   a  in proportion to the applied voltage. Referring to  FIG. 1B , if a voltage higher than the threshold voltage of the TFT (i.e., a positive voltage) is applied to the gate  110 , positive charges (i.e., holes) are pushed upward due to an electric field caused by the applied voltage. As a result, a depletion film which includes no conductive charges is formed near the gate insulating film  120 . In this case, when a voltage is applied between source region  130   a  and drain region  130   b , conductive charge carriers are reduced so that a current flows therebetween, and that current is smaller than when no voltage is applied to the gate  110 . 
     Conversely, referring to  FIG. 1C , if a voltage lower than the threshold voltage (i.e., a negative voltage) is applied to the gate  110 , an electric field is generated in the gate insulating film  120 . The electric field induces conductive charge carriers in the semiconductor active film  140 , and the conductive charge carriers are accumulated between drain region  130   b  and source region  130   a . The accumulated conductive charge carriers form the current channel between drain region  130   b  and source region  130   a.    
     Therefore, the current flowing between source region  130   a  and drain region  130   b  can be controlled by continuously applying a voltage therebetween, and by applying a positive or negative voltage to the gate  110 . The ratio of the current when a positive voltage is applied to the current when a negative voltage is applied is referred to as the on/off ratio. The on/off ratio of an organic TFT is, preferably, as high as possible. 
     Various materials for forming the semiconductor active film  140  of the organic TFT have been developed. The semiconductor active film  140  may be formed of organic semiconductors, such as pentacene, oligo-thiophene, poly(alkyl-thiophene), and poly(thienylenevinylene). Also, the organic semiconductor active film  140  may be formed using vacuum deposition, preferably by thermal evaporation. 
       FIGS. 2A and 2B  are cross-sectional views of an organic TFT according to an embodiment of the present invention. 
     Referring to  FIG. 2A , the organic TFT includes a gate electrode  210  which is located on an insulating substrate  200  formed of silicon, plastic or glass and/or on a buffer film  205 . The buffer film  205  is selectively formed to planarize the substrate  200 , and may be formed of SiO 2  to a thickness of about 3000 Å using plasma enhanced chemical vapor deposition (PECVD), atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or electron cyclotron resonance (ECR). 
     The gate electrode  210  may be formed of a material having a small work function, such as Al, AlNd, or MoW, so as to embody a p-channel TFT. The gate electrode  210  may be formed by stacking a gate metal film using sputtering to a thickness of 300 Å, and etching the film using photolithography. 
     Alternatively, to form the gate electrode  210 , a substrate covered by a shadow mask so as to define a gate electrode is loaded into a vacuum chamber, and a metal for the gate electrode is put into a metal boat. The vacuum chamber is adjusted to a vacuum of about 5×10 −4  Torr or less, preferably about 5×10 −7  Torr. The metal for the gate electrode is deposited at a speed of 3 to 5 Å/sec, thereby forming the gate electrode  210 . As an example, an aluminum gate electrode can be formed to a thickness of about 1700 Å. 
     A gate insulating film  220  is formed so as to cover the substrate  200  and the gate electrode  210 . The gate insulating film  220  may be a silicon oxide film. In addition to Ba x Sr 1-x TiO 3  BST(barium strontium titanate), the gate insulating film  220  may be formed of Ta 2 O 5 , Y 2 O 3 , TiO 2 , ferroelectric insulating materials, PbZr x Ti 1-x O 3 (PZT), Bi 4 Ti 3 O 12 , BaMgF 4 , SrBi 2 (Ta 1-x Nb x ) 2 O 9 , Ba(Zr 1-x Ti x )O 3 (BZT), BaTiO 3 , or SrTiO 3 . 
     On top of the gate insulating film  220 , source electrode  230   a  and drain electrode  230   b  are located. The source electrode  230   a  and drain electrode  230   b  are formed by stacking a source/drain metal film using sputtering to a thickness of 5000 Å, and by etching the film using photolithography. 
     In another method of forming the source electrode  230   a  and drain electrode  230   b , the substrate  200  may be covered by a shadow mask, and metal having a large work function may be vacuum deposited on the substrate  200 . The metal having a large work function may be Au. A vacuum deposition chamber is adjusted to 5×10 −4  Torr or less, preferably about 5×10 −7  Torr. The metal is deposited at a speed of 3 to 5 Å/sec, thereby forming the source electrode  230   a  and drain electrode  230   b  to a thickness of about 1500 Å. 
     An organic acceptor film  235  is coated on the source electrode  230   a  and drain electrode  230   b , and an organic semiconductor film  240  is disposed on the organic acceptor film  235 . 
     In a typical TFT, an electric potential barrier is generated due to the difference in work function between source/drain electrodes  230   a ,  230   b  and an organic semiconductor film  240 , precluding the injection of carriers, and thus increasing contact resistance therebetween. Generally, the contact resistance can be reduced by forming the source/drain electrodes  230   a ,  230   b  from a metal that has a work function similar to that of the organic semiconductor film. In the present invention, by forming the organic acceptor film  235  between the source/drain electrodes  230   a ,  230   b  and the organic semiconductor film  240  using an electron withdrawing material, contact resistance is reduced, injection of carriers is improved, and charge mobility increases. 
     In the organic TFT shown in  FIG. 2A , the organic acceptor film  235  contacts the source electrode  230   a  and drain electrode  230   b , and a portion of the gate insulating film  220  which is exposed between the source electrode  230   a  and drain electrode  230   b . In the organic TFT shown in  FIG. 2B , the organic acceptor film  235  contacts the source electrode  230   a  and drain electrode  230   b . During manufacture, the organic acceptor film  235  may be formed by depositing a single layer formed only from an organic acceptor material, or by co-depositing an organic semiconductor material and the organic acceptor material around an interface between the organic semiconductor film  240  and the source and drain electrodes  230   a ,  230   b . If the organic semiconductor material and the organic acceptor material are co-deposited, the organic acceptor film  235  may be formed of 0.1% to 10% by weight of the organic acceptor material. 
     The organic acceptor film  235  includes a charge transfer material that can form a charge transfer complex on the surface of the source and drain electrodes  230   a ,  230   b . Thus, the organic semiconductor film  240  obtains a channel doping effect to exclude energy barrier characteristics, and the amount of carriers injected into a channel increases. As a result, contact resistance is reduced, injection of the carriers is improved, and charge mobility increases. 
       FIG. 4A  shows the variation of dipole moment of a molecule with respect to an electric field, and  FIG. 4B  shows the formation of charge transfer complex where electrons are donated and withdrawn from an electron donating material and an electron withdrawing material. 
     As can be seen from  FIG. 4A , when the electric field is applied, the dipole moment of the molecule is maximized. As can be seen from  FIG. 4B , when the dipole moment occurs, the charge transfer complex is formed so that electrons are donated and withdrawn from the electron donating material and the electron withdrawing material. 
     The organic acceptor film  235  ( FIGS. 2A and 2B ) maybe formed by depositing at least one electron withdrawing material selected from the group consisting of aromatic compounds, olefine compounds, aromatic-olefine compounds, aromatic-aromatic conjugated compounds, fused aromatic compounds, and hetero cyclic compounds, which contain at least one selected from the group consisting of nitro group (NO 2 ), cyano group (CN), sulfonyl group (SO 2 ), sulfoxide group (SO), carbonyl group (CO), carboxyl group (CO 2 ), ester group (COO), anhydride, imide, imine, halogen group, fluoroalkyl group, and fluoroaromatic group. 
     In an embodiment of the present invention, the electron withdrawing material may be formed of at least one selected from the group consisting of 2,4,7-trinitrofluorenone, 4-nitroaniline, 2,4-dinitroaniline, 5-nitroanthranilonitrile, 2,4-dinitrophenylamine, 1,5-dinitronaphthalene, 4-nitrobiphenyl, 9,10-dicyanoanthracene, 3,5-dinitrobenzonitrile, and N,N′-bis(di-t-buytlphenyl)-3,4,9,10-perylenedicarboxyimide. 
     The organic semiconductor film  240  shown in  FIG. 2A  is disposed on the organic acceptor film  235 . The organic semiconductor film  240  is formed using vacuum deposition, preferably thermal evaporation, on the source and drain electrodes  230   a ,  230   b  on which the electron withdrawing material is coated. The organic semiconductor film  240  is formed of an organic semiconductor material, such as pentacene, oligo-thiophene, poly(alkyl-thiophene), or poly(thienylenevinylene). Preferably, the organic semiconductor film  240  is formed of pentacene. The organic semiconductor film  240  is deposited at a vacuum of 5×10 −4  Torr or less, preferably about 5×10 −7  Torr, and at a speed of 0.5 Å/sec to a thickness of about 1000 Å. 
