Abstract:
The present invention relates to a Field-Effect Transistor (FET) and, more particularly, to a Dielectric-Modulated Field-Effect Transistor (DMFET) and a method of fabricating the same. A DMFET according to an embodiment of the present invention comprises a substrate in which a source and a drain are formed, wherein the source and the drain are spaced apart from each other, a gate formed on a region between the source and the drain, of the substrate, wherein at least part of the gate is spaced apart from the substrate, biomolecules formed below a region spaced apart from the substrate, of the gate, and a linker for combining the gate and the biomolecules.

Description:
[0001]    This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No 10-2007-0096674 filed in Korea on Sep. 21, 2007 and Patent Application No 10-2007-0004292 filed in Korea on Jan. 15, 2007, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a Field-Effect Transistor (FET) and, more particularly, to a Dielectric-Modulated Field-Effect Transistor (DMFET) and a method of fabricating the same. 
         [0004]    2. Description of the Related Art 
         [0005]    As the size of semiconductor devices shrinks, several physical limitations are encountered. A reduction in the device size has reached its limit due to the technical problems of lithography used in the semiconductor process (the wavelength of a light source, scattering of light, NA limit of the lens, the absence of a photoresist, and so on). Further, in the conventional semiconductor devices, an insulating layer was generally made of silicon-oxide (SiO 2 ) . However, as the size of a device shrinks, physical limitations, such as breakdown and tunneling, have appeared. To overcome such physical limitations, active research has been done into devices having a novel structure. Of the devices, a molecule device has been proposed. The molecule device is a new concept of a device employing molecules as channels. 
         [0006]    The molecule device can be used as a biosensor. The biosensor functions to detect specific molecules, such as enzyme or antibody, which constitute an organism. A method of detecting specific molecules comprises chemical, optical, and electrical methods. Of the methods, the electrical detection method can be used when the quantity of detection target samples is small, and is advantageous in that it has rapid detection. In the electrical detection method, a nano-gap is formed in an existing electrical device, a solution comprising a biomaterial is injected into the nano-gap, and specific materials are detected based on a variation in the electrical property of the device. Thus, the biomaterial formed in the nano-gap serves as an electrical sensor. As the size of a nano-gap reduces, sensitivity is increased and therefore a biomaterial can be detected more effectively. 
         [0007]    To fabricate a structure having a nano-size width using the conventional silicon process is inefficient because of several steps of lithography processes, the alignment of a critical value, expensive equipment, environmental limitations of pressure or temperature, a long-time process, and so on. In order to overcome the limitations, a method of employing a breaking phenomenon of a metal nanowire, a method of forming a large-sized gap and reducing the gap size through an electrochemical deposition method, a method of employing ion beam etching, scanning probe lithography, etc., and so on have been introduced. However, the methods or an overall process thereof are problematic in that they are complicated, and have a limit to the formation of a nano-gap with high reproducibility, a low integration level, and a low sensitivity of a sensor. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the present invention is directed to provide a DMFET having a novel structure of a nano-gap with a high reproducibility, and a method of fabricating the same. 
         [0009]    Further, the present invention is directed to provide a DMFET with a high integration level and an improved sensitivity in detecting biomaterials, and a method of fabricating the same. 
         [0010]    A DMFET according to an embodiment of the present invention comprises a substrate in which a source and a drain are formed, the source and the drain being spaced apart from each other, a dielectric layer formed on a region between the source and the drain of the substrate and comprising biomolecules, and a gate formed on the dielectric layer. 
         [0011]    The gate may be made of metal or polysilicon. 
         [0012]    A method of fabricating a DMFET according to an embodiment of the present invention comprises the steps of (a) forming a sacrificial layer on a substrate, (b) forming a gate layer on the substrate and the sacrificial layer, (c) removing the sacrificial layer, and (d) forming a dielectric layer, comprising biomolecules, in a portion from which the sacrificial layer has been removed. 
         [0013]    The method may further comprise the step of pattering the gate layer between the step (b) and the step (c). 
         [0014]    The sacrificial layer may comprise one or more materials of metal oxide such as silicon oxide, Al 2   0   3  or HfO 2 , metal such as Cr, Ti or Al, an organic layer such as a Self-Assembled Monolayer (SAM), and a photoresist. 
         [0015]    In the step (d), the dielectric layer may be formed using a SAM or a dehydration and condensation reaction. 
         [0016]    A DMFET according to another embodiment of the present invention comprises a wafer, a source and a drain formed on the wafer, the source and the drain being spaced apart from each other, a channel portion connecting the source and the drain, gates formed on the wafer and spaced apart from the channel portion, and a dielectric material formed between the channel portion and the gates and comprising biomolecules. 
         [0017]    The gates may be two in number and are opposite to each other on the basis of the channel portion. 
         [0018]    A method of fabricating a DMFET according to still another embodiment of the present invention comprises the steps of (a) sequentially forming a substrate and a first insulating layer on a wafer, (b) patterning a second insulating layer on a region in which a gate and a channel portion will be formed in the first insulating layer, (c) growing the first insulating layer by further forming a constituent material of the first insulating layer on the first insulating layer using a thermal oxidization method, (d) etching the first and second insulating layers until the substrate is exposed, (e) forming the gate and the channel portion by implanting an impurity into the exposed region of the substrate, (f) etching the first insulating layer, and (g) forming a dielectric layer comprising biomolecules between the gate and the channel portion. 
         [0019]    The first insulating layer may comprise silicon-oxide, and the second insulating layer may comprise silicon-nitride. 
         [0020]    In the step (g), the dielectric layer may be formed using a SAM or a dehydration and condensation reaction. 
         [0021]    The biomolecules may comprise one of DNA, RNA, protein, ligand, an antibody-antigen material, and enzyme. 
         [0022]    A DMFET according to further still another embodiment of the present invention comprises a substrate in which a source and a drain are formed, the source and the drain being spaced apart from each other, a gate formed on a region between the source and the drain, of the substrate, at least part of the gate being spaced apart from the substrate, and biomolecules formed below a region spaced apart from the substrate, of the gate. 
         [0023]    A DMFET according to further still another embodiment of the present invention comprises a wafer, a source, a channel portion, and a drain formed in series on the wafer, a gate formed on the other side except for the source side and the drain side of the channel portion on the wafer, the gate being spaced apart from the channel portion, and biomolecules formed between the channel portion and the gate. 
         [0024]    A DMFET according to further still another embodiment of the present invention comprises a source, a channel portion, and a drain formed in series on a wafer, a gate formed over the channel portion, at least part of the gate being spaced apart from the channel portion, and biomolecules formed below a region spaced apart from the channel portion, of the gate. 
         [0025]    The DMFET may further comprise an insulating layer formed on the channel portion, and a sacrificial layer formed between the insulating layer and the gate. The biomolecules may be formed below the gate, which is exposed by etching both ends of the metal layer. 
         [0026]    The DMFET may further comprise a linker between the gate and the biomolecules. 
         [0027]    The biomolecules may comprise one of DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, an antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drug, and enzyme. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a view illustrating a DMFET according to an embodiment of the present invention; 
           [0029]      FIG. 2  is a sectional view sequentially illustrating a method of fabricating the DMFET according to an embodiment of the present invention; 
           [0030]      FIG. 3  is a view illustrating a DMFET according to another embodiment of the present invention; 
           [0031]      FIG. 4  is a sectional view sequentially illustrating a method of fabricating a DMFET according to another embodiment of the present invention; 
           [0032]      FIG. 5  is a graph showing a simulation result of drain currents according to gate voltages in the DMFET shown in  FIG. 3  according to another embodiment of the present invention; 
           [0033]      FIG. 6  is a graph showing a simulation result of threshold voltages according to the type of a dielectric material in the DMFET shown in  FIG. 3  according to another embodiment of the present invention; 
           [0034]      FIG. 7  is a perspective view illustrating a DMFET according to still another embodiment of the present invention; and 
           [0035]      FIG. 8  is a view illustrating an operation of the DMFET according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0036]    A DMFET and a method of fabricating the same according to the present invention will now be described in detail in connection with specific embodiments with reference to the accompanying drawings. In order to facilitate description, same reference numerals are used in different figures to denote similar elements. 
