in the following detailed description , when a layer is described as being formed on another layer or on a substrate , the layer may be formed on the other layer or on the substrate , or a third layer may be interposed between the layer and the other layer or the substrate . fig1 is a schematic top plan view of a fet with a round or circular shaped nano - wire channel in accordance with an embodiment of the invention . fig2 a is a schematic cross - sectional view of one embodiment of the fet of fig1 , taken along line a - a ′ of fig1 . fig2 b is a schematic cross - sectional view of the embodiment of the fet of fig2 a taken along line b - b ′ of fig1 . referring to fig1 , 2 a and 2 b , the fet structure of the invention includes a semiconductor substrate 110 . source / drain regions 14 are formed on the substrate 110 , the source / drain regions 14 include a stacked structure of a first silicon germanium ( sige ) layer 14 a , a silicon layer 14 b and a second sige layer 14 c sequentially stacked as shown . the first and second sige layers and the silicon layers can be epitaxial layers . the silicon layer 14 b includes a round - shaped nano - wire channel region 12 extending across the structure along a longitudinal line x between the source / drain regions 14 . a portion of the substrate 110 protrudes above the surface of the substrate in the region beneath the nano - wire channel region 12 . as noted in fig2 b , the nano - wire channel 12 and the protruding portion of the substrate 110 are separated by a distance d . an isolation region 116 isolates the fet from other devices . a gate dielectric layer 30 made of a material such as silicon oxide surrounds the nano - wire channel region 12 . the gate dielectric layer 30 also surrounds the protruding portion of the substrate . a gate 20 made of a conductive material such as polysilicon , metal or a combination of polysilicon and metal surrounds the nano - wire channel region 12 . the gate 20 is insulated from the nano - wire channel 12 by the gate dielectric layer 30 . fig3 is a schematic cross - sectional view of another embodiment of the fet of fig1 , corresponding to line a - a ′ of fig1 . the device 100 b of fig3 differs from that of fig2 a and 2b in that , instead of the multiple - layer stack having the first and second sige layers 14 a and 14 c and the si layer 14 b , the device of fig3 has only a single si layer 14 d , which serves as the source / drain regions of the device and provides the round - shaped nano - wire channel 12 of the device . fig4 a is a schematic cross - sectional view of another embodiment of the fet in accordance with the invention corresponding to the line a - a ′ of fig1 . fig4 b is a schematic cross - sectional view of the embodiment of the fet of fig4 a corresponding to the line b - b ′ of fig1 . referring to fig4 a and 4b , the structure is different from that of the forgoing embodiments in that the fet 100 c includes multiple , i . e ., two , round - shaped nano - wire channels 112 a and 112 b , instead of a single nano - wire channel 12 . the structure 100 c includes a semiconductor substrate 110 . source / drain regions 114 are formed on the substrate 110 . the source / drain regions 114 are formed of a stacked structure of a first sige layer 114 a , a first si layer 114 b , a second sige layer 114 c , a second si layer 114 d and a third sige layer 114 e sequentially stacked as shown . the first , second and third sige layers 114 a , 114 c and 114 e and the first and second si layers 114 b and 114 d can be epitaxial layers . the first si layer 114 b includes a first round - shaped nano - wire channel region 112 a extending along a longitudinal line x 1 across the structure between the source / drain regions 114 . the second si layer 114 d includes a second round - shaped nano - wire channel 112 b extending along a longitudinal line x 2 across the structure between the source / drain regions 114 . a portion of the substrate 110 protrudes above the surface of the substrate in the region beneath the nano - wire channel regions 112 a and 112 b . as noted in fig4 b , the first nano - wire channel 112 a and the protruding portion of the substrate 110 are separated by a distance d 1 . also , the second nano - wire channel region 112 b and the protruding portion of the substrate 110 are separated by a distance d 2 . an isolation region 116 isolates the fet 100 c from other devices . a gate dielectric layer 30 made of a material such as silicon oxide surrounds the first and second nano - wire channel regions 112 a and 112 b . the gate dielectric layer 30 also surrounds the protruding portion of the substrate . a gate 20 made of a conductive material such as polysilicon , metal or a combination of polysilicon and metal surrounds the nano - wire channel regions 112 a and 112 b . the gate 20 is insulated from the nano - wire channels 112 a and 112 b by the gate dielectric layer 30 . fig5 is a schematic cross - sectional view of another embodiment of the fet of fig1 , corresponding to line a - a ′ of fig1 . the device 100 d of fig5 differs from that of fig4 a and 4 b in that , instead of the multiple - layer stack having the first , second and third sige layers 114 a , 114 c and 114 e and the first and second si layers 114 b and 114 d , the device of fig5 has only a single si layer 114 f , which serves as the source / drain regions of the device and provides the round - shaped nano - wire channels 112 a and 112 b of the device . fig6 a is a schematic cross - sectional view of another embodiment of a fet in accordance with the invention corresponding to the line a - a ′ of fig1 . fig6 b is a schematic cross - sectional view of the fet of fig6 a corresponding to the line b - b ′ of fig1 . referring to fig6 a and 6b , the structure is different from that of the forgoing embodiments in that the fet 100 e includes multiple , i . e ., three , round - shaped nano - wire channels 212 a , 212 b and 212 c , instead of the single nano - wire channel 12 or the two nano - wire channels 112 a and 112 b . the structure 100 e includes a semiconductor substrate 110 . source / drain regions 214 are formed on the substrate 110 . the source / drain regions 214 are formed of a stacked structure of a first sige layer 214 a , a first si layer 214 b , a second sige layer 214 c , a second si layer 214 d , a third sige layer 214 e , a third si layer 214 f and a fourth sige layer 214 g sequentially stacked as shown . the first , second , third and fourth sige layers 214 a , 214 c , 214 e and 214 g and the first , second and third si layers 214 b , 214 d and 214 f can be epitaxial layers . the first si layer 214 b includes a first round - shaped nano - wire channel region 212 a extending along a longitudinal line x 3 across the structure between the source / drain regions 214 . the second si layer 214 d includes a second round - shaped nano - wire channel 212 b extending along a longitudinal line x 4 across the structure between the source / drain regions 214 . the third si layer 214 f includes a third round - shaped nano - wire channel 212 c extending along a longitudinal line x 5 across the structure between the source / drain regions 214 . a portion of the substrate 110 protrudes above the surface of the substrate in the region beneath the nano - wire channel regions 212 a , 212 b and 212 c . as noted in fig6 b , the first nano - wire channel 212 a and the protruding portion of the substrate 110 are separated by a distance d 1 . also , the second nano - wire channel region 212 b and the protruding portion of the substrate 110 are separated by a distance d 2 . also , the third nano - wire channel 212 c and the protruding portion of the substrate 110 are separated by a distance d 3 . an isolation region 116 isolates the fet 100 e from other devices . a gate dielectric layer 30 made of a material such as silicon oxide surrounds the first , second and third nano - wire channel regions 212 a , 212 b and 212 c . the gate dielectric layer 30 also surrounds the protruding portion of the substrate . a gate 20 made of a conductive material such as polysilicon , metal or a combination of polysilicon and metal surrounds the nano - wire channel regions 212 a , 212 b and 212 c . the gate 20 is insulated from the nano - wire channels 212 a , 212 b and 212 c by the gate dielectric layer 30 . fig7 is a schematic cross - sectional view of another embodiment of the fet of fig1 , corresponding to line a - a ′ of fig1 . the device 100 f of fig7 differs from that of fig6 a and 6b in that , instead of the multiple - layer stack having the first , second , third and fourth sige layers 214 a , 214 c , 214 e and 214 g and the first , second and third si layers 214 b , 214 d and 214 f , the device of fig7 has only a single si layer 214 h , which serves as the source / drain regions of the device and provides the round - shaped nano - wire channels 212 a , 212 b and 212 c of the device . fig8 is a logical flow diagram illustrating the process of forming the round - shaped nano - wire channels and gate of the fet according to the invention . fig9 a through 9d are schematic perspective views illustrating the steps in the process of forming the nano - wire channels and gate of the invention . the process of forming the nano - wire channels , the gate and the fet of the invention described herein is applicable to any of the embodiments of the fet described herein . specifically , the formation process is applicable to a fet having any number of round - shaped nano - wire channels . referring to fig8 and 9a through 9 d , in step s 50 , an active si pattern having a polygonal cross - sectional shape is formed . for example , referring specifically to fig9 a , the active si pattern 402 can have flat exterior surfaces 402 b and a substantially square shape 402 a in cross section . next , in step s 60 , a cleaning process is performed to remove any