Abstract:
Provided are a vertical field-effect transistor, a method of manufacturing the same, and a display device having the same. The method is highly reproducible and can be used to manufacture a vertical organic field-effect transistor at a low cost. In addition, the method does not require photolithography and a shadow mask. In the vertical field-effect transistor, a source electrode is formed on a substrate, and an insulating layer and discontinuous gate electrodes are formed. Then, a charge carrier block layer, an organic semiconductor material, and a drain electrode are formed. The gate electrodes are formed using nanoparticles.

Description:
[0001]     This application claims priority to European Patent Application No. EP 040901027, filed on Mar. 11, 2004, and Korean Patent Application No. 2004-52917, filed on Jul. 8, 2004, which are both incorporated herein in their entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention may relate to a field-effect transistor, a display device having the same, and a method of manufacturing the same. More particularly it may relate to a vertical field-effect transistor, a display device having the same, and a method of manufacturing the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Organic electronic engineering may be applied to a display device, such as an organic light emitting diode, and to a field-effect transistor. Organic electronic engineering also may realize a single electronic circuit that can be manufactured without a complicated semiconductor manufacturing process. The circuits manufactured by a method based on an organic polymer semiconductor may be used to activate large size displays and also may be used in the field of transponders.  
         [0006]     In a field-effect transistor structure based on organic materials, a semiconductor organic material may be arranged between a source electrode and a drain electrode. An electric field may be generated in an area (for example, a channel) between the source electrode and the drain electrode by applying a predetermined voltage to a gate electrode. As a result, charge carriers, (for example, electrons or holes) may be introduced into the channel formed of the organic material and may increase conductivity between a source contact and a drain contact. In this case, a line in a channel of a p-type transistor structure may be realized through the holes, and a line in a channel of an n-type transistor structure may be realized through the electrons. The transistor may be controlled by a gate voltage.  
         [0007]     A charge carrier block layer, (for example, a dielectric layer) may be arranged between the channel and the gate electrode, and may prevent the migration of corresponding charges from the gate electrode to the channel. Such migration may deteriorate the quality of a desired electric field and of the transistor structure.  
         [0008]     Such a structure may have a low maximum attainable current. The maximum attainable current may be an important factor of the adaptability of an organic transistor, such as one for use in an active matrix OLED display.  
         [0009]     The maximum attainable current is determined based on the width and the length of a conductive channel (which is formed of an organic semiconductor material) and the charge carrier movement. Examples of organic materials include small molecular compounds like perylene tetra carboxylic acid diimide as an n-type semiconductor and diimide derivative of naphthaline tetra carboxylic acid diimide, as well as pentacenes, tatracences, and oligo thiophenes as p-type semiconductors. Other examples of organic materials may include polymers like a copolymer of alkylfluorene unites of alkythiophenes and polyalkylthiophenes.  
         [0010]     Having the length of the channel small and the width and the movement of the channel large may help to obtain a maximum current. Another factor for obtaining the maximum current is the thickness of the organic semiconductor that defines the thickness of the channel. A channel used for the current modulation between the source electrode and the drain electrode may operate on a very thin layer that is located near the charge carrier block layer. Accordingly, it may not be necessary to increase the thickness of the organic semiconductor. Thicker semiconductor layer may deteriorate the current ratio in a switch-on state and a switch-off state. The optimum thickness of the semiconductor layer may be less than about 100 nanometers.  
         [0011]     The charge carrier movement of the organic semiconductor may be significantly less than the charge carrier movement of an inorganic material, such as silicon. The typical movement of the organic semiconductor may be in a range from about 10 −2  to about 1 cm 2 /Vs. Accordingly, it may be necessary to balance out the small charge carrier movement by reducing the length of the channel in order to obtain a large maximum attainable current of an organic semiconductor that has small charge carrier movement. The typical channel length of an organic field-effect transistor is about 5 to about 100 micrometers. In general, a high resolution process, such as a photolithography, may be performed to lower the channel length to less than about 5 micrometers. Such a method may cost higher and may destroy the value of the organic electronic engineering.  
         [0012]     In order to avoid a photolithography process requiring a high cost and in order to obtain a parallel structure with a high resolution and reduce the channel length in order to obtain a higher current various solutions have been attempted. To this end, a method of depositing a source electrode and a drain electrode of an organic field-effect transistor while the source electrode and the drain electrode are not across each other and adjacent to each other is discussed in “Thin Solid Films, Vol 331(1998), pp. 51-54” by Kudo et al. and “Science, Vol 299(2003), pp. 1881-1884” by Stutzmann et al., which are incorporated herein by reference in their entirety.  
         [0013]     In Kudo, the source electrode and the drain electrode are arranged on the substrate to overlap each other but not to be adjacent to each other. When the gate electrode is continuously arranged to the organic material, the flow of charge carriers from the source electrode to the drain electrode may be disrupted. More specifically, a large leakage current from the gate electrode to the source electrode may destroy the value of a field-effect transistor that can be controlled without using an electric power.  
         [0014]     In Stutzmann a small portion of the surface of the transistor is used as the channel, but a large portion of the surface of the transistor may be needed for use as the channel, in order to obtain a large current by using a small sized transistor.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention provides, inter alia, a vertical organic field-effect transistor having excellent reproducibility and low manufacture cost, a method of manufacturing the same, and a display device having the same. More specifically, the present invention provides a vertical organic field-effect transistor that may be manufactured without using a photolithography or a shadow mask.  
         [0016]     The present invention also provides a method of manufacturing a vertical field-effect transistor. The method may include forming a first electrode on a substrate, forming an insulating layer on one surface of the first electrode, forming discontinuous gate electrodes on the insulating layer using nanoparticles, forming a charge carrier block layer on at least a portion of the discontinuous gate electrodes, forming an organic semiconductor layer covering the discontinuous portion of the discontinuous gate electrodes, and forming a second electrode on the organic semiconductor layer and the discontinuous gate electrodes.  
         [0017]     The present invention may provide a vertical organic field-effect transistor having an excellent reproducibility and a method for manufacturing the same. The present invention may provide a vertical organic field-effect transistor at a low cost by avoiding the use of photolithography or a shadow mask.  
         [0018]     The present invention may provide a vertical organic field-effect transistor having excellent performance by preventing or reducing a current leakage between gate electrodes and a source electrode. This may be accomplished by arranging an insulating layer between the electrodes. The present invention may provide a display device having a vertical field-effect transistor having excellent reproducibility and low manufacture cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a sectional view illustrating a conventional organic field-effect transistor.  
         [0020]      FIG. 2  is a sectional view illustrating a conventional vertical organic field-effect transistor.  
         [0021]      FIGS. 3A, 3B ,  3 C, and  3 D are sectional views illustrating a method of manufacturing an organic field-effect transistor according to a first embodiment of the present invention.  
         [0022]      FIGS. 4A, 4B ,  4 C,  4 D,  4 E, and  4 F are sectional views illustrating a method of manufacturing an organic field-effect transistor according to a second embodiment of the present invention.  
         [0023]      FIGS. 5A, 5B ,  5 C, and  5 D are sectional views illustrating a method of manufacturing an organic field-effect transistor according to a third embodiment of the present invention.  
         [0024]      FIGS. 6A, 6B ,  6 C,  6 D,  6 E, and  6 F are sectional views illustrating a method of manufacturing an organic field-effect transistor according to a fourth embodiment of the present invention.  
         [0025]      FIG. 7  is a sectional view illustrating a vertical organic field-effect transistor and an organic field-effect light-emitting display device having the same according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0027]     As shown in  FIG. 1 , a conventional organic field-effect transistor may include a source electrode  2  and a drain electrode  6  arranged adjacent to each other on a substrate  1 . A gate electrodes  5  and a charge block layer  4  may be arranged on the source electrode  2  and the drain electrode  6 . The current between the source electrode  2  and the drain electrode  6  may be controlled by a voltage applied to the gate electrode  5 . The length of a channel  3  formed of an organic semiconductor material may be defined by the space between the source electrode  2  and the drain electrode  6 . In order to obtain a large current, the length of the channel  3  may be reduced to be smaller than about 5 μm by using a high-resolution method, such as photolithography.  
         [0028]     As shown in  FIG. 2 , a conventional vertical organic field-effect transistor may include a source electrode  2  and a drain electrode  6  separately arranged on a substrate  1 . Here, the length of a channel  3  that is formed of an organic material can be reduced to a nanometer size corresponding to the thickness of the channel  3 . A gate electrode  5  may be formed discontinuously and/or by a porous method. This conventional vertical organic field-effect transistor may differ from the conventional field-effect transistor shown in  FIG. 1 .  
         [0029]     As shown in  FIGS. 3A, 3B ,  3 C, and  3 D a vertical organic field-effect transistor may be manufactured. For example, a source electrode  2  may be deposited on a substrate  1  formed of a material such as glass, silicon, or flexible carrier foil. Here, the source electrode  2  may be deposited on the substrate  1  by vaporization under a high vacuum or by sputtering while including high quality metals (for example, gold, palladium, platinum, and nickel) or indium-tin-oxide (ITO). An insulating layer  8  (which may be, for example, a dielectric layer) may be deposited on the source electrode  2  by vaporization under a high vacuum, sputtering, or chemical vapor deposition (CVD).  
         [0030]     The insulating layer  8  may be formed of, for example, SiO 2 , SiO x , Si 3 N 4 , or Si x O y . An actual functioning layer formed of a dielectric matrix  10  in which metal nanoparticles  13  are scattered may be formed on the insulating layer  8 . Here, the metal nanoparticles  13  may serve as the gate electrodes  5 . The actual functioning layer may be formed by cover vaporizing the metal nanoparticles  13  and the dielectric matrix  10 . The nanoparticles  13  may be formed of, for example, Al, Ti, Ta, Zr, Hf, or Cr, and the dielectric matrix  10  may be formed of SiO x .  
         [0031]     The vertical organic field-effect transistor according to the first embodiment of the present invention may be manufactured by an anisotropic etching. The etching may selectively remove the dielectric matrix  10  and the insulating layer  8  arranged under the dielectric matrix  10  but may not damage the source electrode  2  and the gate electrodes  5 . When the dielectric matrix  10  is SiO x , the etching may be performed by using, for example, an etching gas including fluorine, such as CF 4 , CF 3 H, C 4 F 9 , C 2 F 6 , CF 6 , or NF 3 .  
         [0032]     As shown in  FIG. 3C , a charge carrier block layer  4  (for example, a dielectric layer) may be formed on the gate electrodes  5  by an oxidation process such as thermal oxidation, plasma oxidation, or anodizing. Then, an organic semiconductor  3  may be deposited by vaporization under a high vacuum or spin coating from a solution. The vertical organic field-effect transistor according to the first embodiment of the present invention may be completed by applying a drain contact  6  by vaporization under a high vacuum or sputtering. The drain contact  6  may be formed of, for example, gold, palladium, nickel, or ITO.  
         [0033]     In the case of the vertical organic field-effect transistor, the gate electrodes  5  are formed of metal domains, in other words, the nanoparticles  13 , on which oxide layers like the charge carrier block layers  4  conductively connected to one another and functioning as dielectrics are formed. The organic semiconductor  3  is filled in the spaces between the gate electrodes  5 . In addition, the gate electrodes  5  may easily contact the outside due to the network of composing the metal domains.  
         [0034]     As shown in  FIGS. 4A, 4B ,  4 C,  4 D,  4 E, and  4 F are sectional views illustrating a method of manufacturing a vertical organic field-effect transistor according to a second embodiment of the present invention.  
         [0035]     A source electrode  2 , an insulating layer  8 , and a continuous gate electrode  5  may be deposited on a substrate. The source electrode  2  may be formed of a precious metal, such as Au, Pd, Pt, or Ni, and the insulating layer  8  may be formed of a material such as Al 2 O 3 , SiO 2 , SiO x , Si 3 N x , TiO 2 , Ta 2 O 5 , ZrO 2 , or HfO 2 . The material of the gate electrode  5  may be determined based on the material of the insulating layer  8  (which may function as a dielectric layer). The gate electrode  5  and the insulating layer  8  may be formed of, for example, Al and Al 2 O 3  or Ta and Ta 2 O 5  in order to deposit a metal or a metal oxide without changing a target while performing sputtering.  
         [0036]     Nanoparticles  13  to operate as an etch mask are coated on the surface of the gate electrode  5 . Here, the nanoparticles  13  may be formed of polymer latex particles having a limited size, for example, polystyrene latex. In other case, the nanoparticles  13  may be formed of metal colloids, as well as inorganic nanoparticles, such as SiO 2  and TiO 2 .  
         [0037]     Thereafter, the gate electrode  5  and the insulating layer  8  are etched by using the nanoparticles  13  as the etch mask to obtain a structure shown in  FIG. 4C . The nanoparticles  13  are removed by using a solvent or selective etching to form the structure shown in  FIG. 4D .  
