FIELD OF THE INVENTION 
       [0001]    The disclosed subject matter relates to a photodiode that incorporates a charge balanced set of alternating N and P doped semiconductor regions. 
       BACKGROUND OF THE INVENTION 
       [0002]    Silicon photodiodes are constructed from single crystal silicon wafers similar to those used in the manufacture of integrated circuits. A major difference between the two is that silicon photodiodes require higher purity silicon. The purity of the silicon is directly related to its resistivity, with higher resistivity indicating higher purity. The resistivity could vary from 10 Ohm-cm to 10,000 Ohm-cm. 
         [0003]    When light shines on crystalline silicon, electrons within the crystal lattice may be freed. Only photons within a certain level of energy can free electrons in the semiconductor material from their atomic bonds to produce an electric current. This level of energy, known as the “bandgap energy,” is the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit. To free an electron, the energy of a photon must be at least as great as the bandgap energy. Photons with more energy than the bandgap energy will expend that extra amount of energy as heat when freeing electrons. Crystalline silicon has a bandgap energy of approximately 1.1 electron-volts (eV), which means that the wavelength where it begins to absorb is λ=he/E g , where λ is the wavelength of light, E g  is the bandgap energy of the material, h is Plank&#39;s constant and c is the speed of light. 
         [0004]    The photon energy of light varies according to the different wavelengths of the light. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For example, red light has a photon energy of about 1.7 eV; blue light has a photon energy of about 2.7 eV. 
         [0005]    Only a portion of sunlight exposed to silicon will be absorbed.  FIG. 1  shows the absorption coefficient a versus wavelength λ, where, for silicon, wavelengths beyond about 1 μm are not absorbed. Also, as shown in  FIG. 2 , the depth at which light is absorbed in silicon, and photo-carriers are generated, will also vary. 
         [0006]    It is very important that, when photo-carrier electron-hole (e-h) pairs are generated in the silicon, they are within an electric field. Otherwise, electron-hole pairs will recombine before they can diffuse away from each other. If an electric field exists, then electron-hole pairs will be accelerated away from each other before they can recombine. 
         [0007]    Cross sections of two typical silicon photodiodes are shown in  FIGS. 3 and 4 .  FIG. 3  shows a vertical implementation.  FIG. 4  shows a lateral implementation. 
         [0008]    The  FIG. 3  photodiode implementation includes a vertical P-i-N diode  300  which is reverse biased so that the light-generated carriers are separated before they recombine and are swept with a high electric field to the positive (V+) and negative (V−) contacts. The  FIG. 3  design provides a large depletion region  302 , but requires a discrete process flow. This means that all of the electrons must be added externally, for example on a printed circuit board (PCB). The inherent losses from both resistive ohmic drops and parasitic capacitance, makes the  FIG. 3  photodiode structure less than ideal. 
         [0009]      FIG. 4  shows a traditional lateral photodiode structure  400  that is commonplace in monolithic integrated circuit (IC) designs. In the  FIG. 4  structure  400 , the interface between a Deep N-type region (DNWELL)  402  and a P-type region (PWELL)  404  forms a junction. When light penetrates the silicon surface  406 , it is absorbed by the silicon at different depths according to its wavelength and generates e-h pairs. If the light is absorbed in the silicon region away from the depletion region  408 , then the generated e-h pairs can recombine almost instantly. If the light penetrates to the depletion region  408 , then the generated e-h pairs are separated by the electric field in the depletion region  408  and are swept away to the positive (V+) and negative (V−) contacts to form an electric current. Therefore, it is critical that the ion implants that form the DNWELL  402  region, the PWELL region  404  and the NWELL region  410  be at the particular energy that will form a junction at the depth necessary to absorb the required wavelength(s) of light. As mentioned above, any light that is absorbed outside of a depletion region is “lost.” That is, the generated e-h pairs recombine and no current is collected. The efficiency of the  FIG. 4  photodiode is, therefore, limited. 
         [0010]    Additionally, a silicon nitride, silicon monoxide or silicon dioxide layer may be deposited on top of the silicon surface to serve as protection as well as to act as an anti-reflective coating. This protective layer is then masked and etched so that the area above the collecting junction is open to the light. 
       SUMMARY 
       [0011]    Disclosed embodiments provide a photodiode formed in a semiconductor substrate. The photodiode comprises a first terminal formed in a surface of the substrate; a second terminal formed in the substrate surface and spaced apart from the first terminal and a plurality of adjacent, alternating N-type and P-type diffusion regions formed in the substrate surface between the first terminal and the second terminal. 
         [0012]    The features and advantages of the various embodiments of the invention disclosed herein will be more fully understood and appreciated upon consideration of the following detailed description and the accompanying drawings, which set forth illustrative embodiments of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a graph showing absorption coefficient versus wavelength for identified materials. 
           [0014]      FIG. 2  is a graph showing light penetration depth in silicon versus wavelength. 
           [0015]      FIG. 3  is a cross section drawing illustrating a typical vertical implementation of a silicon photodiode. 
           [0016]      FIG. 4  is a cross section drawing illustrating a typical lateral implementation of a silicon photodiode. 
           [0017]      FIG. 5A  is a perspective drawing illustrating a lateral super-junction LDMOS transistor structure. 
           [0018]      FIG. 5B  is a cross section drawing illustrating a vertical super-junction LDMOS transistor. 
           [0019]      FIG. 5C  is a perspective drawing illustrating a V-groove super-junction LDMOS transistor. 
           [0020]      FIG. 6  is a perspective drawing illustrating a super-junction photodiode structure. 
           [0021]      FIG. 7  is a perspective drawing illustrating the depletion region of the  FIG. 6  super-junction photodiode structure. 
           [0022]      FIG. 8  is a graph showing the responsivity of photodiodes made from silicon and germanium. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The concept of a “super-junction” or charge balanced device is well known, but only as a method by which a high voltage breakdown may be obtained, typically in a laterally diffused metal oxide semiconductor (LDMOS) structure, thereby allowing a reduction in the resistance-area product (RDSON*Area) of the LDMOS device. 
         [0024]    The super-junction LDMOS concept has a number of different known implementations, but fundamentally consists of a series of alternating N- and P-type regions, typically called pillars. These pillars may be arrayed in different configurations, such as laterally, vertically or at an angle, as shown in  FIGS. 5A ,  5 B and  5 C, respectively. In all of each these LDMOS structures, the effect is the same: by adjusting the doping level and the width (Wn and Wp) of the pillar regions, it is possible to cause a state of full depletion either at zero applied bias or with a reverse bias applied across the junction. This state is called “charge balance,” which means that the N and P regions are fully depleted. Once charge balance is achieved, the entire region becomes one large charge collector. 
         [0025]      FIG. 6  shows an embodiment of a super-junction photodiode structure  600  wherein the adjacent, alternating N-pillar diffusions  602  and P-pillar diffusions  604  are formed in a P-type semiconductor substrate  606  between a P+ cathode terminal  608  and an N+ anode terminal  610  and are arrayed across the surface of the device.  FIG. 6  shows 0V applied to the cathode terminal  608  and a positive voltage V+ applied to the anode terminal  610 . The P-N junction  612  is highlighted as bold in the  FIG. 6  drawing. This junction  612  forms the center of the depletion region  614 , which is shown in  FIG. 7 . As is evident from  FIG. 7 , the size of the depletion region  614  has been maximized to the fullest volume possible. Any light that is absorbed from the surface to the bottom of the N- and P-pillar regions  602 ,  604  will cause e-h pair creation. Because of the built-in electric field in the depletion region  614 , all of these carriers are separated before they can recombine and by drift and diffusion, they will reach the anode and cathode terminals. 
         [0026]    It should be noted that, by design, the sensitivity of the super-junction photodiode  600  can be altered. Low doping and smaller pillar widths (Wn, Wp) would allow the silicon to be fully depleted at zero voltage, thereby facilitating a low power solution. Higher doping levels (and/or wider Wn and Wp pillar regions) would give full depletion at some larger reverse bias voltage. This would result in a lower resistance cell (higher conductivity) and the higher voltage would provide higher electric fields for a faster, more sensitive cell. 
         [0027]    Typically, photodiodes are operated in a reverse bias mode. That is, a positive voltage is applied to the N-type regions. This causes the depletion region to expand. It is, therefore, desirable to use a super-junction photodiode design that can sustain a high reverse voltage. However, this is limited to the breakdown voltage of the photodiode junction. By using the charge balance concept described above, the breakdown voltage of the super-junction photodiode is much larger than could otherwise be obtained. In addition, the super-junction structure causes a constant electric field across the drift region (Ldrift in  FIG. 6 ) between the anode and the cathode. The carriers are therefore at a constant rate across the entire depletion region. This means that a large drift region may be used where the electric field accelerates carriers uniformly through the entire volume. This also results in carriers being accelerated faster, which should result in faster operation of the device. 
         [0028]    The super-junction photodiode  600  discussed above assumes that only pure silicon has been used as the material within which the N- and P-type pillars are created  602 ,  604 . Those skilled in the art will appreciate that alternate materials could also be used that have a different bandgap and, therefore, would absorb a different spectrum of light. For example, in  FIG. 7 , instead of a silicon substrate, a germanium substrate could be used, or a layer of germanium or silicon-germanium (SiGe) could be grown on top of the silicon substrate before the implants are performed. The resultant absorbed wavelengths would change, as shown in  FIG. 8 . The wavelength range of the photo-detector  600  would, therefore, shift to higher wavelengths. 
         [0029]    It is also possible to create a photodiode where the N- and P-type pillars shown in the  FIG. 6  embodiment are formed with alternating materials such as, for example, Si/SiGe/Si/SiGe . . . . The resultant super-junction photodiode would absorb light with a much broader spectrum. This type of device could be formed in two ways: etching of the silicon regions and selective epitaxial growth (SEG) of silicon germanium (SiGe), or implanting germanium into certain pillars with the other pillars masked. 
         [0030]    It should be understood that the particular embodiments of the subject matter described above have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope of the claimed subject matter as expressed in the appended claims and their equivalents.
A photodiode comprises a first terminal formed in a surface of a semiconductor substrate; a second terminal formed in the substrate surface and spaced apart from the first terminal; and a plurality of adjacent alternating N-type and P-type diffusion regions formed in the substrate surface between the first terminal and the second terminal.