Patent Publication Number: US-2012025196-A1

Title: Organic thin film transistor and semiconductor integrated circuit

Description:
TECHNICAL FIELD 
     The invention relates to an organic thin film transistor and a semiconductor integration circuit including an organic semiconductor layer. 
     BACKGROUND ART 
     Organic thin film transistors (organic TFTs) using organic semiconductor layers made of organic materials having semiconductive characteristics as channel regions are attracting attention as main devices for printable devices, flexible devices, and the like. In metal oxide semiconductor field effect transistors (MOSFETS) and poly-crystalline silicon thin film transistors (poly-Si TFTs), drain current flows when carrier inversion layers are formed in the semiconductors. In contrast, in organic thin film transistors, drain current flows when carrier accumulation layers are formed. 
     In an organic thin film transistor, when gate voltage is applied to the gate electrode, carriers are accumulated in the organic semiconductor layer. By applying drain voltage across the source and drain electrodes, drain current flows through a part of the organic semiconductor layer serving as a channel region. The operation of such an organic thin film transistor can be simulated by device simulation based on Poisson&#39;s equation and a continuity equation as described in NPL 1, for example. 
     If organic semiconductors are p-type, carriers are holes. Generally, holes in organic semiconductors have low mobility, but some materials found by search, improvement, and the like have high mobility. For example, use of an acene compound such as pentacene allows realization of an organic thin film transistor having a characteristic of mobility of about 1 to 10 cm 2 /V·s (for example, see NPL 2). 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] Y. Nakajima, et al. “Confirmation of electric properties of traps at silicon-on-insulator (SOI)/buried oxide (BOX) interface by three-dimensional device simulation”, Physica E, 24, Jan. 2004, p. 92-95. 
         [NPL 2] Wada, two others, “Prospects for molecular nanoelectronics”, Applied Physics, 2001, vol. 70, No. 12, p. 1395-1406 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the organic thin film transistors having the aforementioned level of characteristics can be used as pixel transistors but are inadequate to be used in peripheral circuits of flexible displays and the like, for example. The characteristics of organic thin film transistors need to be further improved. The reason why conventional organic thin film transistors cannot have adequate characteristics is that lack of carriers in the organic semiconductor layer causes an electric field drop. The organic thin film transistors therefore have small apparent current amplification factors. 
     In the light of the aforementioned problems, an object of the invention is to provide an organic thin film transistor and a semiconductor integration circuit including an organic semiconductor layer with the lack of carriers prevented. 
     Solution of Problem 
     According to an aspect of the invention, an organic thin film transistor is provided, which includes: an organic semiconductor layer; a source electrode and a drain electrode which are separated from each other and are individually in contact with the organic semiconductor layer; a gate insulating film which is in contact with the organic semiconductor layer between the source and drain electrodes; and a gate electrode which is opposed to the organic semiconductor layer and is in contact with the gate insulating film. In the organic thin film transistor, a high-concentration region of the organic semiconductor layer which is located near the source electrode has an impurity concentration higher than an impurity concentration of a low-concentration region of the organic semiconductor layer, the low-concentration region being located near the gate electrode in the thickness direction of the organic semiconductor layer between the source and drain electrodes. 
     According to another aspect of the invention, an organic thin film transistor is provided, which includes: a substrate; and first and second transistors, each including: an organic semiconductor layer; a source electrode and a drain electrode which are separated from each other and are individually in contact with the organic semiconductor layer; a gate insulating film which is in contact with the organic semiconductor layer between the source and drain electrodes; and a gate electrode which is opposed to the organic semiconductor layer and is in contact with the gate insulating film. In the first transistor, a high-concentration region of a first conductivity type in the organic semiconductor layer which is located near the source electrode has an impurity concentration higher than an impurity concentration of a low-concentration region in the organic semiconductor layer, the low-concentration region being located near the gate electrode in the thickness direction of the organic semiconductor layer between the source and drain electrodes. In the second transistor, a high-concentration region of a second conductivity type in the organic semiconductor layer which is located near the source electrode has an impurity concentration higher than an impurity concentration of a low-concentration region in the organic semiconductor layer, the low-concentration region being located near the gate electrode in the thickness direction of the organic semiconductor layer between the source and drain electrodes. 
     According to still another aspect, a semiconductor integrated circuit including the above organic thin film transistor is provided. 
     Advantageous Effects of Invention 
     According to the invention, it is possible to provide an organic thin film transistor and a semiconductor integrated circuit with lack of carriers in an organic semiconductor layer prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to a first embodiment of the invention. 
         FIG. 2  is a schematic cross-sectional view showing a structure of a comparative example. 
         FIG. 3(   a ) is a graph showing device simulation results of the comparative example, and  FIG. 3(   b ) is a graph showing device simulation results of the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 4  is a graph showing device simulation results of the organic thin film transistor according to the first embodiment of the invention for varying thickness of a high-concentration region. 
         FIG. 5  is a graph showing device simulation results of the organic thin film transistor according to the first embodiment of the invention for varying impurity concentration of the high-concentration region. 
         FIG. 6  is a graph showing device simulation results of the organic thin film transistor according to the first embodiment of the invention for varying thickness of the organic semiconductor layer. 
         FIG. 7  is a graph showing device simulation results of the organic thin film transistor according to the first embodiment of the invention for varying thickness of a gate insulation layer. 
         FIG. 8  is a cross-sectional process view (No. 1) for explaining a method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 9  is a cross-sectional process view (No. 2) for explaining the method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 10  is a cross-sectional process view (No. 3) for explaining a method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 11  is a cross-sectional process view (No. 4) for explaining a method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 12  is a cross-sectional process view (No. 5) for explaining a method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 13  is a cross-sectional process view (No. 1) for explaining another method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 14  is a cross-sectional process view (No. 2) for explaining another method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 15  is a cross-sectional process view (No. 3) for explaining another method of manufacturing the organic thin film transistor according to the first embodiment of the invention. 
         FIG. 16  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to a modification according to the first embodiment of the invention. 
         FIG. 17(   a ) is a graph showing device simulation results of the organic thin film transistor shown in  FIG. 1 , and  FIG. 17(   b ) is a graph showing device simulation results of the organic thin film transistor shown in  FIG. 16 . 
         FIG. 18  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to still another modification according to the first embodiment of the invention. 
         FIG. 19  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to still another modification according to the first embodiment of the invention. 
         FIG. 20  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to a second embodiment of the invention. 
         FIG. 21  is a graph showing results of device simulation of the organic thin film transistor according to the second embodiment of the invention for varying carrier concentration of a low-concentration region. 
         FIG. 22  is a graph showing other results of device simulation of the organic thin film transistor according to the second embodiment of the invention for varying carrier concentration of a low-concentration region. 
         FIG. 23  is a cross-sectional process view (No. 1) for explaining a method of manufacturing the organic thin film transistor according to the second embodiment of the invention. 
         FIG. 24  is a cross-sectional process view (No. 2) for explaining the method of manufacturing the organic thin film transistor according to the second embodiment of the invention. 
         FIG. 25  is a cross-sectional process view (No. 3) for explaining the method of manufacturing the organic thin film transistor according to the second embodiment of the invention. 
         FIG. 26  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to a modification according to the second embodiment of the invention. 
         FIG. 27  is a schematic cross-sectional view showing a structure of an organic thin film transistor according to another modification according to the second embodiment of the invention. 
         FIG. 28  is a schematic view showing a configuration example of a semiconductor integrated circuit including the organic thin film transistor according to the second embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, first and second embodiments of the invention are described with reference to the drawings. In the following description of the drawings, same or similar portions are given same or similar reference numerals. It should be noted that the drawings are schematic and that the relation between thickness and planer dimensions, the proportion of thicknesses of layers, and the like are different from the real ones. Accordingly, specific thicknesses and dimensions should be determined with reference to the following description. It is certain that some portions have different dimensional relations and proportions through the drawings. 
     The first and second embodiments shown below show devices and methods to embody the technical idea of the invention by example. The embodiments of the invention do not specify the materials, shapes, structures, arrangements, and the like of the constituent components as shown below. The embodiments of the invention can be variously changed within the scope of claims. 
     First Embodiment 
     As shown in  FIG. 1 , an organic thin film transistor  1  according to the first embodiment of the invention includes: an organic semiconductor layer  90 ; source and drain electrodes  50  and  60  which are separated from each other and are in contact with the organic semiconductor layer  40 ; a gate insulating film  30  which is in contact with the organic semiconductor layer  90  between the source and drain electrodes  50  and  60 ; and a gate electrode  20  which is opposed to the organic semiconductor layer  40  and is in contact with the gate insulating film  30 . The impurity concentration of a high-concentration region  41  of the organic semiconductor layer  40  which is located near the source electrode  50  is set higher than that of a low-concentration region  42  of the organic semiconductor layer  90 . The low-concentration region  42  is located near the gate electrode  20  in the thickness direction of the organic semiconductor layer  90  between the source and drain electrodes  50  and  60 . In other words, the impurity concentration of the high-concentration region  91 , which is located near the source and drain electrodes  50  and  60  in the thickness direction of the organic semiconductor layer  40 , is higher than the impurity concentration of the channel region of the organic thin film transistor  1 . In  FIG. 1 , the thickness direction of the organic semiconductor layer  40  is indicated as a direction y, and the direction of channel length L is indicated as a direction x. 
     In the first embodiment shown in  FIG. 1 , the organic semiconductor layer  40  is located above the gate electrode  20 , and the source and drain electrodes  50  and  60  are located on the organic semiconductor layer  40 . The organic thin film transistor  1  shown in  FIG. 1  therefore includes: the gate electrode  20  located on a substrate  1 ; the gate insulating film  30  located on the gate electrode  20 ; the organic semiconductor layer  40  located on the gate insulating film  30 ; and the source and drain electrodes  50  and  60  which are located on the organic semiconductor layer  40  to be separated from each other. The impurity concentrations of the high-concentration regions  41  of the organic semiconductor layer  40  individually located under the source and drain electrodes  50  and  60  are set higher than the impurity concentration of the low-concentration region  42  of the organic semiconductor layer  40  which is located above the gate electrode  20  between the source and drain electrodes  50  and  60 . 
     In the organic thin film transistor  1  shown in  FIG. 1 , when a predetermined gate voltage Vg is applied to the gate electrode  20 , carriers are accumulated in the gate insulating film  30  side of the organic semiconductor layer  40 . If drain voltage is applied across the source and drain electrodes  50  and  60  with the carriers being accumulated in the organic semiconductor layer  40 , drain current flows between the source and drain electrodes  50  and  60 . In other words, the organic thin film transistor  1  operates using the organic semiconductor layer  40  above the gate electrode  20  as the channel region. Channel length L is a distance between the source and drain electrodes  50  and  60 . 
     The substrate  10  can be an insulator substrate. For example, a plurality of the organic thin film transistors  1  are formed on a silica substrate, and the silica substrate is diced into chips, thus obtaining the individual organic thin film transistors  1 . 
     The gate electrode  20  can be made of a conductive film such as a metallic film of aluminum (Al), molybdenum (Mo), or tungsten (W) or a polysilicon film. The gate insulating film  30  can be a silicon oxide film, a silicon nitride film, a high-k film having a high permittivity, or the like. 
     The organic semiconductor layer  40  is made of an organic material having semiconductor characteristics. The p-type material of the organic semiconductor layer  40  can be pentacene or the like, and the p-type impurities applied to the high-concentration regions  41  can be iodine or ionic molecules such as tetrathiofulvalene (TTF) and tetracyanoquinodimethane (TCNQ), for example. In the following description, the organic semiconductor layer  40  is a p-type semiconductor, or the carriers moving in the channel region are holes. In the example shown in  FIG. 1 , a part of upper portions of the organic semiconductor region  41  in contact with the source and drain electrodes  50  or  60  is the high-concentration region  41 , and the other region is the low-concentration region  42  having a lower impurity concentration than the impurity concentration of the high-concentration region  41 . 
     The source and drain electrodes  50  and  60  can be made of calcium (Ca), Al, gold (Au), or the like, for example. 
       FIG. 2  shows an organic thin film transistor of a comparative example for a comparison of characteristics with the organic thin film transistor  1  according to the first embodiment of the invention. The comparative example shown in  FIG. 2  differs from the organic thin film transistor  1  shown in  FIG. 1  in that the organic semiconductor layer  40  is composed of only the low-concentration region and does not include the high-concentration region. 
     In the organic thin film transistor  1  and comparative example used in device simulation whose results are shown below, the channel length L is 5 μm; channel width is 10 μm; thickness d 2  of the organic semiconductor layer  40  is 50 nm; impurity concentration N 2  of the low-concentration region  42  is 1×10 15  cm −3 ; and thickness dg of the gate insulating film  30  is 300 nm. Thickness d 1  of the high-concentration regions  41  of the organic thin film transistor  1  is 5 nm; and impurity concentration N 1  thereof is 1×10 20  cm −3 . 
       FIGS. 3(   a ) and  3 ( b ) show calculation results of device simulation for the electric characteristics of the comparative example shown in  FIG. 2  and the organic thin film transistor  1  shown in  FIG. 1 , respectively. The electric characteristics shown in  FIGS. 3(   a ) and  3 ( b ) are current-voltage characteristics obtained by calculating drain current Id with drain voltage Vd varying between 0 to −50 V when the gate voltage Vg is set to −10, −20, −30, and −40 V. 
     The comparison between  FIGS. 3(   a ) and  3 ( b ) reveals that the current amplification factor of the organic thin film transistor  1  according to the first embodiment of the invention is about three times as high as that of the comparative example shown in  FIG. 2 . 
     This is because holes as carriers are supplied from the high-concentration regions  41  to the channel region above the gate electrode  20 . The supply of holes prevents lack of carriers in the channel region, and an electric field drop therefore does not occur. Accordingly, the current amplification factor of the organic thin film transistor  1  is higher than that of the comparative example not including the high-concentration regions  41 . Even if only one of the high-concentration regions  41  is formed near the source electrode  50 , the lack of carriers in the channel region is prevented, and the current amplification factor of the organic thin film transistor  1  is improved. 
     The followings show the result of examination on the characteristics by device simulation for the organic thin film transistor  1  shown in  FIG. 1  with the structures being varied. 
     Examination Example 1 
       FIG. 4  shows current-voltage characteristics of the drain current Id and drain voltage Vd which are obtained by device simulation with the thickness d 1  of the high-concentration region  41  of the organic thin film transistor  1  varying between 0.1 and 30 nm. Herein, the thickness d 2  of the organic semiconductor layer  40  is 30 nm; the impurity concentration N 2  of the low-concentration region  42  is 1×10 15  cm −1 ; and the impurity concentration N 1  of the high-concentration regions  41  is 1×10 20  cm −3 . The thickness dg of the gate insulating film  30  is 300 nm; the channel length L is 5 μm; the gate voltage Vg is −30 V; and the drain voltage Vd is 0 to −50V. 
     As shown in  FIG. 4 , the current-voltage characteristics are substantially the same for the thickness d 1  of the high-concentration region  41  varying from 0.1 nm to 30 nm. Herein, when the thickness d 1  is 30 nm, the entire regions of the organic semiconductor layer  40  under the source and drain electrodes  50  and  60  in the thickness direction thereof are the high-concentration regions  41 . Accordingly, it is confirmed that the high-concentration regions  41  are effective on improving the characteristic of the organic thin film transistor  1  when the thickness d 1  of the high-concentration regions  41  is at least not less than 0.1 nm. 
     Examination Example 2 
       FIG. 5  shows current-voltage characteristics of the drain current Id and drain voltage Vd which are obtained by device simulation with the impurity concentration N 1  of the high-concentration region  41  of the organic thin film transistor  1  varying between 1×10 16  and 1×10 20  cm −3 . Herein, the thickness d 2  of the organic semiconductor layer  40  is 30 nm; the thickness d 1  of the high-concentration region  41  is 5 nm; and the impurity concentration N 2  of the low-concentration region  42  is 1×10 15  cm −3 . The thickness dg of the gate insulating film  30  is 300 nm; the channel length L is 5 μm; the gate voltage Vg is −30 V; and the drain voltage Vd is 0 to −50V. 
     As shown in  FIG. 5 , when the impurity concentration N 1  of the high-concentration region  41  is not less than 1×10 17  cm −3 , the current-voltage characteristics thereof are substantially the same regardless of the impurity concentration N 1 . Accordingly, in order to obtain the effect on improving the characteristics of the organic thin film transistor  1 , it is effective that the impurity concentration N 1  of the high-concentration regions  41  is set to 1×10 17  cm −3  or higher. 
     As shown in  FIG. 5 , the effect on improving the characteristics can be also obtained even if the impurity concentration N 1  of the high-concentration region  91  is not less than 1×10 16  cm −3 . This is because the impurity concentration N 2  of the low-concentration region  42  is set to 1×10 15  cm −3 . Therefore, when the impurity concentration N 1  of the high-concentration regions  41  is higher than the impurity concentration N 2  of the low-concentration region  42 , the effect on improving the organic thin film transistor  1  can be obtained not only when the impurity concentration N 2  is 1×10 16  cm −3  or higher regardless of the values of the impurity concentrations N 1  and N 2 . 
     Examination Example 3 
       FIG. 6  shows current-voltage characteristics of the drain current Id and drain voltage Vd which are obtained by device simulation with the thickness d 2  of the organic semiconductor layer  40  of the organic thin film transistor  1  varying between 10 and 100 nm. Herein, the impurity concentration N 2  of the low-concentration region  42  is 1×10 15  cm −3 ; the thickness d 1  of the high-concentration region  41  is 5 nm; and the impurity concentration N 1  of the high-concentration regions  41  is 1×10 2 ° cm −3 . The thickness dg of the gate insulating film  30  is 300 nm; the channel length L is 5 μm; the gate voltage Vg is −30 V; and the drain voltage Vd is 0 to −50V. 
     As shown in  FIG. 6 , the current-voltage characteristics are substantially the same regardless of the thickness d 2  of the organic semiconductor layer  40 . The thickness d 2  of the organic semiconductor layer  40  very little affects the current-voltage characteristic of the organic thin film transistor  1 . Accordingly, the thickness d 2  of the organic semiconductor layer  40  can be arbitrarily set to obtain the effect on improving the characteristic of the organic thin film transistor  1 . 
     Examination Example 4 
       FIG. 7  shows the obtained current-voltage characteristics of the drain current Id and drain voltage Vd which are obtained by device simulation with the thickness dg of the gate insulating film  30  of the organic thin film transistor  1  varying between 50 and 200 nm. Herein, the thickness d 2  of the organic semiconductor layer  40  is 30 nm; the impurity concentration N 2  of the low-concentration region  42  is 1×10 15  cm −3 ; the thickness d 1  of the high-concentration regions  41  is 5 nm; and the impurity concentration N 1  of the high-concentration regions  41  is 1×10 20  cm −3 . The channel length L is 5 μm; the gate voltage Vg is −30 V; and the drain voltage Vd is 0 to −50V. 
     As shown in  FIG. 7 , the magnitude of the drain current Id is inversely proportional to the thickness dg of the gate insulating film  30  for a range of the thickness dg of the gate insulating film  30  from 50 to 300 nm. Such a current-voltage characteristic is similar to that of semiconductor devices including silicon semiconductors, such as MOSFETs. Accordingly, it is apparent that the effect on improving the characteristic of the organic thin film transistor  1  can be also obtained if the gate insulating film  30  is made very thin or if the gate insulating film  30  is composed of insulating film with high permittivity other than silicon oxide film or silicon nitride film. Generally, the thinner the thickness dg of the gate insulating film  30 , the higher the effect on improving the characteristic of the organic thin film transistor  1 . 
     As described above, according to the organic thin film transistor  1  according to the first embodiment of the invention, it is possible to provide an organic thin film transistor with the electric characteristic improved by optimizing the structure and the impurity concentrations of the organic semiconductor layer  40 . Specifically, the impurity concentration N 1  of the high-concentration regions  41  of the organic semiconductor layer  40 , which are located under the source and drain electrodes  50  and  60 , is set higher than the impurity concentration N 2  of the low-concentration region  42  of the organic semiconductor layer  40 , which is located above the gate electrode  20  between the source and drain electrodes  50  and  60 . Therefore, carriers are supplied from the high-concentration region  41  to increase the concentration of carriers in the organic semiconductor layer  40 , thus preventing the lack of carriers in the organic semiconductor layer  40 . This results in an increase in the current amplification factor of the organic thin film transistor  1 . Accordingly, it is possible to constitute a high-performance circuit using the organic thin film transistor  1  with the electric characteristics improved. For example, if the organic thin film transistor  1  is used to constitute each of the pixel transistors and peripheral circuits, a flexible device, a printable device, or the like can be implemented with only organic thin film transistors. 
     With reference to  FIGS. 8 to 12 , a description is given of a method of manufacturing the organic thin film transistor  1  according to the first embodiment of the invention. The following method of manufacturing the organic thin film transistor  1  is shown by example. It is certain that the method of manufacturing the organic thin film transistor  1  can be implemented by other various manufacturing methods including modifications thereof. 
     (a) A thin film  100  for liftoff is formed on the entire surface of the substrate  10  as an insulator substrate, and then using photolithography and etching, a part of the lift-off thin-film  100  is removed to expose a region of the surface of the substrate  10  where the gate electrode  20  is to be formed. As shown in  FIG. 8 , an opening  101  is thus formed. The thin film  100  for liftoff can be made of a photoresist film or the like. 
     (b) As shown in  FIG. 9 , a gate electrode layer  200  having a predetermined thickness is then formed on the substrate  10  and liftoff thin-film  100  so as to fill the opening  101 . As the gate electrode layer  200 , an aluminum film with a thickness of about 0.3 μm is formed, for example. The material and thickness of the gate electrode layer  200  can be arbitrarily selected. 
     (c) The lift-off thin film  100  is removed to form the gate electrode  20  by a lift-off process as shown in  FIG. 10 . 
     (d) As the gate insulating film  30 , a silicon oxide film with a thickness of 300 nm, for example, is formed on the substrate  10  and gate electrode  20 . Furthermore, on the gate insulating film  30 , for example, a pentacene film with a thickness of 30 nm is formed as the organic semiconductor layer  40 . As shown in  FIG. 11 , the high-concentration regions  41  are formed so as to be located at the predetermined position, that is, under the source and drain electrodes  50  and  60  in the organic semiconductor layer  40 . For example, the high-concentration regions  41  are formed by a coating process, for example. 
     (e) As shown in  FIG. 12 , on the organic semiconductor layer  40 , a 10 to 100 nm thick electrode film  500  made of Al, Ca, or the like is formed. Subsequently the electrode film  500  is patterned to form the source and drain electrodes  50  and  60 . The organic thin film transistor  1  according to the first embodiment of the invention is thus completed. 
     The gate insulating film  30 , organic semiconductor layer  40 , electrode film  500  can be formed by spattering, vapor deposition, or the like. The source and drain electrodes  50  and  60  can be formed using photolithography and liftoff or using photolithography and etching. 
     Upper part or all of each predetermined region of the organic semiconductor layer  40  in the thickness direction may be etched using the photoresist film patterned using photolithography as a mask, and the high-concentration regions  41  are formed in the etched regions. Alternatively, the high-concentration regions  41  may be formed by doping p-type impurities in the predetermined regions of the organic semiconductor layer  40 . 
     In the above example, the gate electrode  20  is formed by using a liftoff process. However, the gate electrode  20  may be formed using an etching process. Alternatively, the gate electrode  20  may be formed by using a liftoff process with a double-layer resist process applied thereto. Hereinafter, with reference to  FIGS. 13 to 15 , an example of manufacturing the organic thin film transistor  1  by applying the double layer resist process is described. 
     (a) As shown in  FIG. 13 , polydimethylglutarimide (PMGI) is applied on the substrate  10  up to a thickness of 200 nm by spin coating as a lower resist film  111 . Positive photoresist (OFPR) is applied on the lower resist film  111  by spin coating as an upper resist film  112  up to a thickness of 500 nm. A double layer resist film  110  is thus formed on the substrate  10 . 
     (b) A desired pattern is transferred to the double layer resist film  110  by an ultraviolet exposure process and is then developed to expose a region of the surface of the substrate  10  where the gate electrode  20  is to be placed. The cross-sectional structure shown in  FIG. 14  is thus obtained. At this time, since the etching rate of the lower resist film  111  is higher than that of upper resist film  112 , the upper resist film  112  protrudes in space over the region where the surface of the substrate  10  is exposed, forming an overhang structure. The amount of overhang is determined by the difference between rates at which OFPR and PMGI dissolve in a developer. 
     (c) As shown in  FIG. 15 , the gate electrode layer  200  is formed on the substrate  10  and double layer resist film  110  by vapor deposition process. The double layer resist film  110  is then removed, forming the gate electrode  20  by liftoff in a similar manner to the method described with reference to  FIG. 10 . The subsequent processes are the same as the processes previously described with reference to  FIGS. 10 to 12 . 
     According to the double layer resist process described above, the overhang structure with the upper resist film  112  protrudes into space more than the lower resist film  111 . Accordingly, each edge of the gate electrode  20  has a gentle slope structure as shown in  FIG. 15 . This can prevent a so-called cutting phenomenon in steps in which the gate insulating film  30  and organic semiconductor layer  40  deposited on the gate electrode  20  are not continuous at the edge of the gate electrode  20 . 
     According to the aforementioned method of manufacturing the organic thin film transistor  1 , the impurity concentration N 1  of the high-concentration regions  41  located under the source and drain electrodes  50  and  60  are set higher than the impurity concentration N 2  of the low-concentration region  42  located above the gate electrode  20 . It is therefore possible to provide an organic thin film transistor with the lack of carriers in the organic semiconductor layer  90  prevented. 
     Modification 
       FIG. 16  shows the organic thin film transistor  1  according to a modification of the first embodiment of the invention. The organic thin film transistor  1  shown in  FIG. 16  differs from that shown in  FIG. 1  in that the high-concentration regions  41  are located on the gate insulating film  30  side of the organic semiconductor layer  40 . In the organic thin film transistor  1  shown in  FIG. 1 , the high-concentration regions  41  are in contact with the source and drain electrodes  50  and  60 . On the other hand, in the organic thin film transistor  1  shown in  FIG. 16 , the high-concentration regions  41  are in contact with the gate insulating film  30  and are separated from the source and drain electrodes  50  and  60 . The other configuration thereof is the same as the organic thin film transistor  1  shown in  FIG. 1 . 