         [0038]     As shown in  FIG. 4E , an oxide layer, in other words, a charge carrier block layer  4 , is formed on the gate electrodes  5  to operate as a dielectric layer. Thereafter, an organic semiconductor  3  and a drain electrode  6  as in the vertical organic field-effect transistor according to the first embodiment of the present invention are applied to the structure of  FIG. 4E  to complete the vertical organic field-effect transistor according to the second embodiment of the present invention. Here, holes are etched in the gate electrodes  5  formed of a metal layer. The flanks of the holes and the surfaces of the gate electrodes  5  are coated by using the oxide operating as the charge carrier block layer  4 .  
         [0039]      FIGS. 5A through 5D  are sectional views illustrating a method of manufacturing a vertical organic field-effect transistor according to a third embodiment of the present invention.  
         [0040]     In order to form a vertical organic field-effect transistor shown in  FIG. 5D , the structure shown in  FIG. 5A  is formed. Here, a source electrode  2 , an insulating layer  8 , and a continuous gate electrode  5  are formed on a substrate  1 . The source electrode  2  is formed of a precious metal, such as Au, Pd, Pt, and Ni, and the insulating layer  8  may be formed of Al 2 O 3 , SiO 2 , SiO x , TiO 2 , Ta 2 O 5 , ZrO 2 , or HfO 2 . The material of the gate electrode  5  is determined based on the material of the insulating layer  8 . Here, the gate electrode  5  and the insulating layer  8  may be formed of Al and Al 2 O 3  or Ta and Ta 2 O 5 .  
         [0041]     The structure of  FIG. 5A  is scratched/polished by using the suspension of nanoparticles  13  having a uniform size to form grooves in the insulating layer  8  and the gate electrodes  5 , as shown in  FIG. 5B . Here, the nanoparticles  8  may be formed of silicon oxide, aluminum oxide, or titanium oxide.  
         [0042]     The gate electrodes  5  are anodized and coated by using an oxide layer, which is provided as a charge carrier block layer  4 , as shown in  FIG. 5C . Then, an organic semiconductor  3  is vaporized under a high vacuum or spin coated from a solution, and a drain electrode  6  is applied to the structure in order to complete the vertical organic field-effect transistor. The drain electrode  6  is formed of a metal, which is vaporized under a high vacuum or sputtered. The gate electrodes  5  are formed of a metal layer, and the metal layer is mechanically processed by the nanoparticles  13  to scratch the grooves in the metal layer. In addition, the remaining surface of the gate electrodes  5 , as well as the flanks of the grooves, is coated by the charge carrier block layer  4 , which operates as a dielectric.  
         [0043]      FIGS. 6A through 6F  are sectional views illustrating a method of manufacturing a vertical organic field-effect transistor according to a fourth embodiment of the present invention.  
         [0044]     A source electrode  2  and an insulating layer  8  are formed on a substrate  1  as shown in  FIG. 6A . Here, the source electrode  2  may be formed of gold, nickel, palladium, platinum, or ITO, and the insulating layer  8  may be formed of silicon dioxide. Then, nanoparticles  13  having a predetermined size are formed on the insulating layer  8 . Here, the nanoparticles  13  may be formed of silicon dioxide or polymer latex. In other words, the nanoparticles  13  may be formed of particles that generate a vapor analysis material by a thermal process to remove a covering gate metal, for example, polystyrene.  
         [0045]     Thereafter, a gate electrode  5  is formed by vaporization under a high vacuum or sputtering. In this case, the gate electrode  5  may be formed of aluminum. Here, the nanoparticles  13  operate as a mask for areas on which the gate electrode  5  is not deposited, as shown in  FIG. 6C .  
         [0046]     The particles  13  are removed from the gate electrode  5  through a mechanical process using a cleaner in a supersonic wave bath. When the polymer latex is used as the nanoparticles  13 , the polymer latex may be removed by performing an additional temper process in addition to a pyrolysis at a temperature of 200 to 400° C. while generating a vapor analysis material.  
         [0047]     The nanoparticles  13  remaining in the gate electrode  5  can be etched. In general, the nanoparticles  13  and the insulating layer  8  are etched by using an etchant, which does not etch the gate electrode  5 .  
         [0048]     For example, when the nanoparticles  13  are formed of SiO 2 , a medium including fluorine, for example, CF 4 , is used as the etchant. As a result, the openings of the insulating layer  8  are etched. When the nanoparticles  13  are formed of the polymer latex, the nanoparticles  13  may be oxygen plasma processed by using the admixture of CF 4 .  