         [0037]    In  FIG. 1 , (a) is a plan view schematically showing a DMFET  100  according to an embodiment of the present invention, (b) is a sectional view of the DMFET taken along line A-A′ in (a), and (c) is a sectional view of the DMFET taken along line B-B′ in (a). 
         [0038]    As shown in  FIG. 1(   a ), the DMFET  100  comprises a substrate  110 , a gate  120 , a source  130 , and a drain  140 . The substrate  110  may comprise a silicon substrate, but not limited thereto. The source  130  and the drain  140  are formed by doping a n-type or p-type impurity into the substrate  110 . At this time, it is to be understood that a specific implantation dose and energy can be selected depending on the requirements of a specific end device. 
         [0039]    As shown in  FIG. 1(   b ), the source  130  and the drain  140  are formed in the substrate  110 , a dielectric layer  150  is formed on a region between the source  130  and the drain  140  of the substrate  110 , and the gate  120  is formed on the dielectric layer  150 . The dielectric layer  150  refers to a region between the gate  120  and the substrate  110 , comprising an air gap, biomolecules, a linker for fixing biomolecules to the substrate  110  or the gate  120  or the like. An overall dielectric constant of the dielectric layer  150  can be decided depending on the dielectric constant of an air gap, biomolecules or a linker itself, whether biomolecules are combined with external detection target materials, the dielectric constant of combined detection target materials and/or the like. Hereinafter, biomolecules constituting the dielectric layer  150  are referred to as receptor biomolecules, and external materials combined with the receptor biomolecules are referred to as detection target materials. 
         [0040]    The receptor biomolecules may be DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drugs, enzyme or the like, but may be properly selected according to target materials to be detected. The receptor biomolecules can be fixed to the gate  120  or the substrate  110  using a linker. An example of the linker may comprise a Self-Assembled Monolayer (SAM). When the gate  120  is made of gold (Au), the linker can be formed using a thiol SAM. When the gate  120  is made of silicon, the linker can be formed using a silane SAM. Further, when the receptor biomolecules themselves are material that can be easily fixed to the gate, the linker may not be comprised. In this case, it may be considered that the biomolecules themselves comprise the linker. The thickness of the dielectric layer  150  is a nano-size. The nano-size refers to a thickness range of approximately 10 to 1000 angstrom. This thickness can be controlled easily in the fabrication method of the DMFET according to the present invention, which is described later on. 
         [0041]    As shown in  FIG. 1(   c ), the receptor biomolecules are fixed to the dielectric layer  150  between the substrate  110  and the gate  120 . At this time, in order to form a nano-gap in which the dielectric layer  150  can be formed, the gate  120  may have a shape in which it is coupled to the substrate  110  on both sides of the region in which the dielectric layer  150  is formed. The nano-gap formed due to this shape of the gate  120  has both sides opened, so a material to be formed in the dielectric layer  150  can be introduced easily into the nano-gap. However, if one end of the gate  120  is coupled to the substrate  110  and therefore can support the gate  120 , the shape of the gate layer  120  is not limited. The gate  120  may be made of, preferably, metal or polysilicon. The metal may preferably comprise gold (Au), but not limited thereto. For example, the gate  120  may be made of a material, which can be used to form a gate in a typical MOSFET. 
         [0042]    The above-described DMFET can have a reduced size because the dielectric layer  150  comprises the biomaterial when compared with a conventional biosensor device in which the dielectric layer and the biomaterial are separately formed. In addition, since the biomaterial is directly comprised in the dielectric layer, the electrical property of the device according to detection target materials is directly changed. Accordingly, sensitivity to a variation in the electrical property of the device according to a variation of the biomaterial is increased, enabling further improved detection of the biomaterial. 
         [0043]      FIG. 2  is a sectional view sequentially illustrating a method of fabricating the DMFET according to an embodiment of the present invention. 
         [0044]    The source  130  and the drain  140  (not shown) are formed in a substrate  210  using a Local Oxidation of Silicon (LOCOS) or Shallow Trench Isolation (STI) process. 
         [0045]    Referring to  200   a  of  FIG. 2 , a sacrificial layer  220  is formed on the substrate  210 . The sacrificial layer may be formed from one material of metal oxide such as silicon oxide, Al 2   0   3  or HfO 2 , metal such as Cr, Ti or Al, an organic layer such as SAM, and a photoresist. The thickness of the sacrificial layer  220  becomes the thickness of a nano-gap of the DMFET later on and may be a nano-size. The nano-size may refer to a range of approximately 10 to 1000 angstrom. In order to pattern the sacrificial layer  220 , the sacrificial layer  220  as thick as the nano-gap is formed on the substrate  210  using an Atomic Layer Deposition (ALD) process, a hard mask or soft mask is formed on the sacrificial layer  220 , the sacrificial layer is etched using the mask pattern as an etch-stop layer, and the mask pattern is then removed. The sacrificial layer pattern  220  formed in this process functions to support the gate  230 . 
         [0046]    Referring to  200   b  of  FIG. 2 , a gate  230  is formed on the sacrificial layer  220 . The gate  230  may be made of metal such as, preferably, gold (Au), or polysilicon. Alternatively, the gate  230  may be made of a material, which can be used to form a gate in a typical MOSFET. Since the gate  230  has one end connected to the substrate  210 , it is supported although the sacrificial layer  220  is removed as described above. 
         [0047]    Referring to  200   c  and  200   d  of  FIG. 2 , the gate layer is patterned. A hard mask  240  to define a gate region is stacked on the gate. The gate layer is partially etched using the hard mask ( 240 ) pattern as a mask, and the hard mask ( 240 ) pattern is then removed. 
         [0048]    Referring to  200   e  of  FIG. 2 , the sacrificial layer  220  formed on the substrate  210  is all removed, so that an air gap is formed in a region between the gate  230  and the substrate  210 , from which the sacrificial layer  220  has been removed. 
         [0049]    Referring to  200   f  of  FIG. 2 , a dielectric layer  250  comprising biomolecules is formed in the air gap formed through the removal of the sacrificial layer  220 . That is, the receptor biomolecules are fixed to the gate  230  or the substrate  210  using a linker. The linker may employ a SAM. The receptor biomolecules may be selected properly according to the type of detection target materials, and may comprise one of DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drug, enzyme, and so on. 
         [0050]    The thickness of the dielectric layer  250  can be controlled easily by controlling the thickness of the sacrificial layer  220  formed through the above process. Further, if the gate  230  is formed to have a shape similar to a bridge as described above, the sacrificial layer  220  between the gate layer  230  and the substrate  210  can be removed easily because the lateral sides of the region between the gate  230  and the substrate  210  are opened. Moreover, a biomolecule material of a fluid state can be easily introduced to the region from which the sacrificial layer  220  has been removed and then fixed thereto. 
         [0051]    In  FIG. 3 , (a) is a plan view illustrating a DMFET  300  according to another embodiment of the present invention, (b) is a sectional view of the DMFET taken along line A-A′ in (a), and (c) is a sectional view of the DMFET taken along line B-B′ in (a). 
         [0052]    As shown in  FIG. 3 , the DMFET  300  comprises a wafer  310 , a source  350  and a drain  360  formed on the wafer with them being spaced apart from each other, a channel portion  370  connecting the source  350  and the drain  360 , gates  330  and  340  formed on the wafer  310  with them being spaced apart from the channel portion  370 , and a dielectric layer  380 , which comprises biomolecules and is formed between the channel portion  370  and the gates  330  and  340 . The dielectric layer  380  refers to a region, between the gates  330  and  340  and the channel portion  370 , comprising receptor biomolecules and a linker for fixing the receptor biomolecules to the gates  330  and  340  and is formed. The linker is comprised of a SAM that can be combined with a gate surface and functions to allow a receptor to be easily connected to the gate surface. The receptor biomolecules may be formed from one of DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drug, enzyme, etc., but may be formed from other materials according to target materials to be detected. 
