Abstract:
An UV photo-detector having a GaN-based interlayer is provided. Because of the excellent insulating property of the GaN-based interlayer and an excellent Schottky contact between the GaN-based interlayer and electrodes of the device, the leakage current of the device is substantially reduced. For example, the material of the GaN-based interlayer includes Al x In y Ga 1−x−y N, in which x≧0, y≧0, 1≧x+y. The GaN-based interlayer described above is manufactured without requiring a high temperature treatment process after the epitaxy process, and thus the process flow is simplified. Therefore, an UV photodetector having an excellent performance is obtained.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of Taiwan application serial no.92119489, filed on Jul. 17, 2003.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an ultraviolet (UV) photodetector. More particularly, the present invention relates to an UV photodetector having a high-resistivity GaN-based interlayer for reducing the leakage current.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, a conventional UV photodetector can be classified into three type of devices including a photomultiplier Tube (PMT), a silicon-based UV photodetector and a III-V compound semiconductor UV photodetector such as a GaN UV photodetector. Currently, only the photomultiplier tube and the silicon-based UV photodetector are commercialized and produced under mass production. The GaN UV photodetector is just under a preliminary research and development due to the high cost and the complicated technique.  
         [0006]     In general, the shortcoming of a photomultiplier tube is that the cost is high, the operational voltage is high, and the vacuum tube is fragile, but the advantage is that a precise detecting result can be obtained. The advantage of the silicon-based UV photodetector is that the manufacturing process is simple, the cost is low, the operation voltage is low, and a wavelength of light in visible and infrared can be detected, but the disadvantages are that the rejection ratio of ultraviolet to visible and/or infrared is poor. The advantage of the GaN UV photodetector is that the detecting wavelength of the detector can be adjusted according to the needs during the manufacturing process. For example, when the desired detecting wavelength is set in a range of about 200 nm to about 365 nm, an excellent detecting sensitivity for AlGaN-based UV photodetector can be obtained by adjusting the Al composition of AlGaN absorption layer. Therefore, the AlGaN-based UV photodetector has become the major trend of the UV photodetector in recent years.  
         [0007]      FIG. 1  is cross-sectional view illustrating a conventional Schottky barrier diode (SBD) type UV photodetector. Referring to  FIG. 1 , a conventional Schottky barrier diode type UV photodetector is at least constructed by a substrate  100 , a GaN-based semiconductor layer  102 , a first electrode  104  and a second electrode  106 . The GaN-based semiconductor layer  102  is disposed on the substrate  100 , and the GaN-based semiconductor layer  102  has a first protrusion portion A. The first electrode  104  is disposed on the first protrusion portion A of GaN-based semiconductor layer  102 , and the second electrode  106  is disposed on a portion of the GaN-based semiconductor layer  102  except for the first protrusion portion A. In addition, the first bonding pad  108  and the second bonding pad  110  are disposed on the first electrode  104  and the second electrode  106  respectively.  
         [0008]     Referring to  FIG. 1 , the GaN-based semiconductor layer  102  is generally constructed by a nucleation layer  102   a,  an ohmic contact layer  102   b  and an active layer  102   c.  The nucleation layer  102   a  is disposed on the substrate  100 . The ohmic contact layer  102   b  is disposed on the nucleation layer  102   a,  and the ohmic contact layer  102   b  has a second protrusion portion B. The active layer  102   c  is disposed on the second protrusion portion B. According to the structure described above, the first protrusion portion A of the whole GaN-based semiconductor layer  102  is constructed by the second protrusion portion B of the ohmic contact layer  102   b  and the active layer  102   c.    
         [0009]      FIG. 2  is a perspective view illustrating a conventional metal-semiconductor-metal (MSM) type UV photodetector. Referring to  FIG. 2 , a conventional metal-semiconductor-metal (MSM) type UV photodetector is constructed by a substrate  200 , a GaN-based semiconductor layer  202  and a patterned electrode layer  204 . The GaN-based semiconductor layer  202  is disposed on the substrate  200 . The patterned electrode layer  204  is disposed on the GaN-based semiconductor layer  202 . Moreover, the GaN-based semiconductor layer  202  is constructed from a nucleation layer  202   a  and an active layer  202   b.  The nucleation layer  202   a  is disposed on the substrate  200 , and the active layer  202   b  is disposed on the nucleation layer  202   a.    
         [0010]     Referring to  FIG. 2 , the patterned electrode layer  204  is constructed by a first electrode  206  and a second electrode  208 , and a first bonding pad  210  and a second bonding pad  212  are disposed on the first electrode  206  and the second electrode  208  respectively. Moreover, the first electrode  206  has a plurality of mutually parallel aligned first finger-shaped protrusions  206   a,  and the second electrode  208  has a plurality of mutually parallel aligned second finger-shaped protrusions  208   a.  These first finger-shaped protrusions  206   a  and second finger-shaped protrusion  208   a  are mutually interlaced.  
         [0011]     In a conventional UV photodetector, whether in a Schottky barrier diode (SBD) type UV photodetector, or in a metal-semiconductor-metal (MSM) type UV photodetector, there is an issue of a high leakage current. The leakage current is caused by the thermal emission effect and/or the extraordinary tunneling effect due to the poor Schottky contact property between the semiconductor layer and the electrode. Therefore, if the performance of the Schottky contact between the semiconductor layer and the electrode can be effectively enhanced, the leakage current of the UV photodetector will be drastically reduced.  
       SUMMARY OF INVENTION  
       [0012]     Accordingly, the purpose of the present invention is to provide a Schottky barrier diode (SBD) type UV photodetector that can effectively reduce the leakage current.  
         [0013]     It is another object of the present invention to provide a metal-semiconductor-metal (MSM) type UV photodetector that can effectively reduce the leakage current.  
         [0014]     In order to achieve the above objects and other advantages of the present invention, a Schottky barrier diode (SBD) type UV photodetector is provided. The UV photodetector is at least constructed by a substrate, a GaN-based semiconductor layer, a GaN-based interlayer, a first electrode and a second electrode. The GaN-based semiconductor layer is disposed on the substrate, and the GaN-based semiconductor layer has a first protrusion portion. The GaN-based interlayer is disposed on the first protrusion portion of the GaN-based semiconductor layer, and a material of the GaN-based interlayer includes, for example but not limited to, an Al x In y Ga 1−x−y N, wherein x≧0, y≧0, and 1≧x+y. The first electrode is disposed on the GaN-based interlayer, and the second electrode is disposed on a portion of the GaN-based semiconductor layer except for the first protrusion portion. In addition, in the above-described embodiment of the invention, the first bonding pad and the second bonding pad can be disposed on the first electrode and second electrode respectively.  
         [0015]     In the Schottky barrier diode (SBD) type UV photodetector of the preferred embodiment, the substrate includes, for example but not limited to, an aluminum oxide (sapphire) substrate, a silicon carbide (SiC) substrate, a zinc oxide (ZnO) substrate, a silicon substrate, a gallium phosphide (GaP) substrate, and a gallium arsenide (GaAs) substrate.  
         [0016]     In the Schottky barrier diode (SBD) type UV photodetector of the preferred embodiment, the GaN-based semiconductor layer, for example, is constructed from a nucleation layer, an ohmic contact layer and an active layer. The nucleation layer is disposed on the substrate. The ohmic contact layer is disposed on nucleation layer, and has a second protrusion portion. The active layer is disposed on the second protrusion portion. The first protrusion portion of the whole GaN-based semiconductor layer is constructed by the second protrusion portion of the ohmic contact layer and the active layer. Moreover, a material of the nucleation layer includes, for example but not limited to, Al a In b Ga 1−a−b N semiconductor, wherein a, b≧0 and 0≦a+b≦1. The material of the ohmic contact layer includes, for example but not limited to, N-type Al c In d Ga 1−c−d N semiconductor, wherein c, d≧0 and 0≦c+d≦1. The material of the active layer includes, for example but not limited to, undoped Al e In f Ga 1−e−f N semiconductor, wherein e, f≧0 and 0≦e+f≦1.  
         [0017]     In the Schottky barrier diode (SBD) type UV photodetector of the preferred embodiment, the materials of the first electrode and the second electrode include, for example but not limited to, Ni/Au, Cr/Au, Cr/Pt/Au, Ti/Al, Ti/Al/Ti/Au, Ti/Al/Pt/Au, Ti/Al/Ni/Au, Ti/Al/Ti/Au, Ti/Al/Pd/Au, Ti/Al/Cr/Au, Ti/Al/Co/Au, Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Co/Au, Cr/Al/Ni/Au, Pd/Al/Ti/Au, Pd/Al/Pt/Au, Pd/Al/Ni/Au, Pd/Al/Pd/Au, Pd/Al/Cr/Au, Pd/Al/Co/Au, Nd/Al/Pt/Au, Nd/Al/Ti/Au, Nd/Al/Ni/Au, Nd/Al/Cr/Au Nd/Al/Co/A, Hf/Al/Ti/Au, Hf/Al/Pt/Au, Hf/Al/Ni/Au, Hf/Al/Pd/Au, Hf/Al/Cr/Au, Hf/Al/Co/Au, Zr/Al/Ti/Au, Zr/Al/Pt/Au, Zr/Al/Ni/Au, Zr/Al/Pd/Au, Zr/Al/Cr/Au, Zr/Al/Co/Au, TiN x /Ti/Au, TiN x /Pt/Au, TiN x /Ni/Au, TiN x /Pd/Au, TiN x /Cr/Au, TiN x /Co/Au, TiWN x /Ti/Au, TiWN x /Pt/Au, TiWN x /Ni/Au, TiWN x /Pd/Au, TiWN x /Cr/Au, TiWN x /Co/Au, NiAl/Pt/Au, NiAl/Cr/Au, NiAl/Ni/Au, NiAl/Ti/Au, Ti/NiAl/Pt/Au, Ti/NiAl/Ti/Au, Ti/NiAl/Ni/Au, Ti/NiAl/Cr/Au, N-type conductive indium tin oxide (ITO), cadmium tin oxide (CTO), aluminum zinc oxide (ZnO:Al), indium zinc oxide (ZnO:ln),zinc gallate (ZnGa 2 O 4 ), SnO 2 :Sb, Ga 2 O 3 :Sn, AglnO 2 :Sn, In 2 O 3 :Zn, P-type conductive CuAlO 2 , LaCuOS, NiO, CuGaO 2  or SrCu 2 O 2 .  
         [0018]     In order to achieve the above objects and other advantages of the present invention, a metal-semiconductor-metal (MSM) type UV photodetector is provided. The UV photodetector is constructed by a substrate, a GaN-based semiconductor layer, a GaN-based interlayer and a patterned electrode layer. The GaN-based semiconductor layer is disposed on substrate. The GaN-based interlayer is disposed on GaN-based semiconductor layer, and a material of GaN-based interlayer includes, for example but not limited to, Al x In y Ga 1−x−y N semiconductors, wherein x≧0, y≧0, and 1≧x+y. The patterned electrode layer is disposed on GaN-based interlayer. In addition, the patterned electrode layer of embodiment described above is constructed by a first electrode and a second electrode respectively.  
         [0019]     In the metal-semiconductor-metal (MSM)type UV photodetector of the preferred embodiment, the first electrode, for example, has a plurality of first finger-shaped protrusions which are mutually parallel aligned, and the second electrode, for example, has a plurality of second finger-shaped protrusions which are mutually parallel aligned. Moreover, the first finger-shaped protrusions and second finger-shaped protrusions, for example, are mutually interlaced.  
         [0020]     In the metal-semiconductor-metal type UV photodetector of the present embodiment, the substrate includes, for example but not limited to, an aluminum oxide (sapphire) substrate, a silicon carbide (SiC)substrate, a zinc oxide (ZnO) substrate, a silicon substrate, a gallium phosphide (GaP) substrate, and a gallium arsenide (GaAs) substrate.  
         [0021]     In the metal-semiconductor-metal type UV photodetector of the present embodiment, the GaN-based semiconductor layer, for example is constructed from a nucleation layer and an active layer. The nucleation layer is disposed on the substrate, and the active layer is disposed on the nucleation layer. Moreover, a material of the nucleation layer includes, for example, but not limited to, an Al a In b Ga 1−a−b N semiconductor, wherein a, b≧0 and 0≦a+b≦1. The material of the active layer includes, for example but not limited to, an undoped Al e In f Ga 1−e−f N semiconductor, wherein e, f≧0 and 0≦e+f≦1.  
         [0022]     In the metal-semiconductor-metal type UV photodetector of the present embodiment, a material of the patterned electrode layer includes, for example, but not limited to, Ni/Au, Cr/Au, Cr/Pt/Au, Ti/Al, Ti/Al/Ti/Au, Ti/Al/Pt/Au, Ti/Al/Ni/Au, Ti/Al/Ti/Au, Ti/Al/Pd/Au, Ti/Al/Cr/Au, Ti/Al/Co/Au, Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Co/Au, Cr/Al/Ni/Au, Pd/Al/Ti/Au, Pd/Al/Pt/Au, Pd/Al/Ni/Au, Pd/Al/Pd/Au, Pd/Al/Cr/Au, Pd/Al/Co/Au, Nd/Al/Pt/Au, Nd/Al/Ti/Au, Nd/Al/Ni/Au, Nd/Al/Cr/Au Nd/Al/Co/A, Hf/Al/Ti/Au, Hf/Al/Pt/Au, Hf/Al/Ni/Au, Hf/Al/Pd/Au, Hf/Al/Cr/Au, Hf/Al/Co/Au, Zr/Al/Ti/Au, Zr/Al/Pt/Au, Zr/Al/Ni/Au, Zr/Al/Pd/Au, Zr/Al/Cr/Au, Zr/Al/Co/Au, TiN x /Ti/Au, TiN x /Pt/Au, TiN x /Ni/Au, TiN x /Pd/Au, TiN x /Cr/Au, TiN x /Co/Au, TiWN x /Ti/Au, TiWN x /Pt/Au, TiWN x /Ni/Au, TiWN x /Pd/Au, TiWN x /Cr/Au, TiWN x /Co/Au, NiAl/Pt/Au, NiAl/Cr/Au, NiAl/Ni/Au, NiAl/Ti/Au, Ti/NiAl/Pt/Au, Ti/NiAl/Ti/Au, Ti/NiAl/Ni/Au, Ti/NiAl/Cr/Au, N-type conductive indium tin oxide (ITO), cadmium tin oxide (CTO), aluminum zinc oxide (ZnO:Al), indium zinc oxide (ZnO:In),, zinc gallate (ZnGa 2 O 4 ), SnO 2 :Sb, Ga 2 O 3 :Sn, AglnO 2 :Sn, In 2 O 3 :Zn, P-type conductive CuAlO 2 , LaCuOS, NiO, CuGaO 2  or SrCu 2 O 2 .  
         [0023]     Accordingly, in the present invention, since a high-resistivity GaN-based interlayer is provided, the leakage current of the UV photodetector is thus reduced, and therefore, the performance of the device of the UV photodetector can be enhanced. Moreover, a thermal treatment process after the epitaxy process is not required in the manufacturing of the high-resistivity GaN-based interlayer, therefore the process can be simplified.  
         [0024]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0025]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The following drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0026]      FIG. 1  is cross-sectional view illustrating a conventional Schottky barrier diode (SBD) type UV photodetector.  
         [0027]      FIG. 2  is a perspective view illustrating a conventional metal-semiconductor-metal (MSM) type UV photodetector.  
         [0028]      FIG. 3  is cross-sectional view illustrating a Schottky barrier diode (SBD) type UV photodetector according to a preferred embodiment of the present invention.  
         [0029]      FIG. 4  is a diagram illustrating the current-voltage curves of the Schottky barrier diode (SBD) type UV photodetector of the present invention in comparison with that of a prior art measured under non-illuminated condition.  
         [0030]      FIG. 5  is a perspective view illustrating a metal-semiconductor-metal (MSM) type UV photodetector according to a preferred embodiment of the present invention.  
         [0031]      FIG. 6  is a diagram illustrating the current-voltage curves of the metal-semiconductor-metal (MSM) type UV photodetector of the present invention in comparison with that of a prior art measured under non-illuminated condition. 
     
    
     DETAILED DESCRIPTION  
       [0032]      FIG. 3  is cross-sectional view illustrating a Schottky barrier diode (SBD) type UV photodetector according to a preferred embodiment of the present invention. Referring to  FIG. 3 , a Schottky barrier diode (SBD) type UV photodetector comprises a substrate  300 , a GaN-based semiconductor layer  302 , a high-resistivity GaN-based interlayer  303 , a first electrode  304  and a second electrode  306 . The GaN-based semiconductor layer  302  is disposed on the substrate  300 , and the GaN-based semiconductor layer  302  has a first protrusion portion C. The GaN-based interlayer  303  is disposed on the first protrusion portion C of the GaN-based semiconductor layer  302 , and the material of the GaN-based interlayer  303  includes, for example but not limited to, Al x In y Ga 1−x−y N, wherein x≧0, y≧0, and 1≧x+y. The first electrode  304  is disposed on the GaN-based interlayer  303 , and the second electrode  306  is disposed on a portion of the GaN-based semiconductor layer  302  except for the first protrusion portion C. Moreover, in order to simplify the package process, in the embodiment described above, it is optional that a first bonding pad  308  and a second bonding pad  310  are disposed on the first electrode  304  and the second electrode  306  respectively for the progress of the wire bonding process. The material of the first bonding pad  308  and the second bonding pad  310  includes, for example but not limited to, Ti/Au, Cr/Au, Cr/Pt/Au, or another material that can be incorporated with the first electrode  304  and the second electrode  306 .  
         [0033]     In the present embodiment, the substrate  300  includes, for example, but not limited to, an aluminum oxide (sapphire) substrate, a silicon carbide (SiC)substrate, a zinc oxide (ZnO) substrate, a silicon substrate, a gallium phosphide (GaP) substrate, and a gallium arsenide (GaAs) substrate. The high-resistivity interlayer  303  of the present embodiment is constructed by, for example but not limited to, doping at least one dopant selected from a group consisting of iron (Fe), magnesium (Mg), zinc (Zn), copper (Cu), arsenide (As), phosphorus (P), carbon (C) and beryllium (Be) or by a GaN-based semiconductor layer formed by a low temperature process (a temperature of growth less than 800° C.). The material of the high-resistivity GaN-based interlayer  303  includes, for example but not limited to, Al x In y Ga 1−x−y N, wherein x≧0, y≧0, and 1≧x+y.  
         [0034]     In the present embodiment, the GaN-based semiconductor layer  302  is constructed by, for example, but not limited to, a nucleation layer  302   a,  an ohmic contact layer  302   b  and an active layer  302   c.  The nucleation layer  302   a  is disposed on the substrate  300 . The ohmic contact layer  302   b  is disposed on the nucleation layer  302   a  and has a second protrusion portion D. The active layer  302   c  is disposed on the second protrusion portion D. Referring to  FIG. 3 , a first protrusion portion C of the whole GaN-based semiconductor layer  302  is constructed by the second protrusion portion D of the ohmic contact layer  302   b  and the active layer  302   c.  Moreover, a material of the nucleation layer  302   a  includes, for example, but not limited to, Al a In b Ga 1−a−b N semiconductor, wherein a, b≧0 and 0≦a+b≦1. The material of the ohmic contact layer  302   b  includes, for example, but not limited to, N-type Al c In d Ga 1−c−d N semiconductor, wherein c, d≧0 and 0≦c+d≦1. The material of active layer  302   c  includes, for example but not limited to, undoped Al e In f Ga 1−e−f N semiconductor, wherein e, f≧0 and 0≦e+f≦1.  
         [0035]     In the present embodiment, the materials of the first electrode  304  includes, for example, but not limited to, Ni/Au, Cr/Au, Cr/Pt/Au, Ti/Al, Ti/Al/Ti/Au, Ti/Al/Pt/Au, Ti/Al/Ni/Au, Ti/Al/Ti/Au, Ti/Al/Pd/Au, Ti/Al/Cr/Au, Ti/Al/Co/Au, Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Co/Au, Cr/Al/Ni/Au, Pd/Al/Ti/Au, Pd/Al/Pt/Au, Pd/Al/Ni/Au, Pd/Al/Pd/Au, Pd/Al/Cr/Au, Pd/Al/Co/Au, Nd/Al/Pt/Au, Nd/Al/Ti/Au, Nd/Al/Ni/Au, Nd/Al/Cr/Au Nd/Al/Co/A, Hf/Al/Ti/Au, Hf/Al/Pt/Au, Hf/Al/Ni/Au, Hf/Al/Pd/Au, Hf/Al/Cr/Au, Hf/Al/Co/Au, Zr/Al/Ti/Au, Zr/Al/Pt/Au, Zr/Al/Ni/Au, Zr/Al/Pd/Au, Zr/Al/Cr/Au, Zr/Al/Co/Au, TiN x /Ti/Au, TiN x /Pt/Au, TiN x /Ni/Au, TiN x /Pd/Au, TiN x /Cr/Au, TiN x /Co/Au, TiWN x /Ti/Au, TiWN x /Pt/Au, TiWN x /Ni/Au, TiWN x /Pd/Au, TiWN x /Cr/Au, TiWN x /Co/Au, NiAl/Pt/Au, NiAl/Cr/Au, NiAl/Ni/Au, NiAl/Ti/Au, Ti/NiAl/Pt/Au, Ti/NiAl/Ti/Au, Ti/NiAl/Ni/Au, Ti/NiAl/Cr/Au, N-type conductive indium tin oxide (ITO), cadmium tin oxide (CTO), aluminum zinc oxide (ZnO:Al), indium zinc oxide (ZnO:In), zinc gallate (ZnGa 2 O 4 ), SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, P-type conductive CuAlO 2 , LaCuOS, NiO, CuGaO 2  or SrCu 2 O 2 .  
         [0036]      FIG. 4  is a diagram illustrating the current-voltage curves of the Schottky barrier diode (SBD) type UV photodetector of the present invention in comparison with that of a prior art measured under non-illuminated condition. Referring to  FIG. 4 , the forward current and the reverse current are measured under a non-illuminated condition. It is noted that, under the same bias condition (especially in a bias larger than −3V), the leakage current of a prior art is much larger than that of the present embodiment. In the present embodiment, since the GaN-based interlayer is provided for the Schottky barrier diode (SBD) type UV photodetector, the leakage current is drastically reduced due to the excellent insulating property of the GaN-based interlayer and the excellent Schottky contact formed between the GaN-based interlayer and the electrode  304 .  
         [0037]      FIG. 5  is a perspective view illustrating a metal-semiconductor-metal (MSM) type UV photodetector according to a preferred embodiment of the present invention. Referring to  FIG. 5 , a metal-semiconductor-metal (MSM) type UV photodetector comprises a substrate  400 , a GaN-based semiconductor layer  402 , a GaN-based interlayer  403  and a patterned electrode layer  404 . The GaN-based semiconductor layer  402  is disposed on the substrate  400 . The GaN-based interlayer  403  is disposed on the GaN-based semiconductor layer  402 , and a material of the GaN-based interlayer  403  includes, for example but not limited to, Al x In y Ga 1−x−y N, wherein x≧0, y≧0, and 1≧x+y. The patterned electrode layer  404  is disposed on the GaN-based interlayer  403 . Moreover, in order to simplify the package process, in the present embodiment, a first bonding pad  410  and a second bonding pad  412  can be optionally formed on the first electrode  406  and the second electrode  408  respectively to simplify the wire bonding process. The materials of the first bonding pad  410  and the second bonding pad  412  include, for example, but not limited to, Ti/Au, Cr/Au, Cr/Pt/Au, or another material that can be incorporated with the first electrode  406  and the second electrode  408 .  
         [0038]     Hereinafter, the electrode of the patterned electrode layer  404  and the GaN-based semiconductor layer will be described. Since the materials of the substrate  400  and the patterned electrode layer  404  are the same as that of the substrate and the GaN-based semiconductor layer described in the above embodiments, detailed description of these materials are omitted.  
         [0039]     In the preferred embodiment, the first electrode  406  comprises, for example, a plurality of mutually parallel aligned first finger-shaped protrusions  406   a,  and the second electrode  408  comprises, for example, a plurality of second finger-shaped protrusions  408   a.  Moreover, the first finger-shaped protrusions  406   a  and the second finger-shaped protrusions  408   a  are, for example, mutually interlaced. The high-resistivity interlayer  403  of the embodiment is constructed by doping at least one dopant selected from a group consisting of iron (Fe), magnesium (Mg), zinc (Zn), copper (Cu), arsenide (As), phosphorus (P), carbon (C) and beryllium (Be) or by a GaN-based semiconductor layer formed by a low temperature process (a temperature of growth less than 800° C.). The material of the high-resistivity GaN-based interlayer  403  includes, for example, Al x In y Ga 1−x−y N, wherein x≧0, y≧0, and 1≧x+y.  
         [0040]     In the present embodiment, the GaN-based semiconductor layer  402  is constructed, for example, by a nucleation layer  402   a  and an active layer  402   b.  The nucleation layer  402   a  is disposed on substrate  400 , and the active layer  402   b  is disposed on the nucleation layer  402   a.  Moreover, a material of the nucleation layer  402   a  includes, for example, but not limited to, Al a In b Ga 1−a−b N semiconductor, wherein a, b≧0 and 0≦a+b≦1. The material of the active layer  402   b  includes, for example, but not limited to, undoped Al e In f Ga 1−e−f N semiconductor, wherein e, f≧0 and 0≦e+f≦1.  
         [0041]      FIG. 6  is a diagram illustrating the current-voltage curves of the metal-semiconductor-metal (MSM) type UV photodetector of the present invention in comparison with that of a prior art measured under non-illuminated condition. Referring to  FIG. 6 , the current is measured under a non-illuminated condition. According to  FIG. 6 , under the same bias condition (especially between 0V to 14V), the leakage current of a prior art is much larger than that of the metal-semiconductor-metal (MSM) type UV photodetector of the present invention. In the present embodiment, since the GaN-based interlayer is provided in the metal-semiconductor-metal (MSM) type UV photodetector, the leakage current is drastically reduced due to the excellent insulating property of the GaN-based interlayer and the excellent Schottky contact formed between the GaN-based interlayer and the electrode.  
         [0042]     Accordingly, an UV photodetector provided by the present invention have at least the following advantages. First, since a high-resistivity GaN-based interlayer is provided to reduce the leakage current of the UV photodetector, the performance of the device of the UV photodetector can be enhanced. Moreover, in the present invention, a high temperature thermal treatment process following an epitaxy process is not required during the manufacturing of the GaN-based interlayer, and therefore the process can be simplified.  
         [0043]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.