the present invention now will be described more fully hereinafter with reference to the accompanying drawings , in which 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 embodiments set forth herein . rather , these 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 thickness of layers and regions are exaggerated for clarity . it will also be understood that when an element such as a layer , region or substrate is referred to as being on another element , it can be directly on the other element or intervening elements may also be present . in contrast , if an element such as a layer , region or substrate is referred to as being directly on another element , then no other intervening elements are present . as used herein , the term and / or includes any and all combinations of one or more of the associated listed items . furthermore , relative terms , such as beneath , upper , and / or lower may be used herein to describe one element &# 39 ; s relationship to another element 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 below other elements would then be oriented above the other elements . the exemplary term below , can therefore , encompass both an orientation of above and below . it will be understood that although the terms first and second are used herein to describe various regions , layers and / or sections , these regions , layers and / or sections should not be limited by these terms . these terms are only used to distinguish one region , layer or section from another region , layer or section . thus , a first region , layer or section discussed below could be termed a second region , layer or section , and similarly , a second region , layer or section could be termed a first region , layer or section without departing from the teachings of the present invention . like numbers refer to like elements throughout . 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 ,” 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 . 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 this 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 will not be interpreted in an idealized or overly formal sense unless expressly so defined herein . fig4 is a cross - sectional view illustrating a transistor according to some embodiments of the present invention . referring to fig4 , a field isolation film 52 may be formed on a semiconductor substrate 50 to define an active region 54 . a transistor may be formed at the active region 54 , and a gate electrode 56 may be formed on the active region 54 . shallow low - concentration impurity doped regions 58 of a depth t 1 may be formed in the active region 54 around both sides of the gate electrode 56 . sidewall spacers 60 may be formed at sidewalls of the gate electrode 56 . an epitaxial layer 66 may be selectively grown on a part of each shallow impurity doped region 58 . high - concentration impurity doped regions 68 may be formed at the epitaxial layer 66 . the high - concentration impurity doped regions 68 may extend to a predetermined depth t 2 of the semiconductor substrate 50 . the epitaxial layer 66 may be formed in contact holes 64 penetrating inter - level insulation film 62 covering the semiconductor substrate 50 . accordingly , portions of the inter - level insulation film 62 separate the epitaxial layer 66 and the gate electrode 56 . when a sidewall spacer 60 is formed at sidewalls of the gate electrode 56 , the inter - level insulation film 62 may separate the sidewall spacer 60 and the epitaxial layer 66 . upper portions of the epitaxial layers 66 in the contact holes 64 may be filled with contact patterns 70 . each of the contact patterns 70 may be formed using doped polysilicon and / or a metal . since the epitaxial layer 66 is doped to a higher concentration , the epitaxial layer 66 and a material used as the contact pattern 70 may be in ohmic contact with one another . as shown in fig4 , parasitic capacitors c 1 and c 2 are formed between the epitaxial layer 66 and the gate electrode 56 . the parasitic capacitors c 1 and c 2 may be modeled as serially connected capacitors using an inter - level insulation film and a sidewall spacer as dielectric films . according to embodiments of the present invention , since an inter - level insulation film is provided between a doped epitaxial layer and a sidewall spacer , elevation of the gate voltage may be reduced because of a voltage drop in the inter - level insulation film . a thickness of the sidewall spacer may thus be reduced and / or a separate sidewall spacer may be eliminated to lessen an effective area of the transistor . the high - concentration impurity doped region 68 may have a concentration distribution that is higher away from a boundary with the low - concentrated impurity doped region 58 . this distribution pattern may be achieved by forming an epitaxial layer having an impurity concentration that gradually increases from a lower portion to an upper portion . in other words , since the epitaxial layer 66 may have a concentration distribution that increases gradually from a lower portion to an upper portion , a concentration of the high - concentration impurity doped region 68 formed at the epitaxial layer 66 may gradually increase from a lower portion to an upper portion . as the high - concentration impurity doped region 68 extending to the semiconductor substrate 50 comes nearer to the boundary with the low - concentrated impurity doped region 58 , the concentration thereof may gradually reduce . fig5 through 9 are cross - sectional views illustrating steps of fabricating a transistor according to embodiments of the present invention . referring to fig5 , field isolation film ( s ) 52 are formed in a semiconductor substrate 50 to define an active region 54 . a gate insulation layer 51 is formed on the active region 54 , and a gate electrode 56 is formed on the gate insulation layer 51 . low - concentration impurities are implanted into the semiconductor substrate 50 at both sides of the gate electrode 56 to form low - concentration impurity doped regions 58 . in addition or in an alternative , impurity doped regions 58 may be formed using diffusion . to reduce extension of the low - concentration impurity doped regions 58 to a lower portion of the gate electrode 56 , the low - concentration impurities may be shallowly implanted . additionally , sidewall spacers 60 can be formed at sidewalls of the gate electrode 56 . referring to fig6 , an inter - level insulation film 62 may be formed on an entire surface of the semiconductor substrate 50 . the inter - level insulation film 62 may be patterned to form contact holes 64 exposing portions of the impurity doped regions 58 of a lower concentration . epitaxial layers 66 may be formed on portions of the semiconductor substrate 50 exposed through contact holes 64 . the epitaxial layers 66 can be grown using selective epitaxial growth . during growth of the epitaxial layers 66 , the epitaxial layers 66 may be doped in situ using an impurity source during deposition . in addition or in an alternative , the epitaxial layer may be doped using ion implantation and / or diffusion . during growth of the epitaxial layer 66 , the epitaxial layer 66 may have an impurity concentration distribution that gradually increases from the lower portion to the higher portion . by doing this , a high - concentration impurity doped region to be formed later may provide a coupling with the low - concentration impurity doped region 58 without an abrupt variation of an electric field . since the epitaxial layers 66 may be partially formed in the contact hole 64 , a partially elevated source / drain structure can be formed on the active region 54 . referring to fig7 , by implanting a high - concentration of impurities into a resulting structure in which the epitaxial layers 66 have been formed , an impurity doped layer 68 having a relatively high concentration may be provided at the epitaxial layers 66 . since the epitaxial layers 66 may provide a predetermined depth , boundaries may be defined in the low - concentration impurity doped regions 58 into which the high - concentration impurities may extend . prior to forming the high - concentration impurity doped regions 68 , the epitaxial layers 66 may be formed with a concentration profile that decreases from upper portions to the lower portions . thus , the high - concentration impurity doped regions 68 may also have concentration profiles that decrease gradually from the upper portion of the epitaxial layer 66 to the lower portion thereof . even when there has not been any prior doping step for the epitaxial layer ( s ), the high - concentration impurity doped region ( s ) 68 formed by an impurity implant and / or diffusion may have a concentration profile that is lower near the low - concentration diffusion region ( s ) 58 . the high - concentration impurity doped region 68 may extend into the semiconductor substrate 50 a predetermined depth . in this case , the nearer the high - concentration impurity doped regions 68 to the low - concentration impurity depend region 58 , the lower the concentration thereof . accordingly , the high - concentration impurity doped regions 68 may have a concentration distribution profiles that are higher away from the boundary of the low - concentration impurity doped region 58 . referring to fig8 , the contact holes 64 are filled with a conductive film to form contact patterns 70 connected to the epitaxial layers 66 . the contact patterns 70 may be polysilicon plugs . at this time , the polysilicon plugs may be in - situ doped or doped by ion implantation . in an alternative , the contact patterns 70 can be formed of metal . referring to fig9 , contact holes 64 may be filled with metal to form the contact patterns 70 . the contact patterns 70 may include metal barrier layers 70 a and metal core layers 70 b . the metal barrier layers 70 a may conformally cover inner walls of the contact holes 64 and upper surfaces of the epitaxial layers 66 . the metal barrier layer ( s ) 70 a may be a titanium / titanium nitride film ( s ). the contact holes 64 in which the metal barrier layers 70 a are formed may be filled with the metal layer ( s ) 70 b . the metal layers 70 b may include tungsten , tungsten nitride aluminum , and / or copper . in this case , the contact patterns 70 and the epitaxial layers 66 may be in ohmic contact with each other . the epitaxial layers 66 may be doped at a higher concentration and a metal silicide may be formed at the boundary between the epitaxial layers 66 and the metal barrier layers 70 a allowing the contact patterns 70 and the epitaxial layers 66 to be in ohmic contact with each other . as discussed above , an epitaxial layer is not formed on portions of the semiconductor substrate exposed at opposite sides of a gate electrode before forming an inter - level insulation film . contact holes exposing a part of the semiconductor substrate may be formed on opposite sides of the gate electrode , and the epitaxial layers may be formed on exposed portions of the substrate . accordingly , the epitaxial layers may be formed at portions of impurity doped regions having a lower concentration but the epitaxial layers may be spaced apart from portions of the substrate in the vicinity of a gate electrode and / or a sidewall spacer . such a structure according to embodiments of the present invention may provide a potential barrier by an inter - level insulation film separating the epitaxial layer ( s ) and the gate electrode . accordingly , although a high voltage may be applied to the epitaxial layer ( s ), a voltage drop due to an inter - level insulation film separating the epitaxial layer ( s ) and the gate electrode may reduce voltage increases at the gate electrode . in addition , since there may be a parasitic capacitor of relatively low capacitance between the gate electrode and the epitaxial layer , fluctuations of a gate potential due to electrical signals from the source and / or drain regions may be reduced . while the present invention has been particularly shown and described with reference to embodiments thereof , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents .