Abstract:
An image sensor including a pixel array, each pixel including, in a substrate of a doped semiconductor material of a first conductivity type, a first doped region of a second conductivity type at the surface of the substrate; an insulating trench surrounding the first region; a second doped region of the first conductivity type, more heavily doped than the substrate, at the surface of the substrate and surrounding the trench; a third doped region of the second conductivity type, forming with the substrate a photodiode junction, extending in depth into the substrate under the first and second regions and being connected to the first region; and a fourth region, more lightly doped than the second and third regions, interposed between the second and third regions and in contact with the first region and/or with the third region.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 1161775, filed on Dec. 16, 2011, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure generally relates to integrated photodetectors, for example, photodetectors of an image sensor. More specifically, the present disclosure relates to the protection—or hardening—of such photodetectors against ionizing radiations. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Image sensors may comprise photosensitive sites and transistors formed in a semiconductor substrate, for example, silicon. More specifically, such image sensors comprise a pixel array, each pixel comprising at least one photodiode comprising a P-N junction formed in the substrate. Photons reaching the pixel may cause the forming of electron/hole pairs in the substrate. The electron/hole pairs may form in the photodiode depletion area. The electric field present in the depletion area then directs electrons towards the N-type doped region and holes towards the P-type doped region. The electron/hole pairs may form in the substrate in a P-type or N-type doped region. When an electron/hole pair forms at a shorter distance from the depletion area than the minority carrier diffusion distance, the minority carrier (electron or hole) may diffuse all the way to the depletion area. The electric field present in the depletion area then directs the minority carrier towards the P-type or N-type region where it is a majority carrier. The collection of electrons and holes by the photodiode appears as a measurable variation of the voltage across the photodiode. 
         [0006]    The quantum efficiency of the pixel corresponds to the ratio of the number of electrons that can be collected by the photodiode to the number of photons reaching the pixel. It is desirable to have the highest possible quantum efficiency. 
         [0007]    The dark current of the pixel corresponds to the signal provided by the photodiode in the absence of any lighting. It results from the forming of electron/hole pairs in the pixel in the absence of any lighting, which electrons and holes may be collected by the photodiode. Thermal agitation generally causes the forming of electron/hole pairs in the substrate. The presence of defects at the interface between the semiconductor substrate and a portion of an insulating material increases the electron/hole pair forming speed in the substrate at the level of this interface, especially in a depletion area. Such is especially the case at the interface between the semiconductor substrate and the insulating layer covering the substrate or at the interface between the semiconductor substrate and insulating trenches formed in the substrate to insulate doped regions. 
         [0008]    Image sensors may be submitted to an ionizing radiation, for example, a gamma radiation, in particular when they are used for space applications. 
         [0009]    The dark current of the pixel may vary under the effect of the ionizing radiation. Two phenomena may cause a variation of the dark current. First, the state of the interfaces between the insulating portions and the semiconductor substrate may damage under the action of the ionizing radiation, which increases the electron/hole forming speed at these interfaces. Second, the ionizing radiation may cause the forming of positive charges in the insulating portions of the pixel. By electrostatic effect, such positive charges push back the holes present at the interface of the P-type doped regions. This causes a local increase of the dimensions of the depletion area in the P-type doped region close to the interface with the insulating portion, and thus an increase of the number of electron/hole pairs formed in the absence of any lighting at the interface between the substrate and the insulating portion in the depletion area. 
         [0010]    The pixel hardening especially aims at decreasing the variation of the dark current of the pixel in the presence of an ionizing radiation. 
         [0011]    The pixel of an image sensor may comprise the three following electronic components: the photodiode, a charge reading transistor, a reset transistor, and a selection transistor. The photodiode is in charge of collecting and storing the charges photogenerated in the pixel (for example, electrons). The photodiode is connected to the charge reading transistor (follower-assembled transistor) and to the reset transistor. The line selection transistor allows a sequential line-by-line reading. 
         [0012]    For many image sensors, the substrate is doped with a first conductivity type and the photodiode of the pixel is obtained by forming, at the substrate surface, a doped region of the second conductivity type. The doped region is laterally insulated by an insulating trench formed in the substrate. The quantum efficiency of the photodiode may be increased by increasing the surface of the P-N junction, that is, by a lateral extension of the doped region. This however increases the insulating trench dimensions. The dark current thus increases, as well as the sensitivity of the dark current to ionizing radiations. 
         [0013]    There thus is a need for a photodiode simultaneously having a high quantum efficiency and a decreased sensitivity of the dark current to ionizing radiations. 
       SUMMARY 
       [0014]    Embodiments aim at, at least partly, addressing one or several issues in the prior art. 
         [0015]    Thus, an embodiment provides an image sensor comprising a pixel array, each pixel comprising, in a substrate of a doped semiconductor material of a first conductivity type, a first doped region of a second conductivity type at the surface of the substrate; a trench of an insulating material surrounding the first region; a second doped region of the first conductivity type, more heavily doped than the substrate, at the surface of the substrate and surrounding the trench; a third doped region of the second conductivity type, forming with the substrate a photodiode junction, extending in depth into the substrate under the first and second regions and being in contact with the first region or connected to the first region by one or several additional regions of the second conductivity type; and a fourth doped region of the first or of the second conductivity type, more lightly doped than the second and third regions, interposed between the second and third regions and in contact with the first region and/or with the third region. 
         [0016]    According to an embodiment, the ratio of the external lateral perimeter of the trench to the external lateral perimeter of the third region is smaller than or equal to 5. 
         [0017]    According to an embodiment, the dopant concentration of the fourth region is smaller than or equal to 10 16  atoms/cm 3 . 
         [0018]    According to an embodiment, the dopant concentration of the second region is smaller than or equal to 10 17  atoms/cm 3 . 
         [0019]    According to an embodiment, the dopant concentration of the third region is smaller than or equal to 10 16  atoms/cm 3 . 
         [0020]    According to an embodiment, the dopant concentration of the substrate is smaller than or equal to 10 16  atoms/cm 3 . 
         [0021]    According to an embodiment, the dopant concentration of the first region is greater than the dopant concentration of the third region. 
         [0022]    According to an embodiment, the image sensor comprises a fifth region of the second conductivity type interposed between the first region and the third region, the dopant concentration of the fifth region being smaller than the dopant concentration of the first region and greater than or equal to the dopant concentration of the third region. 
         [0023]    Another embodiment provides a camera comprising an image sensor, such as defined hereabove, capable of providing images and a device for storing said images. 
         [0024]    An embodiment provides a method for manufacturing an image sensor comprising a pixel array, the method comprising the steps, for each pixel in a substrate of a doped semiconductor material of a first conductivity type, of: 
         [0025]    forming a first doped region of a second conductivity type at the surface of the substrate; 
         [0026]    forming a trench of an insulating material surrounding the first region; 
         [0027]    forming a second doped region of the first conductivity type, more heavily doped than the substrate, at the surface of the substrate and surrounding the trench; 
         [0028]    forming a third doped region of the second conductivity type, forming with the substrate a photodiode junction, extending in depth into the substrate under the first and second regions and being in contact with the first region or connected to the first region by one or several regions of the second conductivity type; and 
         [0029]    forming a fourth doped region of the first or second conductivity type, more lightly doped than the second and third regions, interposed between the second and third regions and in contact with the first region and/or with the third region. 
         [0030]    According to an embodiment, the fourth region is doped with the second conductivity type, the third and fourth regions being formed in a single step of implantation of dopants of the second conductivity type. 
         [0031]    According to an embodiment, the method further comprises the step of forming a fifth region of the second conductivity type interposed between the first region and the third region, the dopant concentration of the fifth region being smaller than the dopant concentration of the first region and greater than or equal to the dopant concentration of the third region, the fifth region being obtained by several implantations of dopants of the second conductivity type having different implantation energies. 
         [0032]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a simplified cross-section view of an example of a photodiode formed in a substrate; 
           [0034]      FIG. 2  is a simplified top view of the substrate of the photodiode of  FIG. 1 ; 
           [0035]      FIGS. 3 and 4  are simplified cross-section views of other examples of photodiodes; 
           [0036]      FIG. 5  is an embodiment of a hardened photodiode; 
           [0037]      FIG. 6  shows the variation of the dopant concentration versus the depth of the hardened photodiode of  FIG. 5  along line VI-VI; and 
           [0038]      FIG. 7  shows another embodiment of a hardened photodiode. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. Further, only those elements which are necessary to the understanding of the embodiments have been shown. 
         [0040]      FIG. 1  is a cross-section view illustrating a pixel  10  of an image sensor comprising a photodiode  11  formed in a substrate  12  of a semiconductor material, for example, silicon. The image sensor may comprise a pixel array. The substrate may be a lightly-doped P-type silicon layer  12  (P − ), for example, an epitaxial layer formed on a silicon wafer. Substrate  12  comprises an upper surface  13 .  FIG. 2  is a simplified top view of surface  13 . 
         [0041]    Photodiode  11  is provided at the surface of substrate  12  and is delimited by an insulating trench  14 . Insulating trench  14  is formed of an insulating material, for example, silicon dioxide, and may be of STI (Shallow Trench Insulation) type. Insulating trench  14  may be arranged in top view as a square or a rectangle. Photodiode  11  comprises an N-type doped region  16  formed in substrate  12  in the opening delimited by insulating trench  14 . A heavily-doped N-type region  18  (N + ) is provided at the surface of region  16  to ease the forming of an ohmic contact. Surface  13  of substrate  12  is covered with a stack  20  of insulating layers for example, made of silicon dioxide. Conductive tracks and vias, not shown, are formed in stack  20  of insulating layers and especially connect the terminals of photodiode  11 . Further, the other electronic components of the pixel have not been shown. A heavily-doped P-type region  22  (P + ) is provided at the surface of substrate  12  around insulating trench  14 . A depletion area  24  forms at the junction between region  16  and substrate  12 . The limits of depletion area  24  are schematically shows by dashed lines in  FIG. 1 . 
         [0042]    When light rays reach pixel  10 , electron/hole pairs form in substrate  12 . The charges photogenerated in depletion area  24  are collected and stored in the P-N junction with a maximum efficiency. The charges photogenerated outside of depletion area  24  are collected due the mechanism of diffusion of minority carriers in the P-type doped regions. The charge collection efficiency is thus decreased and the quantum efficiency of the pixel is decreased. Statistically, the electrons photogenerated at a distance from depletion area  24  shorter than the minority carrier diffusion distance are collected by photodiode  11 . The quantum efficiency of pixel  10  shown in  FIG. 1  especially depends on the surface of the P-N junction between region  16  and substrate  12 . 
         [0043]    The dark current of pixel  10  is due to the charges collected by photodiode  11  in the absence of any lighting. Indeed, the defects electrically active at the silicon-oxide interfaces result in the creation of electron/hole pairs.  FIG. 1  shows two types of interfaces, the interface between insulating layer  20  and silicon substrate  12  and the interface between insulating trench  14  and silicon substrate  12 . An efficient solution to limit the dark current at the insulation oxide/silicon interface is to create a region having a high hole or electron concentration at this interface. This region is formed by enhancing the P-type doping (region  22 ) or the N-type doping (region  18 ). However, depletion area  24  of a P-N junction at the oxide/silicon interface is, by definition, an area without free carriers and thus is an area with a very high dark current generation rate. The electron/hole pairs which form in the semiconductor material of depletion area  24  in contact with insulating trench  14  may be collected by photodiode  11 . 
         [0044]    It is possible to form several photodiodes  11  such as shown in  FIG. 1  within a same pixel to increase the quantum efficiency. It is further possible to increase the dimensions of each photodiode  11 . 
         [0045]      FIG. 3  shows an example of a pixel  110  comprising a photodiode  111  of improved quantum efficiency. Each element of  FIG. 3  identical or similar to an element of  FIG. 1  by its structure or its function is designated with the reference numeral used in  FIG. 1  preceded by “1”. Photodiode  111  illustrated in  FIG. 3  differs from photodiode  11  illustrated in  FIG. 1  by the transverse dimensions of insulating trench  114 , of N-type doped region  116 , and of heavily-doped N-type region  118 . 
         [0046]    A disadvantage of photodiode  111  is that the surface of insulating trench  114  in contact with the semiconductor material and, in particular, in contact with depletion area  124  of photodiode  111 , is increased. This causes, in the presence of an ionizing radiation, an increase of the dark current. The same disadvantage appears when the number of photodiodes  11  per pixel is increased. 
         [0047]    To increase the quantum efficiency of the photodiode without increasing the surface area of the insulating trenches, a possibility is not to modify the dimensions of insulating trench  14  of photodiode  11  shown in  FIG. 1  and of only increasing the lateral dimensions of N-type doped region  16 . 
         [0048]      FIG. 4  is a cross-section view illustrating a photodiode  210  having an improved quantum efficiency without increasing the dark current. Each element of  FIG. 4 , identical or similar to an element of  FIG. 1  by its structure or its function, is designated with the reference numeral used in  FIG. 1  preceded by “2”. 
         [0049]    The dimensions of insulating trenches  214  and of heavily-doped N-type regions  218  are identical to those of photodiode  11 . Region  216  extends laterally beyond insulating trench  214 . A depletion area  226  thus forms at the P-N junction between heavily-doped P-type region  222  and N-type region  216  around insulating trench  214 . The limit of depletion area  226  is schematically shown in  FIG. 4  by a dashed line. 
         [0050]    The method for manufacturing region  216  generally corresponds to a method for forming a so-called “well” structure comprising successive implantations at different implantation energies and which results in the forming of a region having a substantially constant dopant concentration over most of the depth of region  216 . 
         [0051]    Generally, width W of a depletion area is provided by the following relation (1): 
         [0000]    
       
         
           
             
               
                 
                   W 
                   = 
                   
                     
                       2 
                        
                       
                         
                           
                             ɛ 
                             S 
                           
                            
                           
                             ɛ 
                             0 
                           
                         
                         q 
                       
                        
                       
                         ( 
                         
                           
                             1 
                             
                               N 
                               A 
                             
                           
                           + 
                           
                             1 
                             
                               N 
                               D 
                             
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             V 
                             0 
                           
                           - 
                           V 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0052]    where ∈ 0  is the dielectric permittivity of vacuum, ∈ S  is the relative permittivity of the semiconductor material, q is the charge of an electron, N D  is the N-type dopant concentration (for example, phosphorus) of the N-type doped region forming the P-N junction, N A  is the P-type dopant concentration (for example, boron) of the P-type doped region forming the P-N junction, V 0  is the voltage induced by the forming of the depletion area of the photodiode when the photodiode is not biased, and V is the photodiode bias voltage. The depletion area extends more in the more lightly doped region forming the P-N junction. 
         [0053]    Thereby, depletion area  226  resulting from the P-N junction between P-type region  222  and N-type region  216  is narrow, for example, smaller than 100 nm, since the dopant concentrations are high in these regions, for example, on the order of 10 18  atoms/cm 3  for P-type region  222  and on the order of 10 18  atoms/cm 3  for N-type region  216 . 
         [0054]    Junction capacitance C associated with a depletion area is provided by the following relation (2): 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         ɛ 
                         S 
                       
                        
                       A 
                     
                     W 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0055]    where A is the surface area of the P-N junction, that is, the contact surface area between the P-type and N-type regions. Junction capacitance C is thus directly dependent on surface area A and inversely dependent on width W of depletion area. To maximize the gain of conversion of the charge into a voltage, the capacitance of the P-N junction must be as low as possible. 
         [0056]    Thereby, for photodiode  211 , a compromise must be found between the increase of the quantum efficiency of photodiode  211  and the resulting increase of the junction capacitance of photodiode  211 . 
         [0057]      FIG. 5  shows an embodiment of a photodiode  311  of a pixel  310  simultaneously having an improved quantum efficiency, a small variation of the dark current in the presence of an ionizing radiation, and a junction capacitance having a reduced and controlled increase. Each element of  FIG. 5 , identical or similar to an element of  FIG. 1  by its structure or its function, is designated with the reference numeral used in  FIG. 1  preceded by “3”. 
         [0058]    As compared with photodiode  211  illustrated in  FIG. 4 , region  316  of photodiode  311  has decreased lateral dimensions and does not extend much beyond insulation trench  314 . Photodiode  311  comprises, under region  316  and in contact therewith, an N-type region  327  which forms a P-N junction with substrate  312 . N-type region  327  may have the same dopant concentration as N-type region  316  or be more lightly doped than region  316 . A lightly-doped N-type region  328  (N − ) is interposed between P-type region  322  and N-type region  327 . Region  328  forms a P-N junction with region  322 . A depletion area  330  forms at the junction between region  327  and substrate  312  and a depletion area  332  forms at the junction between regions  324  and  313 . The limits of depletion areas  330  and  332  are shown by dashed lines in  FIG. 5 . 
         [0059]    As an example, the dopant concentrations have the following orders of magnitude: 
         [0060]    P-type doped region  322 :  10   18  atoms/cm 3 ; 
         [0061]    N-type doped region  327 :  10   16  atoms/cm 3 ; 
         [0062]    N-type doped region  316 :  10   18  atoms/cm 3 ; 
         [0063]    lightly-doped N-type region  328 :  10   15  atoms/cm 3 ; 
         [0064]    heavily-doped N-type region  318 :  10   19  atoms/cm 3 ; and 
         [0065]    lightly-doped P-type region  312 :  10   15  atoms/cm 3 . 
         [0066]    As an example, the depths of the doped regions with respect to surface  313  of substrate  312  have the following orders of magnitude: 
         [0067]    P-type doped region  322 : 0.1 μm; 
         [0068]    insulating trench  314 : 0.3 μm; 
         [0069]    N-type doped region  327 : from 1.5 to 2 μm; 
         [0070]    N-type doped region  316 : from 0.5 to 1.5 μm; 
         [0071]    lightly-doped N-type region  328 : from 0.5 to 1 μm; 
         [0072]    heavily-doped N-type doped region  318 : 0.3 μm; and 
         [0073]    lightly-doped P-type region  312 : greater than 3 μm. 
         [0074]    Pixel  310  may comprise several photodiodes  311 . As a variation, pixel  310  may comprise a single region  327  and several assemblies, each comprising insulating trench  314  and N-type doped regions  316 ,  318 . Regions  316  of these assemblies then are in contact with region  327  at different locations. 
         [0075]    An example of a method for manufacturing photodiode  311  of pixel  310  comprises the steps of: 
         [0076]    forming substrate  312  on a silicon wafer by epitaxy; 
         [0077]    forming insulating trench  314 ; 
         [0078]    forming N-type doped regions  327  and  328 , for example, in a single implantation step with an implantation energy ranging from 400 keV to 2 MeV and a phosphorus dose ranging from 10 11  to 10 12  atoms/cm 2 ; 
         [0079]    forming N-type doped region  316 , by several successive implantations; 
         [0080]    forming heavily-doped N-type region  318 ; 
         [0081]    forming heavily-doped P-type region  322  which delimits lightly-doped N-type region  328 ; and 
         [0082]    forming stack  320  of insulating layers and the conductive tracks and vias. 
         [0083]    The order of some of the steps of the previously-described manufacturing method may be modified. 
         [0084]      FIG. 6  shows the variation of the dopant concentration (Conc) according to the depth measured from surface  313  of substrate  312  at the level of line VI-VI of  FIG. 5  when the previously-described manufacturing method is implemented. 
         [0085]    Lightly-doped N-type region  328  may be formed simultaneously to more heavily-doped N-type region  327 . Indeed, the step of implantation of N-type dopants is carried out by multiple dose and energy implantations resulting in the forming of an area  334 , where the N-type dopant concentration is maximum, preceded by an area  336  where the N-type dopant concentration is smaller than the maximum concentration and which extends all the way to surface  313 . Region  327  then corresponds to the N-type dopant concentration peak. The implantation of P-type dopants results in the forming of region  322  and delimits lightly-doped N-type region  328 . The implantation of P-type dopants may be performed before or after the step of N-type dopant implantation for the forming of regions  327  and  328 . The method has the advantage of having a small number of additional steps with respect to the method for manufacturing photodiode  11  shown in  FIG. 1 . 
         [0086]    Given that the dopant concentration of region  328  of photodiode  311  is smaller, for example, by a factor one thousand, than the dopant concentration of region  216  of photodiode  211 , the thickness of depletion area  332  is greater, for example, by a factor thirty, than the thickness of depletion area  226  of photodiode  211  shown in  FIG. 4 . As an example, the total thickness of depletion area  334  may be greater than 1 μm. Thereby, the capacitance of the P-N junction between regions  322  and  328  is small and only slightly increases the total capacitance of photodiode  311 . 
         [0087]    Further, the charge collection efficiency of the photodiode is mainly defined by depletion area  330  which forms at the junction between region  327  and substrate  312 . Since regions  316  and  318  have a small part in the collection efficiency of photodiode  311 , the dimensions of insulating trench  314  and of region  318  may be decreased to a minimum while taking into account the constraints due to the methods used to manufacture photodiode  311 . This enables to decrease the value of the dark current due to insulating trench  314  and also the sensitivity to the dark current to ionizing radiations. As an example, the ratio between the external lateral perimeter of N-type doped region  327  and the external lateral perimeter of trench  314  is greater than or equal to twenty, for example, greater than or equal to one hundred. 
         [0088]    The lateral dimensions of region  316  may be the smallest possible dimensions allowed by the methods used to manufacture photodiode  311 . However, the constraints due to the manufacturing technology of photodiode  311  may impose for region  316  to laterally extend beyond insulating trench  314  as shown in  FIG. 5 . A P-N junction is then present between heavily-doped P-type region  322  and N-type region  316 . Depletion area  337  associated with this P-N junction is much narrower than depletion area  330 . Thereby, the resulting increase of the dark current is reduced. Further, the surface area of the junction between heavily-doped P-type region  322  and N-type region  316  being decreased, its contribution to the total capacitance of photodiode  311  is decreased. 
         [0089]      FIG. 7  shows another embodiment of a photodiode  411  of a pixel  410  simultaneously having an improved quantum efficiency, a small variation of the dark current in the presence of an ionizing radiation, and a decreased junction capacitance. Each element of  FIG. 5 , identical or similar by its structure or its function to an element of  FIG. 1 , is designated with the reference numeral used in  FIG. 1  preceded by “4”. 
         [0090]    Photodiode  411  has a structure similar to photodiode  311 , with the difference that lightly-doped N-type diode  328  is replaced with a lightly-doped P-type region  428  (P − ). 
         [0091]    Region  428  may be formed by a specific P-type dopant implantation step. As a variation, region  428  may correspond to a portion of lightly-doped P-type substrate  412  which is delimited at the forming of regions  427  and  422 . As an example, the P-type dopant concentration of region  428  is approximately 10 15  atoms/cm 3 . Depletion area  438  associated with the P-N junction between P-type doped region  428  and N-type doped region  427  has a significant thickness since the P-type dopant concentration of region  428  is low. This junction thus has a decreased capacitance. Further, the dark current due to this junction is decreased. 
         [0092]    Specific embodiments of have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, region  316  is mainly used to provide an electric continuity between region  327  and region  318  and may be different from what has been previously described in relation with  FIGS. 5 and 7  as long as it carries out this function. As an example, region  316  may stop laterally at the level of the insulating trench. Further, the conductivity types of the substrate and of the doped regions formed in the substrate may be inverted with respect to what has been previously described. 
         [0093]    It should further be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. In particular, lightly-doped N-type region  328  of photodiode  311  may be replaced with a stack of a lightly-doped P-type region in contact with heavily-doped P-type region  322  and of a lightly-doped N-type region in contact with N-type doped region  327 , these two lightly-doped regions forming a P-N junction. 
         [0094]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.