         [0053]    The two gates  330  and  340  are opposite to each other on the basis of the channel portion  370 . A double gate type having the two gates  330  and  340  has been described as an example, but the number of the gates is not limited to two. The gates  330  and  340 , the source  350 , the drain  360 , and the channel portion  370  are formed by doping a n-type or p-type impurity into the substrate  320 , which is thinly formed on a bulk silicon wafer or a Silicon-On-Insulator (SOI) wafer. It is to be noted that a specific implantation dose and energy can be selected depending on the requirements of a specific end device. The substrate  320  may comprise a silicon substrate. 
         [0054]    The gap between the gates  330  and  340  and the channel portion  370  is nanometer in size. The nano-size may be in the range of approximately 10 to 1000 angstrom. The gap may be decided through an e-beam lithography process or a similar process. The dielectric layer  380 , which is formed between the gates  330  and  340  and the channel portion  370  and comprises the biomolecules, is fixed to the channel portion  370 , the substrate  320  or the gates  330  and  340  using a SAM or through a similar process. 
         [0055]    The substrate  320  and the gates  330  and  340  may comprise other semiconductor materials such as germanium (Ge), or other metal materials such as gold (Au). An oxide film is formed on the substrate  320 , a polysilicon or poly-germanium thin film is formed using a Chemical Vapor Deposition (CVD) method, and the source  350 , the drain  360 , the gates  330  and  340 , etc. are then formed in the same manner as the SOI wafer process. In this case, the channel region is not excellent in the current property, mobility, and the gain property since it has not a single crystal structure as in the bulk silicon wafer or the SOI wafer, but an amorphous or polycrystalline structure, but is advantageous in that it can form a cheap substrate in the application fields of a sensor. 
         [0056]    Referring to  FIG. 3(   b ), the source  350 , the channel portion  370 , and the drain  360  are formed on the substrate  310 . Referring to  FIG. 3(   c ), the two gates  330  and  340 , which are opposite to each other with the channel portion  370  intervened therebetween, are formed with them being spaced apart from the channel portion  370 . The nano-gap having a nano-size width is formed between the gates  330  and  340  and the channel portion  370 . The dielectric layer  380  comprising the biomolecules is formed in the nano-gap. If voltage applied to the gates  330  and  340  forms an electric field, the formed electric field forms a depletion layer in the channel region through the dielectric layer  380 . The depletion layer formed as described above hinders a channel from being formed. The formation is dependent on whether the depletion layer exists. This operation principle is very similar to that of a Junction Field Effect Transistor (JFET). 
         [0057]      FIG. 4  is a sectional view sequentially illustrating a method of fabricating a DMFET according to another embodiment of the present invention. In a method of forming a nano-gap according to an embodiment of the present invention, a substrate and a first insulating layer are sequentially formed on a wafer. A second insulating layer is patterned on a region in which a gate and a channel portion will be formed in the first insulating layer. A constituent material of the first insulating layer is further formed on the first insulating layer using a thermal oxidization method in order to grow the first insulating layer. The first and second insulating layers are etched until the substrate is exposed. An impurity is implanted into the exposed region of the substrate in order to form the gate and the channel portion. The first insulating layer is then etched. A dielectric layer comprising biomolecules is formed between the formed gate and the formed channel portion. Each of the above processes is described in detail with reference to  FIG. 4 . 
         [0058]    Referring to  400   a  of  FIG. 4 , a substrate  420  and a first insulating layer  430  are sequentially formed over a wafer  410 . The wafer  410  may be a SOT wafer or a bulk-silicon wafer, and the substrate  420  may be a silicon substrate. Second insulating layers  440  are patterned on the first insulating layer  430 . At this time, the second insulating layer  440  is patterned so that regions in which gates  460  and  470  and a channel portion  480  will be formed finally are defined, and gaps  450  between the second insulating layer ( 440 ) patterns are patterned to have a nano-size. To pattern the second insulting layer( 440 ), a hard mask may be formed on the second insulating layer  440 , thus defining the regions in which the gates and the channel portion will be formed, and then the second insulating layers  440  may be patterned using the hard mask pattern as a mask. The first insulating layer  430  may be made of silicon-oxide, and the second insulating layers  440  may be made of silicon-nitride. 
