in the following description , numerous specific details are set forth , such as particular structures , components , materials , dimensions , processing steps and techniques , in order to provide a thorough understanding of the present invention . however , it will be appreciated by one of ordinary skill in the art that the invention may be practiced with a wide range of specific details . in other instances , well - known structures or processing steps have not been described in detail in order to avoid obscuring the invention . fig1 shows a dual dielectric tri - gate structure according to one embodiment of the present invention . structure 10 comprises a base semiconductor substrate 12 , an insulator layer 14 , a plurality of semiconductor fins 16 , hi - k dielectric 20 , metal layer 22 , top gate dielectric 24 , and gate electrode 26 . the base semiconductor substrate layer 12 may comprise any semiconductor material including , but not limited to : si , sic , sige , sigec , ge alloys , gaas , inas , inp , other iii - v or ii - vi compound semiconductors , or organic semiconductor structures . in some embodiments of the present invention , the base semiconductor substrate layer 12 may be comprised of a si - containing semiconductor material , i . e ., a semiconductor material that includes silicon . further , the base semiconductor substrate layer 12 may be doped or contain both doped and undoped regions . although the base semiconductor substrate layer 12 may be a bulk semiconductor structure , it may also include a layered structure with one or more buried insulator layers ( not shown ). the insulator layer 14 may comprise any suitable insulator material ( s ), and it typically comprises a buried oxide ( box ), a nitride , or an oxynitride in either a crystalline phase or a non - crystalline phase . the buried insulator layer 14 may be a homogeneous , continuous layer , or it may contain relatively large cavities or micro - or nano - sized pores ( not shown ). the physical thickness of the buried insulator layer 14 may vary widely depending on the specific applications , but it typically ranges from about 10 nm to about 500 nm , with from about 20 nm to about 200 nm being more typical . the present invention , in an embodiment , may utilize a bulk substrate , referred to as bulk finfet or trigat / finfet on bulk substrate , discussed in more detail below . the semiconductor fins 16 may comprise any semiconductor material including , but not limited to : si , sic , sige , sigec , ge alloys , gaas , inas , inp , other iii - v or ii - vi compound semiconductors , or organic semiconductor structures . in some embodiments of the present invention , it may be preferred that the semiconductor fins 16 be comprised of a si - containing semiconductor material , i . e ., a semiconductor material that includes silicon . further , the semiconductor fins 16 may be doped or contain both doped and undoped regions therein . the physical thickness of the fins 16 may vary widely depending on the specific applications . as will be understood by those of ordinary skill in the art , fins 16 may be formed in other ways . for example , side wall image transfer ( sit ) may be used to define the fins . gate dielectric layer 20 extends over the sidewalls of semiconductor fins 16 and on or adjacent the insulating layer 14 . gate dielectric layer 20 can be any suitable dielectric material . for instance , the gate dielectric layer may be a silicon dioxide ( sio 2 ), silicon oxynitride ( sio x n y ) or a silicon nitride ( si 3 n 4 ) dielectric layer . in an embodiment of the present invention , the gate dielectric layer 20 may be a silicon oxynitride film formed to a thickness of between 5 - 20 å . in an embodiment of the present invention , gate dielectric layer 20 may be a high k gate dielectric layer , such as a metal oxide dielectric , such as but not limited to tantalum pentaoxide ( ta 2 o 5 ), and titanium oxide ( tio 2 ). gate dielectric layer 20 can be other types of high k dielectric , such as but not limited to pzt . metal layer 22 extends over gate dielectric layer 20 , and this layer 22 may be formed of a variety of suitable materials such as , but not limited to , tungsten , tantalum , titanium , and their nitrides . as another example , layer 22 may comprise polycrystalline silicon doped to a concentration density between 10 19 - 10 20 atoms / cm 3 . also , layer 22 need not necessarily be a single material and can be a composite stack of thin films , such as but not limited to a polycrystalline silicon / metal electrode or a metal / polycrystalline silicon electrode . top gate dielectrics 24 are positioned on or adjacent top surfaces of fins 16 . dielectrics 24 , similar to dielectric layer 20 , can be any suitable dielectric material ; and , for example , dielectrics 24 may be a silicon oxynitride sion or a silicon nitride dielectric layer . in an embodiment of the invention , the gate dielectric 24 may be a silicon oxynitride film formed to a thickness of between 5 - 20 a . the gate electrode layer 26 may comprise polycrystalline silicon ( poly - silicon ), metal such as al , cu , w , ti , ta , tin , tan , nisi , cosi , and / or other proper conductive materials . the gate electrode layer 26 may be formed by cvd , pvd , plating , ald , and other suitable processes . the gate electrode layer 26 may have a multilayer structure and may be formed in a multiple - step process . fig2 - 6 illustrate processing steps in the fabrication of the structure 10 shown in fig1 , and fig7 shows a fabrication flow chart diagram according to an embodiment of the invention . generally , in embodiments of the invention , conventional fabrication steps can be used to form semiconductor substrate 12 , insulator layer 14 and fins 16 , as shown in fig2 . for example , in the manufacture of the device 10 , a silicon substrate 12 forming a silicon semiconductor body may be provided with an insulating layer 14 and on top thereof a monocrystalline silicon layer . such a semiconductor body can , for example , be obtained by implanting oxygen ions into a monocrystalline silicone substrate . however , other techniques to obtain such a start - point semiconductor body are feasible , such as using thermal oxidation of a semiconductor substrate . subsequently , an implant may be performed to tune the electrical properties of the semiconductor / silicon layer 12 . after this , a hard mask layer , e . g ., of silicon nitride or a silicon oxide , may be deposited and patterned on the semiconductor layer at the location at which fins are to be formed and where source and drains regions are envisaged for forming a finfet device . this may be followed by an etching step to form the fins 16 . optionally , this may be followed by a surface treatment like an h 2 annealing step . then , a poly silicon layer or hard mask layer is deposited and patterned , after which source and drain implants are done for forming source and drain regions that border the fin . during each of these two implants , the other regions of the structure are protected by , for example , a photo resist spot . after the source and drain implants are completed , the hard mask layer n is removed , also by ( selective ) etching . with reference to fig3 and 7 , after the formation of structure 30 of fig2 , high - k dielectric layer 20 is formed at step 102 . this may be done , for example , by a chemical vapor deposition ( cvd ) or an atomic layer deposition ( ald ) hi - k hfo 2 , zio 2 or hf / zr silicate deposition . the high - k dielectric layer 20 may contain any of the materials known in the art , including , but not limited to oxides of zr , hf , ai , hfsi , hfsin , and combinations thereof . the thickness of high - k dielectric layer 20 may be between about 1 . 0 nm and about 2 . 5 nm . an optional step 104 is to depost a band - edge metal on the hi - k dielectric layer 20 . for an nfet , this may be done , for instance , by depositing any ii / ii column element such as la , mg , or ba . for a pfet , alo 2 or rh may be deposited to form a base - edge metal . step 106 , illustrated in fig4 , is a metal deposition step to form metal layer 22 . this layer extends over gate dielectric layer 20 , and the metal layer 22 may be formed of a variety of suitable materials such as , but not limited to , tungsten , tantalum , titanium , and their nitrides . as another example , layer 22 may comprise polycrystalline silicon doped to a concentration density between 10 19 - 10 20 atoms / cm 3 . also , layer 22 need not necessarily be a single material and can be a composite stack of thin films , such as but not limited to a polycrystalline silicon / metal electrode or a metal / polycrystalline silicon electrode . at step 110 , silicon dioxide ( sio 2 ), shown in fig5 at 32 , is deposited to fill the trenches between the fins 16 and to cover layer 22 . in one embodiment , a silicon oxide layer is thickly deposited to cover the entire structure . subsequently , chemical - mechanical polishing ( cmp ) is performed to planarize the silicon oxide layer and to expose the tops of the fin - shaped structures 16 . at step 112 , the oxide is removed from the trenches , and top gate dielectrics 24 are then grown , at step 114 , on the tops of fins 16 , as shown in fig6 . these top gate dielectrics can be any suitable dielectric material , and for example , dielectrics 24 may be a silicon oxynitride sion or a silicon nitride dielectric layer . in an embodiment of the invention , the gate dielectric 24 may be a silicon oxynitride film formed to a thickness of between 5 - 20 a . dielectrics 24 may be formed , for example , by rapid thermal processing ( rtp ) oxide , decoupled plasma nitridation ( dpn ) or by rapid thermal oxidation using no gas ( rtno ). at step 116 , polysilicon gate 26 ( shown in fig1 ) is deposited over the metal - hi - k ( mhk ) sidewalls and the sion top gate dielectrics 24 . this gate electrode layer 26 may comprise polycrystalline silicon ( poly - silicon ), metal such as al , cu , w , ti , ta , tin , tan , nisi , cosi , and / or other proper conductive materials . the gate electrode layer 26 may be formed by cvd , pvd , plating , ald , and other suitable processes . also , the gate electrode layer 26 may have a multilayer structure and may be formed in a multiple - step process . with the above - described design , fet device 10 essentially has three separate channels and gates . each fin 16 forms a top channel and two side channels . the top dielectric 24 and polysilicon material 26 form a first , top gate with a threshold voltage vt1 , and metal - hi - k dielectric layer 20 , 22 and the polysilicon material 26 form two additional side gates with a threshold voltage vt2 . as mentioned above , in an embodiment , the invention may be fabricated using a bulk substrate , referred to as a bulk fin fet or trigat / finfet on bulk si substrate . fig8 shows such a bulk substrate , with fins 42 . any suitable bulk substrate may be used , and fins 42 may be formed thereon in any suitable way . as shown in fig9 , an oxide insulator layer 44 is deposited on substrate 44 , between the fins 42 . any suitable oxide material may be used , and the oxide layer 44 may be formed or deposited on substrate 40 in any suitable manner . after layer 44 is formed , the resulting structure may be processed , for example , as described above in connection with fig2 - 7 to fabricate a dual dielectric trigate field effect transistor . embodiments of the invention have significant utility . for instance , in an embodiment of the invention , the top surface of the gated region may be engineered to have a threshold voltage vt1 that is less than the threshold voltage vt2 of the metal high - k gated side surfaces . a device with these properties will operate excellently in low vdd ( vt2 & gt ; vdd & gt ; vt1 ), low power mode , and when vdd is increased above vt2 , the device will operate in a high performance mode . in the low power mode , the device will also consume lower active power , as the gate capacitance of polysilicon gated sion fets will be much lower than mhk gated devices . while it is apparent that the invention herein disclosed is well calculated to fulfill the objects discussed above , it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art , and it is intended that the appended claims cover all such modifications and embodiments as fall within the true scope of the present invention .