       FIGS. 17(   a ) and  17 ( b ) show current-voltage characteristics of the drain current Id and drain voltage Vd which are obtained by device simulation for the organic thin film transistors  1  shown in  FIGS. 1 and 16  with the channel length L varying between 5 and 20 μm, respectively. The thickness d 2  of the organic semiconductor layer  40  is 30 nm; the impurity concentration N 2  of the low-concentration regions  42  is 1×10 15  cm −3 ; the thickness d 1  of the high-concentration region  41  is nm; and the impurity concentration N 1  of the high-concentration regions  41  is 1×10 20  cm −3 . The thickness dg of the gate insulating film  30  is 300 nm; the gate voltage Vg is −30 V; and the drain voltage Vd is 0 to −50V. 
     The comparison between  FIGS. 17(   a ) and  17 ( b ) reveals that the organic thin film transistors  1  thereof have the same current-voltage characteristics. In other words, the effect on improving the performance can be provided regardless of the positions of the high-concentration regions  41  in the organic semiconductor layer  40  in the thickness direction if the high-concentration regions  41  are located near the source electrode  50  and near the drain electrode  60 . 
     The organic thin film transistor  1  shown in  FIG. 16  is manufactured by forming the high-concentration regions  41  in the predetermined regions on the gate insulating film  30  by coating or the like and forming a pentacene film on the high-concentration regions  41  by vapor deposition or the like as the organic semiconductor layer  40 . 
       FIG. 1  shows the example in which the high-concentration regions  41  are in contact with the source and drain electrodes  50  and  60 , and  FIG. 16  shows the example in which the high-concentration regions  41  are in contact with the gate insulating film  30 . However, in order to supply and accumulate carriers in the organic semiconductor layer  40 , the high-concentration regions  41  only need to be individually located near the source electrode  50  and drain electrode  60 . Accordingly, the high-concentration regions  41  may be surrounded by the low-concentration region  42 . 
     Alternatively, the high-concentration regions  41  may be the entire regions of the organic semiconductor layer  40  under the source and drain electrodes  50  and  60  in the thickness direction. In this case, the high-concentration regions  41  are in contact with the source and drain electrodes  50  and  60  and are also contact with the gate insulating film  30 . Moreover, the high-concentration regions  41  may be formed in regions of the organic semiconductor layer  40  which are in contact with the source and drain electrodes  50  and  60  partially in the planar direction and in the thickness direction. 
     The positional relationship of the gate electrode  20 , organic semiconductor layer  40 , and source and drain electrodes  50  and  60  is not limited to the first embodiment shown in  FIG. 1 . The organic thin film transistor  1  only should have an organic TFT structure in which the high-concentration regions  41  are located near the source electrode  50 . In the first embodiment shown in  FIG. 1 , the organic semiconductor layer  40  is located on the gate insulating film  30  above the gate electrode  20 , and the source and drain electrodes  50  and  60  are located on the organic semiconductor layer  40 . However, as shown in  FIG. 18 , for example, the organic thin film transistor  1  may have a structure in which the organic semiconductor layer  40  is located on the source and drain electrodes  50  and  60 . Specifically, the organic thin film transistor  1  shown in  FIG. 18  includes: the gate electrode  20  located on the substrate  10 ; the gate insulating film  30  located on the gate electrode  20 ; the source and drain electrodes  50  and  60  located on the gate insulating film  30  so as to be separated from each other; and the organic semiconductor layer  40  continuously located on the source and drain electrodes  50  and  60 . The impurity concentration N 1  of the high-concentration regions  41  of the organic semiconductor layer  40 , which are located on the source electrode  50  and the drain electrode  60 , is set higher than the impurity concentration N 2  of the low-concentration region  42  of the organic semiconductor layer  40 , which is located above the gate electrode  20  between the source and drain electrodes  50  and  60 . 
     Alternatively, as shown in  FIG. 19 , the organic thin film transistor  1  may have a structure in which the organic semiconductor layer  40  is located on the source and drain electrodes  50  and  60  and the gate insulating film  30  and gate electrode  20  are located on the organic semiconductor layer  40 . The organic thin film transistor  1  shown in  FIG. 19  includes: the source and drain electrodes  50  and  60  located on the substrate  10  so as to be separated from each other; the organic semiconductor layer  40  continuously located on the source and drain electrodes  50  and  60 ; the gate insulating film  30  located on the organic semiconductor layer  40 ; and the gate electrode  20  located on the gate insulating film  30 . The impurity concentration N 1  of the high-concentration regions  41  of the organic semiconductor layer  40 , which are individually located on the source and drain electrodes  50  and  60 , is set higher than the impurity concentration N 2  of the low-concentration region  42  of the organic semiconductor layer  40 , which is located under the gate electrode  20  between the source and drain electrodes  50  and  60 . 
       FIGS. 18 and 19  show the examples in which the high-concentration regions  41  are in contact with the source and drain electrodes  50  and  60 . However, as previously described, the high-concentration regions  41  only should be individually located near the source and drain electrodes  50  and  60 . 
     In the modifications of the first embodiment shown in  FIGS. 18 and 19 , the impurity concentration N 1  of the high-concentration regions  41  located near the source and drain electrodes  50  and  60  can be set higher than the impurity concentration N 2  of the low-concentration region  42  in which the channel region is formed. Accordingly, it is possible to implement the organic thin film transistor  1  in which carriers are supplied to the channel region from the high-concentration regions  41  to prevent the lack of carriers in the organic semiconductor layer  40 . 
     Second Embodiment 
     An organic thin film transistor according to a second embodiment of the invention is a complementary organic thin film transistor including: an organic thin film transistor having majority carriers of a first conductivity type as main current; and an organic thin film transistor having majority carriers of a second conductivity type as main current. The first and second conductivity types are opposite to each other. The first conductivity type is n-type when the second conductivity type is p-type, and the first conductivity type is p-type when the second conductivity type is n-type. According to the complementary organic thin film transistor including an organic thin film transistor performing a p-channel operation using holes as the majority carriers (hereinafter, referred to as an p-channel organic thin film transistor) and an organic thin film transistor performing an n-channel operation using electrons as the majority carriers (hereinafter, referred to as an n-channel organic thin film transistor) which are formed on a same substrate, a high-performance circuit can be implemented similarly to silicon complementary MOS (CMOS) circuits. 
       FIG. 20  shows an example of a complementary organic thin film transistor  1 A in which an n-channel organic thin film transistor  1   n  and a p-type organic thin film transistor  1   p  are formed on a same substrate  10 . Each of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  has the same structure as the organic thin film transistor  1  shown in  FIG. 1 . High-concentration regions  41   n  of an organic semiconductor layer  410  of the n-channel organic thin film transistor  1   n  are n-type conductors, and high-concentration regions  41   p  of an organic semiconductor layer  420  of the p-channel organic thin film transistor  1   p  are p-type conductors. The n-channel and p-channel thin film transistors  1   n  and  1   p  are the same only excepting the conductivity types of the high-concentration regions  41   n  and  41   p.    