     The organic acceptor film  235  is formed by depositing a single layer formed of only an organic acceptor material, or by co-depositing an organic semiconductor film forming material (e.g., pentacene) and an organic acceptor material. In the latter case, the organic semiconductor film forming material and the organic acceptor material are co-deposited around an interface between the organic semiconductor film  240  and the source and drain electrodes  230   a ,  230   b . In the organic TFT shown in  FIGS. 2A and 2B , since the source electrode  230   a  and drain electrode  230   b  are located below the organic semiconductor film  240 , the organic acceptor film  235  can be formed by depositing 0.1% to 10% by weight of the organic acceptor material around the interface between the organic semiconductor film  240  and the source and drain electrodes  230   a ,  230   b  when the formation of the organic semiconductor film  240  begins. 
       FIG. 5  is a graph of current density versus voltage for an organic TFT according to the present invention. In  FIG. 5 , the data marked with squares (▪) show the case wherein no organic acceptor film  235  is coated, while the data marked with Xes (x) shows the case where the organic acceptor film  235  is coated. 
     Referring to  FIG. 5 , under the same conditions, when the organic acceptor film  235  is coated, the turn-on voltage is reduced from 4 V to 3.2 V, and the current density measured at 5V is greatly increased from 0.1 to 1 mA/cm 2 . Thus, the amount of carriers injected into a channel increases greatly, and this results in an increase in charge mobility. 
       FIGS. 3A and 3B  are cross-sectional views of an organic TFT according to another embodiment of the present invention. In the organic TFT, an organic semiconductor film  240  is interposed between a gate electrode  210  and source and drain electrodes  230   a ,  230   b.    
     Referring to  FIG. 3A , the organic TFT includes the gate electrode  210 , which is located on an insulating substrate  200  formed of silicon, plastic, or glass and/or a buffer film  205 . The buffer film  205  is selectively formed to planarize the substrate  200 , and is formed of SiO 2  to a thickness of about 3000 Å using PECVD, APCVD, LPCVD, or ECR. 
     The gate electrode  210  is formed of a material having a small work function, such as Al, AlNd, or MoW, so as to embody a p-channel TFT. A gate insulating film  220  is formed to cover the substrate  200  and the gate electrode  210 . The gate insulating film  220  may be a silicon oxide film. 
     The organic semiconductor film  240  is formed on the gate insulating film  220 . 
     The organic semiconductor film  240  is formed using vacuum deposition, preferably thermal evaporation, on the source electrode  230   a  and drain electrode  230   b  on which an electron withdrawing material is coated. The organic semiconductor film  240  is formed of an organic semiconductor material, such as pentacene, oligo-thiophene, poly(alkyl-thiophene), or poly(thienylenevinylene). Preferably, the organic semiconductor film  240  is formed of pentacene. The organic semiconductor film  240  is deposited at a degree of vacuum of 5×10 −4  Torr or less, preferably about 5×10 −7  Torr, and at a speed of 0.5 Å/sec to a thickness of about 1000 Å. 
     As can be seen from  FIGS. 3A and 3B , the source electrode  230   a  and drain electrode  230   b  are formed on the organic semiconductor film  240 , and an organic acceptor film  235  is interposed between the source and drain electrodes  230   a ,  230   b  and the organic semiconductor film  240 . The organic acceptor film  235  is formed by depositing a single layer formed of only an organic acceptor material, or by co-depositing an organic semiconductor film forming material (e.g., pentacene) and an organic acceptor material. In the organic TFT shown in  FIGS. 3A and 3B , since the source electrode  230   a  and drain electrode  230   b  are located above the organic semiconductor film  240 , the organic acceptor film  235  can be formed by depositing 0.1 to 10% by weight of the organic acceptor material around the interface between the organic semiconductor film  240  and the source and drain electrodes  230   a ,  230   b  when the formation of the organic semiconductor film  240  is completed. 
     The organic acceptor film  235  includes a charge transfer material that can form a charge transfer complex on the surface of the source electrode  230   a  and drain electrode  230   b . Thus, the organic semiconductor film  240  obtains a channel doping effect to exclude energy barrier characteristics, and the amount of carriers injected into a channel increases. As a result, contact resistance is reduced, injection of the carriers is improved, and charge mobility increases. The organic acceptor film  235  should be formed to a small thickness of 100 Åor less such that injection of carriers is improved without raising the conductance of an active film. 
     The organic acceptor film  235  is formed by depositing at least one electron withdrawing material selected from the group consisting of aromatic compounds, olefine compounds, aromatic-olefine compounds, aromatic-aromatic conjugated compounds, fused aromatic compounds, and hetero cyclic compounds, which contain at least one selected from the group consisting of nitro group (NO 2 ), cyano group (CN), sulfonyl group (SO 2 ), sulfoxide group (SO), carbonyl group (CO), carboxyl group (CO 2 ), ester group (COO), anhydride, imide, imine, halogen group, fluoroalkyl group, and fluoroaromatic group. 
     In an embodiment of the present invention, the electron withdrawing material is formed of at least one selected from the group consisting of 2,4,7-trinitrofluorenone, 4-nitroaniline, 2,4-dinitroaniline, 5-nitroanthranilonitrile, 2,4-dinitrophenylamine, 1,5-dinitronaphthalene, 4-nitrobiphenyl, 9,10-dicyanoanthracene, 3,5-dinitrobenzonitrile, and N,N′-bis(di-t-buytlphenyl)-3,4,9,10-perylenedicarboxyimide. 
     The source electrode  230   a  and drain electrode  230   b  are formed on the organic acceptor film  235 . To form the source electrode  230   a  and drain electrode  230   b , a source/drain metal film is stacked to a thickness of 5000 Å using sputtering, and is etched using photolithography. In another method, the substrate  200  is covered by a shadow mask, and a metal having a large work function is vacuum deposited on the substrate  200 , thereby forming the source electrode  230   a  and drain electrode  230   b  of the TFT. The metal having a large work function may be Au. 
     In another embodiment, as shown in  FIGS. 6A  thru  7 B, before or after the organic semiconductor film  240  is deposited, a surface processing material, such as octadecyltrichlorosilane (OTS), is coated on the gate insulating film  220 , thus forming a surface processing film  232 . 
     The surface processing film  232  reinforces interfacial adhesion and crowds organic molecules. This surface processing film  232  maybe interposed between the gate insulating film  220  and the organic acceptor film  235  (refer to  FIG. 6A ), or between the gate insulating film  220  and the organic semiconductor film  240  (refer to  FIGS. 6B ,  7 A, and  7 B). 
     The surface processing film  232  may include one selected from the group consisting of trichlorosilyl-(—SiCl 3 ), trimethoxysilyl-(—Si(OMe) 3 ), and mercapto-(—SH) moieties. 
       FIG. 8  shows the coupling structure of the surface of an SiO 2  film processed with OTS. In this case, a hydrophilic tail, which faces an oxide film, strongly interacts with the surface of the SiO 2  film, while a hydrophobic tail strongly interacts with pentacene of the organic semiconductor film  240 . 
     Thus, field effect mobility is greatly improved to about 0.3 cm 2 /V.sec, the on/off ratio that determines the characteristic of a switching device is markedly increased to 10 6 , and the turn-off leakage current is reduced to about 10 −11  A. These parameters satisfy the requisites for a switching device, i.e., a field effect mobility of at least 0.1 cm 2 /V.sec and an on/off ratio of 10 6  to 10 8 , and are achieved by crowding the pentacene molecules and reinforcing the adhesion between pentacene and OTS molecules. 
     In an organic TFT of the present invention, despite the energy barrier required to form a carrier accumulation film, an organic acceptor film which can form a charge transfer complex is coated so as to produce a channel doping effect and exclude energy barrier characteristics. Thus, by increasing the amount of carriers injected into the channel of an active film, contact resistance is reduced and charge mobility increases. 
     Also, the organic acceptor film which can form a charge transfer complex is coated to a small thickness, so that a doping effect is achieved, but the conductivity of the active film is unchanged. Thus, the on/off ratio of the organic TFT is not reduced. 
     The organic TFT of the present invention further includes a surface processing film which reinforces adhesion between a gate insulting film formed of SiO 2  and an adjacent layer, and also crowds pentacene molecules, thereby further improving charge mobility. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 
     For example, although it is described in the embodiments and shown in the drawings that the organic semiconductor film  240  is located on or under the source electrode  230   a  and drain electrode  230   b , the present invention can be also applied to coplanar, inverse coplanar, staggered, and inverse staggered types. In addition, a portion of the organic semiconductor film  240  may extend to the lateral surface of the source and drain electrodes  230   a ,  230   b.