the invention will be described more fully hereinafter with reference to the accompanying drawings , in which example embodiments of the invention are shown . this invention may , however , be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein . rather , the disclosed embodiments are provided so that this disclosure will be thorough and complete , and will fully convey the scope of the invention to those skilled in the art . in the drawings , the size and relative sizes of layers and regions may be exaggerated for clarity . moreover , each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well . like numbers refer to like elements throughout . it will be understood that when an element or layer is referred to as being “ on ,” “ connected to ,” “ coupled to ” or “ responsive to ” ( and / or variants thereof ) another element , it can be directly on or directly connected , coupled or responsive to the other element or intervening elements may be present . in contrast , when an element is referred to as being “ directly on ,” “ directly connected to ,” “ directly coupled to ” or “ directly responsive to ” ( and / or variants thereof ) another element , there are no intervening elements present . as used herein , the term “ and / or ” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. it will be understood that , although the terms first , second , third , etc . may be used herein to describe various elements , components , regions , layers and / or sections , these elements , components , regions , layers and / or sections should not be limited by these terms . these terms are only used to distinguish one element , component , region , layer or section from another region , layer or section . thus , a first element , component , region , layer or section discussed below could be termed a second element , component , region , layer or section without departing from the teachings of the present invention . the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention . as used herein , the singular forms “ a ,” “ an ” and “ the ” are intended to include the plural forms as well , unless the context clearly indicates otherwise . it will be further understood that the terms “ comprises ” and / or “ comprising ” ( and / or variants thereof ), when used in this specification , specify the presence of stated features , integers , steps , operations , elements , and / or components , but do not preclude the presence or addition of one or more other features , integers , steps , operations , elements , components , and / or groups thereof . in contrast , the term “ consisting of ” ( and / or variants thereof ) when used in this specification , specifies the stated number of features , integers , steps , operations , elements , and / or components , and precludes additional features , integers , steps , operations , elements , and / or components . furthermore , relative terms , such as “ lower ” or “ bottom ” and “ upper ” or “ top ,” may be used herein to describe one element &# 39 ; s relationship to another elements as illustrated in the figures . it will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures . for example , if the device in one of the figures is turned over , elements described as being on the “ lower ” side of other elements would then be oriented on “ upper ” sides of the other elements . the exemplary term “ lower ”, can therefore , encompasses both an orientation of “ lower ” and “ upper ,” depending of the particular orientation of the figure . similarly , if the device in one of the figures is turned over , elements described as “ below ” or “ beneath ” other elements would then be oriented “ above ” the other elements . the exemplary terms “ below ” or “ beneath ” can , therefore , encompass both an orientation of above and below . example embodiments of the invention are described herein with reference to cross - section illustrations that are schematic illustrations of idealized embodiments ( and intermediate structures ) of the invention . as such , variations from the shapes of the illustrations as a result , for example , of manufacturing techniques and / or tolerances , may be expected . thus , the disclosed example embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein unless expressly so defined herein , but are to include deviations in shapes that result , for example , from manufacturing . for example , an implanted region illustrated as a rectangle will , typically , have rounded or curved features and / or a gradient of implant concentration at its edges rather than a binary change from implanted to non - implanted region . likewise , a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place . thus , the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention , unless expressly so defined herein . unless otherwise defined , all terms ( including technical and scientific terms ) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs . it will be further understood that terms , such as those defined in commonly used dictionaries , should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present application , and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein . as is described in more detail below , embodiments of the present invention may provide improved edge termination of semiconductor devices , such as p — n , schottky , pin or other such semiconductor devices . particular embodiments of the present invention provide edge termination for silicon carbide ( sic ) devices . for example , embodiments of the present invention may be utilized as edge termination for sic schottky diodes , junction barrier schottky ( jbs ) diodes , pin diodes , thyristors , transistors , or other such sic devices without departing from the scope of the present invention . in particular , some embodiments of the present invention provide an improved edge termination for high voltage silicon carbide devices using double guard ring termination (“ double gr termination ”) as will be discussed in detail below with respect to fig3 through 15 . commonly assigned u . s . pat . no . 7 , 026 , 650 to ryu et al . ( hereinafter “ the ′ 650 patent ), which has been incorporated herein by reference above , discusses robust guard ring termination (“ robust gr termination ”) used in conjuction with sic power devices . as discussed therein , a surface charge compensation layer , such as a thin p - type layer , is provided in addition to the multiple floating guard rings . the guard ring termination structure discussed in the &# 39 ; 650 patent has shown higher blocking capabilities than the conventional guard ring termination , including junction termination extension ( jte ). however , the maximum breakdown voltage may still be less than the value predicted in theory . accordingly , some embodiments of the present invention provide double guard ring ( dgr ) termination structures for highly doped and lightly doped implantations , respectively . in particular , the highly doped portion of the guard ring may be achieved by aluminum ( al ) implants , while the lightly doped portion of the guard ring may be achieved by boron ( b ) implant . providing both highly doped and lightly doped implants in accordance with some embodiments of the present invention may provide a doping gradient from the main junction to the termination edge such that the electrical field can be further reduced in devices according to some embodiments of the present invention . in other words , the mean doping concentration of the guard ring is greater at the main junction of the device and decreases toward the periphery of the device . the doping gradient in accordance with some embodiments of the present invention is especially useful for boron in sic , which can diffuse during a high temperature activation process . details with respect to some embodiments of the present invention will be discussed further herein with respect to fig3 through 15 . referring now to fig3 , a cross section of a silicon carbide semiconductor device 20 illustrating some embodiments of the present invention will be discussed . as illustrated in fig3 , a silicon carbide layer 30 , such as a lightly doped n - type silicon carbide layer , has formed therein a main junction 32 , for example , of p - type silicon carbide , and a plurality of double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 , such as p - type silicon carbide floating guard rings , in accordance with some embodiments of the present invention . although four double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 are illustrated in fig3 , embodiments of the present invention are not limited to this configuration . any number of double guard rings may be included without departing from the scope of the present invention . as further illustrated in fig3 , an insulating layer 26 , such as an oxide layer , is provided on the silicon carbide layer 30 . the insulating layer 26 may be a deposited or grown oxide and may be fabricated utilizing techniques known to those of skill in the art . in particular embodiments of the present invention , the insulating layer 26 may be an oxide , such as sio 2 , a nitride , such as si 3 n 4 , an oxide - nitride - oxide structure and / or an oxynitride or organic films such as a polyimide layer . as is further illustrated in fig3 , the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 each include first and second portions 34 and 36 , respectively . in some embodiments of the present invention , the first and second portions 34 and 36 of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 may be p - type silicon carbide . although the first and second guard rings dgr 1 and dgr 2 are illustrated as having a distance d 1 between them equal to zero , embodiments of the present invention are not limited to this configuration . the distance between the first and second guard rings dgr 1 and dgr 2 can be greater than zero without departing from the scope of the present application . further illustrated are the distances between the second dgr 2 and third guard dgr 3 rings and the third dgr 3 and fourth dgr 4 guard rings d 2 and d 3 , respectively . the distances d 1 , d 2 and d 3 may all be different distances without departing from the scope of the present invention . for the structure illustrated in fig3 where p - type silicon carbide double guard rings are provided in an n - type silicon carbide layer , the dose charge ( concentration × depth = dose ) of the highly doped portion ( p + ) 34 of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 may be from about 5 . 0 × 10 18 to about 1 . 0 × 10 20 cm − 3 . the lightly doped portion ( p − ) 36 may be from about 5 . 0 × 10 16 to about 5 . 0 × 10 17 cm − 3 . as used herein , “ n + ” or “ p + ” refer to regions that are defined by higher carrier concentrations than are present in adjacent or other regions of the same or another layer or substrate . the highly doped portions 34 and the lightly doped portions 36 of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 may extend a distance d 4 from about 0 . 3 to about 0 . 8 μm into the silicon carbide layer 30 . although fig1 illustrates that the highly doped portion 34 and the lightly doped portion 36 extend into the substrate a same distance d 4 , embodiments of the present invention are not limited to this configuration . alternative embodiments will be discussed further below with respect to fig1 . the highly doped 34 and / or lightly doped 36 portions of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 may have higher doping concentrations closer to the main junction 32 of the device 20 and lower doping concentrations at the periphery of the device 20 . in some embodiments of the present invention , aluminum ions are implanted to achieve the highly doped portions 34 and boron ions are implanted to achieve the lightly doped portions 36 . providing the lightly doped portions 36 in a second guard ring adjacent the highly doped guard ring allows a doping gradient from the main junction 32 to the termination edge to be provided , especially for boron in sic which can diffuse during high temperature activation process . thus , the electric field may be further reduced in accordance with some embodiments of the present invention . the oxide - semiconductor interface is expected to have from about 1 . 0 × 10 12 to about 2 . 0 × 10 12 cm − 3 of positive charge . the surface of the surface low does portions 36 of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 will , typically , be depleted by the positive surface charges , and the negative charges in the depletion region low does portions 36 of the double guard rings dgr 1 , dgr 2 , dgr 3 and dgr 4 will reduce the e - field lines originating from the oxide interface charges , and possibly neutralize the negative effects of the positive interface charges . as discussed above , in some embodiments of the present invention , the double guard rings 34 / 36 may be uniformly spaced , non - uniformly spaced or combinations of uniformly and non - uniformly spaced . in other words , the lengths of d 1 , d 2 , d 3 and the like may vary and may not be constant . additionally , in certain embodiments of the present invention , from about 1 to about 100 guard rings 34 / 36 may be provided . the guard rings 34 / 36 may extend a distance of from about 10 μm to about 1000 μm from the main junction of the device . in fabricating devices according to certain embodiments of the present invention , the lightly doped portions 36 may be formed before or after formation of the highly doped portions 34 . both the highly doped portion 34 and the lightly doped portion 36 may be provided by , for example , ion implantation , or other techniques known to those of skill in the art . alternatively , the lightly doped portion 34 or the highly doped portion 36 may be an epitaxially grown layer of sic or deposited layer of sic that is formed on the layer 30 and , in the case of the regions , patterned to provide the desired surface charge compensation regions and / or layers . in such case , the guard rings may be formed prior to formation of the sic layer or after formation of the sic layer . while embodiments of the present invention have been illustrated with reference to a p — n main junction , as will be appreciated by those of skill in the art in light of the present disclosure , edge termination techniques according to embodiments of the present invention may be utilized with other devices and / or junction types , such as schottky junctions . as discussed above with respect to fig3 , some embodiments of the present invention provide a novel edge termination structure including double guard rings for the high and low implant doses to further reduce the electrical field . in particular , as discussed above , some embodiments of the present invention provide a gradient of the lightly doped portion 34 of the guard ring , which can further improve the blocking capability of power devices . furthermore , in accordance with some embodiments of the present invention , the lightly doped portion 34 may have a wide range of tolerance in processing such as misalignment , opening definition , and the like . double guard ring termination structures according to some embodiments of the present invention may be processed using existing processes and , therefore , may not increase the processing steps and difficulty of processing these devices . a higher blocking capability provided by devices according to some embodiments of the present invention may result in the improvement in other parameters of power devices , such as reducing on - resistance by a thinner drift layer . a thinner drift layer may reduce the power device die size further to achieve a higher die yield . methods of fabricating double guard ring edge termination structures according to some embodiments of the present invention will now be discussed with respect to fig4 a through 4d . as seen in fig4 a , a silicon carbide layer 30 has formed in it a junction 32 and spaced apart highly doped portions 34 of the double guard ring structure . such regions may be formed , for example , by ion implantation into a silicon carbide substrate and / or epitaxial layer . for example , aluminum ions having a doping concentration of from about 5 . 0 × 10 18 to about 1 . 0 × 10 20 cm − 3 may be implanted into the silicon carbide layer 30 to provide the highly doped portions 34 illustrated in fig4 a . referring now to fig4 b , a mask layer 100 may be formed and patterned on the silicon carbide layer 30 and may correspond to the junction 32 and highly doped portions of the guard ring 34 . the mask layer 100 may be made of conventional mask materials and may , for example , be patterned using conventional photolithography or other such techniques known to those of skill in the art . the mask layer 100 opens windows adjacent the junction 32 and the highly doped portions of the guard rings 34 . as illustrated in fig4 c , the lightly doped portions 36 of the guard rings may be formed through ion implantation using the mask layer 100 as an ion implantation mask . for example , boron ions having a doping concentration of from about 5 . 0 × 10 16 to about 5 . 0 × 10 17 cm − 3 may be implanted into the silicon carbide layer 30 to provide the lightly doped portions 34 illustrated in fig4 c . the mask layer 100 may then be removed and the insulating layer 26 may be formed on the resulting structure as illustrated in fig4 d . the insulating layer 26 may , for example , be formed by thermal oxidation and / or depositing an oxide on the resulting structure . while embodiments of the present invention have been described with reference to particular operations in fabrication , particular mask patterns and the like , as will be appreciated by one of skill in the art in light of the present disclosure , other operations , sequences of operations , mask patterns and the likes may be utilized while still benefiting from the teachings of the present invention . for example , a different sequence of implantation of guard rings and surface charge compensation regions may be provided . furthermore , the particular operations in fabrication of the device may depend on the device being fabricated . thus , for example , the fabrication of a transistor may have different fabrication steps than the fabrication of a diode . accordingly , embodiments of the present invention should not be construed as limited to particular operations in fabrication but may encompass and fabrication operations that provide edge termination structures as described herein . referring now to fig5 , a mask used to form double guard ring termination structures in accordance with some embodiments of the present invention will be discussed . as illustrated in fig5 , the mask 500 has been patterned to form the lightly doped portion 34 , for example , boron , of the double guard rings and the highly doped portion 36 , for example , aluminum , of the double guard rings . the portions of the mask 500 used to implant the highly doped portions 36 are set out in fig5 . furthermore , the portions of the mask 500 used to implant the lightly doped portions 34 are set out in fig5 . accordingly , double guard ring termination in accordance with some embodiments of the present invention may allow the dose of the lightly doped guard ring ( b ) to be gradually reduced from the main junction of the device to the periphery using one time implantation . referring now to fig6 , a blocking histogram for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention will be discussed . as illustrated by the histogram , a blocking voltage of about 130 v higher was achieved using the double guard ring termination structure in accordance with some embodiments of the present invention . fig7 is a graph illustrating representative reverse iv curves for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention . fig8 is a graph illustrating simulations of sic jbs diode blocking characteristics for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention . the devices were fabricated using the same wafer . fig9 and 10 are graphs illustrating potential distribution of jbs with double guard ring termination and robust guard ring termination at 680v , respectively , according to some embodiments of the present invention . fig1 and 12 are graphs illustrating electrical field distribution comparisons underneath termination junctions , and at the sio 2 / sic interface , respectively , according to some embodiments of the present invention . it will be understood that a lower electric field yields a lower leakage current . referring now to fig1 , a cross section illustrating an edge termination structure 20 ′ according to some embodiments of the present invention . like reference numerals refer to like elements throughout , accordingly details with respect to like numbered elements discussed above with respect to fig3 will not be repeated herein in the interest of brevity . as illustrated in fig1 , the lightly doped portion of the double guard ring structure has first and second portions 46 and 47 . as illustrated in fig1 , the first portion of the lightly doped portion 46 may be a p − − layer and may have a doping concentration of about 1 . 0 × 10 17 cm − 3 . the first portion of the lightly doped portion may extend about 0 . 5 μm into the semiconductor layer 30 . furthermore , the second portion of the lightly doped portion 47 may be a p − layer and have a doping concentration of about 1 . 4 × 10 17 cm − 3 . the second portion of the lightly dopes portion 47 may extend about 0 . 8 μm into the semiconductor layer 30 and may extend beneath the highly doped portion 36 . embodiments of the present invention illustrated in fig1 may exhibit improvement in electric field characteristics as illustrated in fig1 and 15 discussed below . fig1 is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in fig1 . fig1 is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in fig1 . in particular , fig1 is a graph illustrating robust gr termination and double guard ring termination with a retrograde profile of the second doping concentration . although embodiments of the present invention are discussed above primarily with respect to silicon carbide semiconductor devices , embodiments of the present invention are not limited to silicon carbide devices . for example , devices according to some embodiments of the present invention may be silicon ( si ), gallium nitride ( gan ) or gallium arsenide ( gaas ) without departing from the scope of the present invention . in the drawings and specification , there have been disclosed typical preferred embodiments of the invention and , although specific terms are employed , they are used in a generic and descriptive sense only and not for purposes of limitation , the scope of the invention being set forth in the following claims .