         [0059]    Referring to  400   b  of  FIG. 4 , the same material as that constituting the first insulating layer  430  is further formed on the first insulating layer  430  using a thermal oxidization method, thus growing the first insulating layer  430 . At this time, the material constituting the first insulating layer  430  may also be formed on the second insulating layers  440 . If the first insulating layer is grown using the thermal oxidization method as described above, the silicon substrate  420  below the first insulating layer  430  is also oxidized and is thus changed to the same material as that of the first insulating layer  430 . In the thermal oxidization process, the plurality of second insulating layers  440  serve as a mask, so that the first insulating layer  430  below the second insulating layers  440  is not thermally oxidized. Accordingly, the silicon substrate  420  can be divided into several isolated elements. 
         [0060]    Referring to  400   c  of  FIG. 4 , the second insulating layers  440 , and the first insulating layer  430  below the second insulating layers  440  are sequentially etched so that the silicon substrate  420  is exposed. At this time, when a material constituting the first insulating layer  430  is stacked on the second insulating layers  440 , the material is first etched. Further, the first insulating layer  430  is also etched to a certain extent even in the regions in which the second insulating layers  440  are not formed, but only the first insulating layer  430  is thickly formed, in such regions, the first insulating layer  430  remains to some degree even when the silicon substrate  420  is exposed, and thus serves as a mask in a subsequent process of implanting an impurity into the silicon substrate  420 . 
         [0061]    Referring to  400   d  of  FIG. 4 , an impurity is implanted into the silicon substrate  420  using the remaining first insulating layer  430  as a mask and then activated, thus forming the gates  460  and  470  and the channel portion  480 . 
         [0062]    Referring to  400   e  of  FIG. 4 , the first insulating layer  430  formed on the silicon substrate  420  is all etched. Accordingly, nano-gaps  450  are formed between the gates  460  and  470  and the channel portion  480 . A depth of the nano-gap  450  is identical to a thickness of the initial substrate  420 , and a width of the nano-gap  450  is decided according to a gap between the second insulating layers  440 . The width of the nano-gap  450  may have a nano-size in the range of approximately 10 to 1000 angstrom. 
         [0063]    Referring to  400   f  of  FIG. 4 , a dielectric layer  490  comprising biomolecules is formed in the nano-gaps  450 . In other words, the receptor biomolecules are fixed to the gates  460  and  470  or the channel  480  using a linker. A SAM may be used as the linker. The receptor biomolecules may be selected appropriately according to the type of detection target materials, and may comprise one of DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drug, enzyme, and so on. 
         [0064]      FIG. 5  is a graph showing a simulation result of drain currents according to gate voltages in the DMFET shown in  FIG. 3  according to another embodiment of the present invention. In the present simulation, ATLAS simulator of Silvaco Data Systems Inc. was used. In the present simulation, the length of the channel portion  370  was  300  rim and the width of the nano-gap was 40 nm. All the gates  350  and  360  and the channel portion  370  were doped to be an n-type and the doping concentration was 5×0 15 . If 5×0 15 . If voltage is applied to the gates  350  and  360 , an electric field is formed through the dielectric layer  380 . A depletion layer is formed in the channel portion  370  by means of the electric field. If the gate voltage exceeds a certain range, the depletion layer formed in the channel portion  370  by means of the gate voltage prevents the flow of electrons in the channel portion  370 , thus deciding the flow of current between the source  350  and the drain  360 . It can be said that this electrical operation property of the device is similar to that of the JFET. Current values flowing between the source  350  and the drain  360  when the voltage applied to the gates  350  and  360  is changed to −2V, −1, −0.5V, 0V or 0.5V are shown  FIG. 5 . From  FIG. 5 , it can be seen that when the voltage applied to the gates  350  and  360  exceeds −2V, current flows through the source  350  and the drain  360 . 
         [0065]      FIG. 6  is a graph showing a simulation result of threshold voltages according to the type of a dielectric material in the DMFET shown in  FIG. 3  according to another embodiment of the present invention.  FIG. 6  is a graph showing a difference in the threshold voltage when the dielectric layer  380  is (1) in an air gap state (the air is introduced to the nano-gap) (ε Γ =1) and (2) comprises biomolecules (ε Γ =25) in the DMFET shown in  FIG. 3  according to another embodiment of the present invention. The amount of the electric field formed by the gate voltage is varied according to a material forming the dielectric layer  380 , which makes different the thickness of the depletion layer formed in the channel portion  370 . Due to this difference, there occurs a difference in the gate voltage, which prevents the flow of current between the source  350  and the drain  360  when compared with the air gap device. This difference is varied according to which molecules are filled in the nano gap between the gate and the channel portion. 
         [0066]      FIG. 7  is a serspective view sequentially illustrating a method of fabricating a DMFET according to still another embodiment of the present invention. The DMFET shown in  FIG. 7  can be fabricated using the conventional Complementary Metal Oxide (CMOS) semiconductor process in the same manner as the above-described DMFET. Referring to  FIG. 7 , a source  702 , a channel portion  704 , and a drain  703  are formed over a SOI substrate  701  into which a P type impurity has been implanted. A gate  707  is formed over the channel portion  704 . The gate  707  is spaced apart from and opposite to the channel portion  704 . A sacrificial layer  706  may be formed between the channel portion  704  and the gate  707 . As an embodiment, the sacrificial layer  706  may be made of chrome (Cr). Both ends of the sacrificial layer  706  are etched, thus forming a nano-sized air gap between the channel portion  704  and the gate  707 . A depth of a horizontal direction of the air gap may be decided according to the extent that both ends of the sacrificial layer  706  are etched. Alternatively, the sacrificial layer  706  may be fully etched and removed. A linker is formed at the bottom of the gate  707  exposed by the formed air gap. The linker may be formed from a SAM. When the gate  707  is made of gold (Au), the linker may be formed from a thiol SAM. When the gate  707  is made of silicon, the linker may be formed from a silane SAM. The linker is combined with biomolecules  708  and then fixed thereto. The biomolecules  708  may be formed from one of DNA, RNA, nucleotide analogs, protein, peptide, amino acid, ligand, an antibody-antigen material, a sugar structure, an organic/inorganic compound, vitamin, drug, enzyme, and so on, but not limited therto. The biomolecules may be properly selected according to target materials to be detected. Alternatively, an insulating layer  705  may be formed on the channel portion  704 . The insulating layer  705  functions to prevent the gate  707  and the channel portion  704  from being shortened by the biomolecules  708 , or the biomolecules  708  and detection target materials combined with the biomolecules  708 . 
         [0067]      FIG. 8  is a view illustrating an operation of the DMFET according to the present invention. A SAM  801  is formed on a gate as shown in  FIG. 8(   a ). Biomolecules  802  are fixed to the SAM  801  as shown in  FIG. 8(   b ). Accordingly, as shown in  FIG. 8(   c ), when detection target materials  803  exist in an environment in which a DMFET is placed, the detection target materials  803  are fixed to an air gap of the DMFET through coupling of the detection target materials  803  and the biomolecules  802 , so the property of the DMFET is changed. 
         [0068]    As described above, in accordance with the structure of the DMFET and the method of fabricating the same according to an embodiment of the present invention, the thickness of the nano-gap can be controlled easily by controlling the thickness of the sacrificial layer. Further, since the gate layer is formed to have a shape similar to a bridge, the sacrificial layer can be removed easily, and biomolecule materials of a fluid state can be easily introduced to a region from which the sacrificial layer has been removed and then fixed thereto. Accordingly, a DMFET having a novel structure of a nano-gap with a high reproducibility can be provided. 
         [0069]    In addition, the size of the DMFET can be reduced compared with a conventional biosensor device in which a dielectric layer and a biomaterial exist separately, sensitivity to a variation in the electrical property of a device according to a change of the biomaterial can be increased, and further improved detection of the biomaterial is possible. 
         [0070]    In accordance with the structure of the DMFET and the method of fabricating the same according to another embodiment of the present invention, the size of the nano-gap can be controlled easily by means of isolation through a lithography process. Accordingly, a DMFET having a nano-gap with a high reproducibility can be fabricated. 
         [0071]    While the invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.