     The n-channel organic thin film transistor  1   n  includes: the organic semiconductor layer  410 ; source and drain electrodes  510  and  610  which are separated from each other and are in contact with the organic semiconductor layer  410 ; a gate insulating film  310  which is in contact with the organic semiconductor layer  410  between the source and drain electrode  50  and  60 ; and a gate electrode  210  which is opposed to the organic semiconductor layer  40  and is in contact with the gate insulating film  310 . The impurity concentration of high-concentration regions  41   n  of the n-type conductor in the organic semiconductor layer  410 , which are located near the source and drain electrodes  510  and  610 , is set higher than the impurity concentration of a low-concentration region  412  in the organic semiconductor layer  910 , which is located near the gate electrode  20  in the thickness direction of the organic semiconductor layer  910  between the source and drain electrodes  510  and  610 . The impurity concentration of the high-concentration regions  41   n , which are located near the source and drain electrodes  510  and  610  in the thickness direction of the organic semiconductor layer  410 , is higher than the impurity concentration of the channel region of the n-channel organic thin film transistor  1   n . The low-concentration region  412  may be either a p-type or an n-type conductor. 
     On the other hand, the p-channel organic thin film transistor  1   p  includes: the organic semiconductor layer  420 ; source and drain electrodes  520  and  620  which are separated from each other and are in contact with the organic semiconductor layer  420 ; a gate insulating film  320  which is in contact with the organic semiconductor layer  420  between the source and drain electrodes  520  and  620 ; and a gate electrode  220  which is opposed to the organic semiconductor layer  420  and is in contact with the gate insulating film  320 . The impurity concentration of high-concentration regions  42   p  of the p-type conductor in the organic semiconductor layer  420 , which are individually located near the source and drain electrodes  520  and  620 , is set higher than the impurity concentration of a low-concentration region  422  of the organic semiconductor layer  420 , which is located near the gate electrode  220  in the thickness direction of the organic semiconductor layer  420  between the source and drain electrodes  520  and  620 . In other words, the impurity concentration of the high-concentration regions  42   p , which are located near the source and drain electrodes  520  and  620  in the thickness direction of the organic semiconductor layer  420 , is set higher than the impurity concentration of the channel region of the p-channel organic thin film transistor  1   p . The low-concentration region  422  may be either a p-type or an n-type conductor. 
     To be more specific, in the complementary organic thin film transistor  1 A shown in  FIG. 20 , the gate electrodes  210  and  220  are located on the substrate  10 , and the low-concentration regions  412  and  422  are located on the gate insulating films  310  and  320 , respectively. On the organic semiconductor layers  910  and  420 , the source electrodes  510  and  520  and the drain electrodes  610  and  620  are located. The high-concentration regions  41   n  as the n-type carrier high-concentration regions are individually located in contact with the source and drain electrodes  510  and  610 . The high-concentration regions  42   p  as the p-type carrier high-concentration region are individually located in contact with the source and drain electrodes  520  and  620 . 
       FIGS. 21(   a ) and  21 ( b ) show results of device simulation for the n-channel and p-channel organic thin film transistors  1   n  and  1   p , respectively.  FIG. 21(   a ) shows device simulation results of the drain current-drain voltage characteristics of the n-channel organic thin film transistor  1   n  when the n-type carrier concentration of the high-concentration regions  41   n  is set to 1×10 20  cm −3 . In the device simulation, the low-concentration region  412  constituting the channel region is a p-type conductor, and the p-type carrier concentration is varied to 1×10 10 , 1×10 11 , 1×10 15 , 1×10 16 , and 1×10 17  cm −3 .  FIG. 21(   b ) shows device simulation results of the drain current-drain voltage characteristics of the p-channel organic thin film transistor  1   p  when the p-type carrier concentration of the high-concentration regions  42   p  is set to 1×10 20  cm −3 . In the device simulation, the low-concentration region  422  constituting the channel region is a p-type conductor, and the p-type carrier concentration is varied to 1×10 12 , 1×10 13 , 1×10 15 , 1×10 16 , and 1×10 17  cm −3 . The gate voltage in  FIG. 21(   a ) is set to 50V, and the gate voltage in  FIG. 21(   b ) is set to −50V. The carrier mobilities thereof are set to 0.3 cm 2 /Vs. 
     As apparent from  FIGS. 21(   a ) and  21 ( b ), when the concentrations of the high-concentration regions  41   n  and  42   p  are set to 1×10 20  cm −3 , the n-channel and p-channel organic thin film transistors  1   n  and  1   p  in which the low-concentration regions  412  and  422  have concentrations in a range between 1×10 10  and 1×10 16  cm −3  have substantially a same drain current-drain voltage characteristic. 
     However, if the carrier concentration of the low-concentration region  412  is increased to 1×10 17  cm −3 , the carriers begin to be recombined to reduce the drain current in the n-channel organic thin film transistor  1   n . If the carrier concentration of the low-concentration region  422  is increased to 1×10 17  cm −3 , leak current began to flow to increase the drain current in the p-channel organic thin film transistor  1   p.    
     Based on the device simulation results shown in  FIGS. 21(   a ) and  21 ( b ), even if the organic semiconductor layer constituting the channel region is a p-type conductor, the complementary organic thin film transistor operation can be implemented by locating the n-type and p-type high-concentration regions near the source and drain electrodes. When the carrier concentrations of the high-concentration regions  41   n  and  41   p  are about 1×10 20  cm −3 , it is preferable that the carrier concentrations of the low-concentration regions  412  and  422  are set to not more than 1×10 17  cm −3 . In other words, it is preferable that the carrier concentrations of the low-concentration regions  412  and  422  are lower. Similarly, even if the organic semiconductor layer constituting the channel region is an n-type conductor, the complementary organic thin film transistor operation can be implemented by locating the n-type and p-type high-concentration regions near the source and drain electrodes. 
       FIG. 22(   a ) shows device simulation results of the drain current-drain voltage characteristics of the n-channel organic thin film transistor in when the n-type carrier concentration of the high-concentration regions  41   n  is 1×10 17  cm −3 .  FIG. 22(   b ) shows device simulation results of the drain current-drain voltage characteristics of the p-channel organic thin film transistor  1   p  when the p-type carrier concentration of the high-concentration region  41   p  is 1×10 17  cm −3 . The graph of  FIG. 22(   a ) shows results of device simulation with the low-concentration region  412  being a p-type conductor and the p-type carrier concentration varying to 1×10 10 , 1×10 11 , 1×10 15 , and 1×10 16  cm −3 . The graph of  FIG. 22(   b ) shows results of device simulation with the low-concentration region  422  being a p-type conductor and the p-type carrier concentration thereof varying to 1×10 12 , 1×10 13 , 1×10 15 , 1×10 16 , and 1×10 17  cm −3 . 
     As shown in  FIGS. 21(   a ) and  21 ( b ), the characteristics of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  are substantially the same when the carrier concentrations of the high-concentration regions  41   n  and  42   p  are 1×10 20  cm 3 . However, as shown in  FIGS. 22(   a ) and  22 ( b ), when the carrier concentrations of the high-concentration regions  41   n  and  42   p  are set to 1×10 17  cm −3 , the drain current of the p-channel organic thin film transistor  1   p  is about three times as large as the drain current of the n-channel organic thin film transistor in, thus providing a better characteristic. This is because, in the case where the low-concentration regions  412  and  422  are made of p-type materials, the characteristic of the n-channel organic thin film transistor in is degraded due to the influence of the recombination of carriers in the low-concentration region  412  unless the carrier concentration of the high-concentration region  41   n  is made high enough. 
     Accordingly, it is preferable that the carrier concentration of the high-concentration regions  41   n  is two orders of magnitude higher than the carrier concentration of the low-concentration region  412 . On the other hand, in the case where the low-concentration regions  412  and  422  are made of n-type, materials, the characteristic of the p-channel organic thin film transistor  1   p  is degraded unless the carrier concentration of the high-concentration  42   p  is made high enough. Accordingly, it is necessary to properly select the circuit configuration and parameters including the carrier concentrations of the high-concentration regions  41   n  and  42   p  according to whether each of the low-concentration regions  412  and  422  is an n-type or p-type conductor. 
     The n-channel and p-channel organic thin film transistors  1   n  and  1   p  shown in  FIG. 20  are manufactured by the same method as the method of manufacturing the organic thin film transistor  1  described with reference to  FIGS. 8 to 12  and  13  to  15 . The complementary organic thin film transistor  1 A is manufactured as follows, for example. 
     As shown in  FIG. 23 , a conductor layer is formed on the substrate  10  made of an insulator and is then patterned to form the gate electrodes  210  and  220 . On the gate electrode  20 , an insulating film  300  is formed. On the insulating film  300 , an organic semiconductor film  400  is formed. 
     The substrate  10  may be a substrate of a silicon wafer with a thermal oxide film or the like formed thereon, a glass or crystal oxide substrate such as a silica glass or sapphire substrate, a plastic sheet, or the like. In short, the substrate  10  can be composed of any substrate made of an insulator. 
     The materials and thicknesses of the gate electrodes  210  and  220  are determined in the light of the desired transistor characteristics, structures of the gate electrodes  210  and  220  facilitating formation of the organic semiconductor film  400 , and the like. The gate electrodes  210  and  220  can be formed by forming a metallic layer of Al, nickel (Ni), or the like by vapor deposition process, by applying fine particles of silver (Ag) or the like, and using an organic conductor such as polyacetylene. 
     The insulating film  300  is formed to a predetermined thickness so as to prevent occurrence of defects such as pin holes. The insulating film  300  can be formed using a process such as sputtering, vapor deposition, or coating, for example. The material of the insulating film  300  can be an insulator material generally used in a gate oxide film, such as an inorganic insulator including a silicon oxide film, a high-dielectric material such as tantalum oxide film, and an organic insulator. 
     The method of growing the organic semiconductor film  400  is not particularly limited and only should be a method capable of forming the organic semiconductor film  400  uniformly. The organic semiconductor film  400  can be formed by sputtering, laser deposition, CVD, coating, or the like, for example. The material of the p-type conductor can be pentacene, ruburene, or the like, and the material of the n-type conductor can be C60 or the like. The organic semiconductor film  400  can be made of a material generally used as an organic semiconductor. The organic semiconductor film  400  constituting the channel region can be made of an organic semiconductor which is conventionally not used because of the low carrier concentration thereof. This is because carriers in the channel region are supplied from the high-concentration regions  41   n  and  42   p . This is a characteristic of the embodiment of the invention. 
     As shown in  FIG. 24 , the insulating film  300  and organic semiconductor film  400  are divided corresponding to the positions of the n-channel and p-channel organic thin film transistors  1   n  and  1   p . The gate insulating films  310  and  320  and organic semiconductor layers  410  and  420  are thus formed. 
     Thereafter, the n-type high-concentration regions  41   n  are formed near regions where the source and drain electrodes  510  and  610  of the n-channel organic thin film transistor  1   n  are to be located. The p-type high-concentration regions  42   p  are formed near regions where the source and drain electrodes  520  and  620  of the p-channel organic thin film transistor  1   p  are to be located. The high-concentration regions  41   n  and  42   p  are formed by addition of a carrier inducing agent, deposition of a high-carrier concentration material, or the like. The material of the n-type high-concentration regions  41   n  can be alkali metal such as cesium. The material of the p-type high-concentration region  92   p  can be a halogen such as bromine, an oxide such as vanadium oxide, or the like. Forming the high-concentration regions  41   n  and  41   p  by using materials generating n-type or p-type carriers in the organic semiconductor layers  410  and  420 , such as elements, compound materials, or organic materials, is within the scope of the organic thin film transistor according to the embodiment of the invention. 
     The high-concentration regions  41   n  and  42   p  of the n-channel and p-channel organic thin film transistors  1   n  need to include different types of impurities at high-concentrations. Accordingly, as shown in  FIG. 25 , the region other than where the high-concentration regions  41   n  are to be formed is covered with a mask  700  to form the high-concentration regions  41   n  by screen printing or the like. In a similar manner, the region other than where the high-concentration regions  42   p  are to be formed is covered with a mask to form the high-concentration regions  42   p.    
     Subsequently, the source electrodes  510  and  520  and the drain electrodes  610  and  620  are formed at predetermined positions. In such a manner, the complementary organic thin film transistor  1 A shown in  FIG. 20  is completed. In the above-described method, the insulating film  300  and organic semiconductor film  400  are divided before the high-concentration regions  41   n  and  42   p  are formed. However, the insulating film  300  and organic semiconductor film  400  may be divided after the high-concentration regions  41   n  and  41   p  are formed. 
     As described above, the organic thin film transistor according to the second embodiment of the invention is characterized in that the material supplying electrons and supplying holes are selectively formed in the regions where the high-concentration regions  41   n  and  42   p  of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  are to be located, respectively. The complementary organic thin film transistor  1 A can be therefore manufactured using the organic semiconductor layers  410  and  420  of a same conductivity type. Accordingly, it is possible to implement a high-performance complementary organic thin film transistor while significantly reducing manufacturing cost. 
     In the complementary organic thin film transistor  1 A shown in  FIG. 20 , the high-concentration regions  41   n  and  42   p  are respectively formed in the organic semiconductor layers  410  and  402  partially in the thickness direction. The regions where the high-concentration regions  41   n  and  42   p  are to be formed are not limited to the example shown in  FIG. 20 . The high-concentration regions  41   n  and  42   p  only should be located near the source electrodes  510  and  520  and the drain electrodes  610  and  620 , respectively, and do not need to be in contact with the source electrodes  510  and  520  and the drain electrodes  610  and  620 . Accordingly, the high-concentration regions  41   n  and  42   p  may be located so as to be surrounded by the low-concentration regions  412  and  422  or may be located near the interface between the organic semiconductor layer  410  and gate insulating film  310  and the interface between the organic semiconductor layer  420  and gate insulating film  320 . The high-concentration regions  41   n  and  42   p  may be formed in the organic semiconductor layers  410  and  420  entirely in the thickness directions thereof. Alternatively, the high-concentration region  41   n  may be formed very thinnly in the interfaces between the source electrode  510  and the organic semiconductor layer  410  and between the drain electrode  610  and the organic semiconductor layer  410  while the high-concentration region  42   p  may be formed very thinnly in the interfaces between the source electrode  520  and the organic semiconductor layer  420  and between the drain electrode  620  and the organic semiconductor layer  420 . For example, the high-concentration regions  41   n  and  42   p  have thicknesses of 0.1 nm. 
     As shown in  FIG. 26 , the high-concentration regions  41   n  and  42   p  may be formed in portions of the organic semiconductor layers  410  and  402  which are in contact with the source electrodes  510  and  520  and the drain electrodes  610  and  620  partially in the planer direction and thickness direction. Alternatively, as shown in  FIG. 27 , the high-concentration regions  41   n  and  42   p  may be formed only near the source electrodes  510  and  520 . 
     Generally, organic semiconductors are p-type, and it is very difficult to form n-type and p-type organic semiconductor layers from a same organic semiconductor material by controlling doping of impurities like silicon. Moreover, it is technically difficult to separately form organic semiconductors in the n-type and p-type regions on a plane. 
     However, in the complementary organic thin film transistor  1 A according to the second embodiment of the invention, the organic semiconductor layers  410  and  420  of a same conductivity type include the high-concentration regions  41   n  and  42   p , respectively. This allows the n-type region and p-type region to be separately formed in a plane. It is therefore possible to easily implement the complementary organic thin film transistor  1 A including the n-channel and p-channel organic thin film transistors formed on a single substrate. 
     Furthermore, it is possible to implement a semiconductor integrated circuit which includes a plurality of the complementary organic thin film transistors  1 A combined to execute various functions. 
       FIG. 28  shows an example in which the complementary organic thin film transistor  1 A used as an inverter, for example. The source electrodes  510  and  520  of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  are connected to a ground line GND and a power supply line V DD , respectively. The drain electrodes  610  and  620  of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  are connected to an output terminal P. If a signal is inputted to the gate electrodes  210  and  220  of the n-channel and p-channel organic thin film transistors  1   n  and  1   p , an inverted signal of the inputted signal is outputted to the output terminal P. 
     As described above, it is known that combinations of the complementary transistors can constitute memory devices, logic circuits, and the like and can be applied to various usages. 
     In the example shown in  FIG. 28 , the p-channel and n-channel organic thin film transistors  1   p  and  1   n  are used as a load transistor and a drive transistor, respectively. However, it is certain that the configuration shown in  FIG. 28  is one example. As previously described, the characteristics of the organic thin film transistors depend on the difference between carrier concentrations of the n-channel and p-channel thin film transistors  1   n  and  1   p  and the like. Accordingly, one of the n-channel and p-channel organic thin film transistors  1   n  and  1   p  which has larger drain current and better characteristics should be used as a drive transistor. In other words, using the n-channel organic thin film transistors in as a load resistor and p-channel organic thin film transistors  1   p  as a drive resistor is included in the second embodiment of the invention. 
     As described above, according to semiconductor integrated circuits including the complementary organic thin film transistors  1 A, all the properties of CMOS circuit designs which have been accumulated can be used with silicon integrated circuit technologies. The complementary organic thin film transistor  1 A can be therefore used in a significantly wider range of applications. Furthermore, since the complementary organic thin film transistor  1 A can be manufactured by a non-expensive technique such as screen printing, it is possible to implement a fordable complementary organic thin film transistor at low cost. This enables printable and flexible systems. 
     As described above, the organic thin film transistor according to the second embodiment of the invention can implement the complementary organic thin film transistor  1 A including the n-channel and p-channel organic thin film transistors  1   n  and  1   p  formed on the same substrate  10  by using the low-concentration regions  412  and  422 , which are organic semiconductors with low carrier concentrations, as the channel regions and forming the high-concentration regions  41   n  of the n-type conductor with a high carrier concentration and high-concentration regions  41   p  of the p-type conductor with a high carrier concentration near the source electrodes. At this time, the high-concentration regions  41   n  and  42   p  only should be provided near the source electrodes. The complementary organic thin film transistor  1 A is not limited to a top contact type in which the source and drain electrodes are provided above the organic semiconductor layer as shown in  FIG. 20 . In other words, even a complementary organic thin film transistor of a bottom contact type in which the source and drain electrodes are provided below the organic semiconductor layer as shown in  FIGS. 18 and 19 , for example, can also provide the above-described characteristic of the complementary organic thin film transistor  1 A. As described above, the complementary organic thin film transistor  1 A according to the second embodiment of the invention has a structure which has a very high flexibility in device manufacturing and is easily realized industrially. 
     Other Embodiments 
     The invention is described with the first and second embodiments in the above, but it should not be understood that the invention is limited by the description and drawings constituting a part of this disclosure. From this disclosure, those skilled in the art will understand various substitutions, examples, and operational techniques. 
     In the above description of the first embodiment, the organic semiconductor layers  410  and  420  are p-type conductors. The organic semiconductor layers  410  and  420  may be n-type conductors. For example, the n-type high-concentration regions  41   n  and p-type high-concentration regions  41   p  may be formed in the predetermined regions of the organic semiconductor layers  410  and  420  made of fullerene (C60). Alternatively, the high-concentration regions  41   n  and low-concentration regions  412  may be configured to have different conductivity types while the high-concentration regions  42   p  and low-concentration region  422  are configured to have different conductivity types. The high-concentration regions  41   n  and low-concentration region  412  may be configured to have a same conductivity type while the high-concentration regions  42   p  and low-concentration region  422  are configured to have a same conductivity type. 
     As described above, it is certain that the invention includes various embodiments and the like not described in this disclosure. The technical scope of the invention is therefore determined by the features of the invention according to the claims which are appropriate based on the above description. 
     INDUSTRIAL APPLICABILITY 
     The organic thin film transistor of the invention is applicable to electronic industries including manufacture manufacturing electronic devices such as flexible devices and printable devices including organic thin film transistors.