         [0049]     The structure of  FIG. 6D  is anodized to form a structure of  FIG. 6E , and a charge carrier block layer  4 , in other words, a dielectric layer, is formed through a metal oxidation. When the gate electrode  5  is formed of aluminum, the anodizing may be performed in a diammonium citrate solution as an electrolyte by using an end voltage of 80 V.  
         [0050]     Then, an organic semiconductor  3  is formed by vaporization under a high vacuum or spin coating from a solution. When the organic semiconductor  3  is formed by spin coating, regio-regular polyalkylthiophene, which can be deposited from an organic solution, may be used as the organic semiconductor  3 . A drain electrode  6  is vaporized under a high vacuum to complete the structure of  FIG. 6F . The drain electrode  6  may be formed of gold, nickel, palladium, platinum, or ITO.  
         [0051]      FIG. 7  is a sectional view illustrating a display device having a vertical field-effect transistor according to the present invention.  
         [0052]     A source electrode  120   a  as a first electrode is formed on one surface of a glass substrate  110 . An insulating layer  130  is formed on the surface of the source electrode  120   a . Here, the insulating layer  130  may be formed of Al 2 O 3 , SiO 2 , SiO x , Si 3 N x , TiO 2 , Ta 2 O 5 , ZrO 2 , or HfO 2 , as described above.  
         [0053]     Gate electrodes  140  are formed on the insulating layer  130 , and charge carrier block layers  150  are formed on the surfaces of the gate electrodes  140  in order to insulate the gate electrodes  140  from other layers. Here, the charge carrier block layers  150  may be formed by thermal oxidation, plasma oxidation, or anodizing. An organic semiconductor layer  160  is formed on the gate electrodes  140  and in the spaces between the gate electrodes  140 . A drain electrode  120   b  is formed on the organic semiconductor layer  160 , and a transistor insulating layer  170  as an insulating layer for protecting and insulating the structure under the transistor insulating layer  170  is formed on the drain electrode  120   b.    
         [0054]     Here, the transistor insulating layer  170  may be formed of an inorganic material, such as SiNx, or formed by arranging an organic layer, such as benzocyclobutene (BCB) or acryl, on the inorganic material. The transistor insulating layer  170  may be formed of a single layer or multi-layers. The transistor layer can be formed by the method of manufacturing the vertical field-effect transistor described above.  
         [0055]     A pixel layer including a first pixel electrode  210 , a second pixel electrode  240 , and an organic field-effect light-emitting unit  230  therebetween is formed on the transistor insulating layer  170 . The first pixel electrode  210  and the second pixel electrode  240  may include more than one of ITO, Al, and Mg—Ag, and the structures of the first and second pixel electrodes  210  and  240  depend on the light-emitting type of the display device, for example, a front plate light-emitting type or a rear plate light-emitting type. The first pixel electrode  210  is electrically connected to the drain electrode  120   b  through a via hole  171  of the transistor insulating layer  170 . The pixels of emitting light are defined by pixel define layers  220 .  
         [0056]     The organic field-effect light-emitting unit  230  may be formed of a small molecular organic layer or a polymer organic layer. When the small molecular organic layer is used, one or more than one of a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL) are used. In addition, the small molecular organic layer is formed of copper phthalocyanine (CuPc), N,N′-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), and tris-8-hydroxyquinoline aluminium (Alq3). The smaller molecular organic layer is formed by a vacuum deposition.  
         [0057]     When the polymer organic layer is used, the layer is formed as the HTL or the EML. Here, PEDOT is used as the HTL, and a polymer organic material of poly-phenylenevinylene (PPV) group or polyfluorene group is used as the EML. In addition, the polymer organic layer may be formed by screen printing or inkjet printing.  
         [0058]     The operation of the organic field-effect display device according to the present invention will now be described. An electric signal from the source electrode  120   a , which is formed by an electric signal applied to the gate electrodes  140 , is transferred to the drain electrode  120   b  through a channel formed adjacent to the gate electrodes  140  of the organic semiconductor layer  160 . The electric signal transferred to the drain electrode  120   b  is transferred to the first pixel electrode  210  through the via hole  171 . Accordingly, the organic field-effect light-emitting unit  230 , which is interposed between the first pixel electrode  210  and the second pixel electrode  240 , emits light based on the transferred electric signal.  
         [0059]     The vertical field-effect transistor may be applied to an inorganic field-effect display device and/or a liquid crystal display (LCD), as well as the organic field-effect display device.  
         [0060]     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 details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.