oxide that may have formed on the active si pattern 402 . next , in step s 70 , etching is performed to remove square corners from the active pattern 402 . an annealing step is performed to complete the round - shaped si nano - wire channel 404 . as shown in fig9 b , the channel 404 has a substantially round shape 404 a in cross section . next , in step s 80 , a gate dielectric layer 406 is formed surrounding the round - shaped nano - wire channel region 404 , as shown in fig9 c . next , in step s 90 , a gate electrode 408 is formed surrounding the gate dielectric layer 406 and the round - shaped or cylindrical si nano - wire channel 404 , as shown in fig9 d . the gate electrode 408 is made of a conductive material such as polysilicon , metal , or a combination of polysilicon and metal . fig1 is a detailed flow diagram illustrating the process of forming the round nano - wire channel 404 from the square nano - wire channel 402 . fig1 will be described below in detail . fig1 a through 11m are schematic perspective views illustrating steps in a process of manufacturing the fet of the invention , in accordance with an embodiment of the invention . referring to fig1 a , a silicon substrate 500 is provided . a first silicon germanium ( sige ) layer 512 is formed on top of the substrate 500 . the first sige layer 512 can have a thickness of 5 - 50 nm and can contain 15 - 20 % germanium . an active silicon layer 514 is formed over the first sige layer . the silicon layer 514 is the layer of which the round - shaped nano - wire channel region of the fet will eventually be formed . a second sige layer 516 is formed on the silicon layer 514 . the second sige layer 516 can be formed to a thickness of 5 - 50 nm and can contain 5 - 10 % germanium . the second sige layer 516 may have a lower concentration of germanium such that during a subsequent etching process , the rate at which the upper layer of sige is consumed is lower to prevent damage to the silicon layer 514 to ensure a good nano - wire channel . in one embodiment , the first and second sige layers 512 and 516 and the silicon layer 514 are grown epitaxially to the thickness of 5 - 50 nm . a capping layer 518 is then formed over the second sige layer 516 . the capping layer may be a buffer oxide layer made of a material having a high etch selectivity with respect to silicon nitride ( sin ), such as silicon oxide , for a subsequent etching process . next , a hard mask layer , made of a material such as sin , is formed on the buffer oxide layer 518 . the hard mask layer is patterned such as by photolithographic and etching processes to form a hard mask pattern 520 on the buffer oxide layer 518 . referring to fig1 b , the structure is etched using the hard mask pattern 520 as an etching mask to form a sti trench 522 . in one embodiment , the trench depth is 150 - 350 nm . referring to fig1 c , next , a shallow trench isolation ( sti ) 524 is formed in the trench 522 . the sti is formed by a high density plasma ( hdp ) process to deposit an oxide . after the hdp process , a chemical mechanical polishing ( cmp ) process is carried out to expose the top surface of the hard mask pattern 520 . as shown in the drawing , the slurry used during the cmp process has a higher polishing rate on the hdp oxide 524 than it has on the hard mask pattern 520 such that a step is created between the hdp oxide 524 and the hard mask pattern 520 . next , referring to fig1 d , the hard mask pattern 520 and a portion of the hdp oxide is removed using phosphoric acid , leaving the buffer oxide layer 518 and the top surface of the hdp oxide 524 exposed on the top of the structure . next , referring to fig1 e , a second hard mask pattern 530 is formed on the top surface of the structure by forming a hard mask layer made of sin and then applying photolithographic and etching processes to pattern the hard mask layer . next , referring to fig1 f , the structure is etched using the second hard mask pattern 530 as an etching mask to form a sti recess 532 . the depth of the recess 532 is controlled to be deeper than the total thickness of the first and second sige layers 512 , 516 and the si layer 514 . that is , the recess 532 extends down into the structure deeper than the bottom of the epitaxial sige and si layers . the width of the remaining epitaxial sige and si layers , as well as a portion of the substrate beneath the epitaxial layers is indicated in the figure as w 1 . next , referring to fig1 g , the remaining portion of the sige epitaxial layers 512 and 516 , the remaining portion of the active si layer 514 a and the raised portion of the substrate beneath the epitaxial layers having the width w 1 are optionally trimmed by etching , such that the remaining portion of the epitaxial layers and the raised portion of the substrate beneath the epitaxial layers has a width w 2 . the etching is preferably a chemical dry etch ( cde ), which can be carried out , for example , at 400 w , 225 mtorr , 250 degrees c . for 20 seconds , in an atmosphere containing cf 4 and o 2 at flow rates of 60 and 150 sccm , respectively . the width w 2 is selected based on a desired size of the final nano - wire channel to be formed . the cde is performed to reduce the channel width to w 2 and to make the channel square in cross - section . fig1 h is the structure of fig1 g rotated 90 degrees to clearly illustrate the resulting structure . next , referring to fig1 i , the sacrificial sige layers 512 and 516 on the top and bottom of the channel region 514 a are removed to completely expose the channel region 514 a . it is also noted that a rectangular portion of the substrate 500 also remains beneath the channel region 514 a following this step . this step is performed by a chemical wet etch using a chemical including ch 3 coooh ( or ch 3 cooh )+ hf + diw ( deionized water ) (+ h2o2 + surfactant , etc .). next , referring to fig1 j , the square cross - sectional channel region 514 a is formed into a nano - wire channel 514 b having a round or circular cross - section . this is performed by etching and annealing the square cross - sectional channel region 514 a until it becomes the round nano - wire channel 514 b . fig1 is a flow diagram illustrating the process of forming the round nano - wire channel 514 b from the square nano - wire channel 514 a . first , an optional cleaning step s 100 is performed in an atmosphere of h2 . in one embodiment , the cleaning is performed in an atmosphere of 100 % h2 . alternatively , the atmosphere can also contain ar and / or he with or without the h2 . in one embodiment , the cleaning is performed at a pressure of 0 . 1 to 10 torr and a temperature of 600 - 900 degrees c . in one particular embodiment , the cleaning is performed at a temperature of 700 - 800 degrees c . the gas flow rate during the cleaning step can be 1 - 500 sccm , and the process time can be 1 - 5 minutes . after the cleaning step , the four corners of the square nano - wire channel 514 a are etched in step s 200 . the etching is carried out by introducing a combination of hcl and h2 gas into the process chamber . in one particular embodiment , the gas flow rate of hcl during the etching is 100 - 2000 sccm , and the gas flow rate of the h2 is 100 - 2000 sccm . the flow rate ratio of hcl : h2 can be in a range of 5 : 5 to 3 : 7 . in one particular embodiment , the flow rate ratio of hcl : h2 is 300 sccm : 500 sccm . the etch temperature can be in the range of 600 - 900 degrees c ., and the pressure can be 10 - 100 torr . the time of the etch can be in the range of 1 - 120 seconds . the etch of step s 200 can be performed under one of at least three possible sets of conditions . for a relatively long - duration , low - temperature etch , the etch temperature can be in the range of 600 - 700 degrees c . for a relatively short - duration , high - temperature etch , the etch temperature can be in the range of 850 - 900 degrees c . between the two process conditions above , the middle - duration , middle - temperature etch can be carried out at a temperature between 750 and 820 degrees c . after the etching process , a low - temperature annealing is carried out in step s 300 to form the round - shaped nano - wire channel region 514 b . the annealing is performed in an atmosphere of h2 gas . in one embodiment , the annealing is performed at a pressure of 0 . 1 - 10 torr . if the pressure is lowered , the process time can also be lowered . in one embodiment , the annealing is performed at a temperature of 600 - 900 degrees c . for a period of 10 - 800 seconds . in one embodiment , the h 2 gas flow rate is 1 - 500 sccm . in one particular embodiment , the annealing is carried out at a temperature of about 810 degrees c ., at a pressure of 5 torr and for a period of 500 seconds . the etching and annealing steps can be repeated as many times as are needed to form the final round - shaped nano - wire channel 514 b . between the annealing step s 300 and the next repeated etching step s 200 , a purging step s 400 can be performed to remove the remaining annealing h 2 gas from the process chamber . the purge can be performed using at least one of ar , he and h 2 gas . after the purging step s 400 , a determination is made in step s 500 as to whether the channel 514 b is the desired size and / or shape . if the channel 514 b is of the proper shape and size , the process ends . if not , the process returns to step s 200 to begin another cycle of etching , annealing ( s 300 ) and optional purging ( s 400 ). referring to fig1 k , after the round - shaped nano - wire channel 514 b is formed , a gate dielectric layer is formed on the structure , including surrounding the nano - wire channel 514 b . the gate dielectric can be formed by growing sio2 on the structure using o2 gas . next , a gate material such as polysilicon or a metal layer with polysilicon is formed surrounding the nano - wire channel 514 b . then the gate material is planarized such as by chemical mechanical polishing ( cmp ) to form the gate 540 surrounding the nano - wire channel 514 b . next , referring to fig1 l , the second hard mask pattern 530 is removed . fig1 m illustrates the final structure with the gate pattern 540 shown in phantom . fig1 m shows the round - shaped nano - wire channel 514 b and the raised portion of the substrate 500 , which have both been formed into a round shape by the etching and annealing steps of the invention . it is noted that this embodiment of manufacturing a fet is applicable to formation of any number of channel regions . where more channel regions are to be formed , more alternating layers of sige and si are initially formed . it should be noted that fig2 a corresponds to a cross - sectional view of fig1 l , taken along line iia - iia ′ of fig1 l . likewise , fig2 b corresponds to a cross - sectional view of fig1 l , taken along line iib - iib ′ of fig1 l . fig1 a through 12k are views illustrating steps in a process of manufacturing the fet of the invention , in accordance with another embodiment of the invention . the initial steps used in the embodiment of fig1 a through 12k are the same as those illustrated in the previously described embodiment for steps 11 a through 11 d . for the remaining steps , where a step is analogous to a step described in connection with the embodiment of fig1 a through 11m , the step is carried out in similar fashion . therefore , description of those steps will not be repeated . fig1 a - 12f are schematic cross - sectional views of the structure of fig1 d , taken along line xii - xii ′ of fig1 d , illustrating steps in the process of manufacturing a fet according to the embodiment of the invention . fig1 g - 12k are schematic perspective views illustrating steps in the process of manufacturing a fet according to the embodiment of the invention . referring to fig1 a , the structure resulting after performance of steps 11 a through 11 d is illustrated . next , referring to fig1 b , a hard mask pattern 630 is formed on the structure . the hard mask pattern 630 can be formed by patterning a layer of sin . next , referring to fig1 c , recessed regions 632 are formed in the structure . this can be performed by etching a trench to a depth deeper than the first epitaxial sige layer 512 using the hard mask pattern 630 as an etching mask . next , referring to fig1 d , the recess 632 is partially filled by epitaxially growing a si layer 640 in the recess . in one embodiment , the epitaxial si layer 640 is grown to a depth higher than the second epitaxial sige layer 516 . next , referring to fig1 e , another hard mask pattern made of , for example , sin is formed above the epitaxial si layer 640 and adjacent to the hard mask pattern 630 . referring to fig1 f and 12g , the hard mask pattern 630 is then removed , exposing the top or second epitaxial sige layer 516 and a portion of the top surface of the sti dielectric layer 524 . next , referring to fig1 h , the structure is etched to remove a portion of the sti dielectric layer 524 to form a damascene recessed region 532 to expose the sides of the stack of the first and second sige layers 512 and 516 , the si layer 514 and a portion of the substrate 500 beneath the stacked epitaxial layers 512 , 514 and 516 . next , referring to fig1 i , the structure is etched , preferably by a chemical dry etch to trim the first and second sacrificial sige layers 512 and 516 , the si layer 514 and the protruding portion of the substrate 500 . the trimming also exposes a portion of the epitaxial si layer 640 as shown . next , referring to fig1 j , the sacrificial sige layers 512 and 516 on the top and bottom of the channel region 514 a are removed to completely expose the channel region 514 a . it is also noted that the rectangular protruding portion of the substrate 500 also remains beneath the channel region 514 a following this step . this step is performed by a chemical wet etch using a chemical including ch 3 coooh ( or ch 3 cooh )+ hf + diw ( deionized water ) (+ h2o2 + surfactant , etc .). next , referring to fig1 k , the channel region 514 a is cleaned , etched and annealed in accordance with the description of fig1 to form the round - shaped nano - wire channel region 514 b . it is noted that in this embodiment , the source / drain regions of the fet are made from the single epitaxial si layer 640 , as opposed to the stacked structure of epitaxial sige and si layers . it is also noted that this embodiment , like the previously described embodiment , is applicable to formation of any number of channel regions . where more channel regions are to be formed , more alternating layers of sige and si are initially formed . while the present invention has been particularly shown and described with reference to exemplary embodiments thereof , it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims .