number 20 in fig3 indicates a wafer of semiconductor material , comprising a stack of superimposed layers , including : a first layer 21 of monocrystalline silicon ; a layer 22 of electrically insulating material , but , of high thermal conductivity ( preferably over 10 w / m ° k . ); a bond layer 23 ; and a second layer 24 of monocrystalline silicon . more specifically , insulating layer 22 may include silicon carbide ( sic ), silicon nitride ( si 3 n 4 ) or a ceramic material such as beryllium oxide ( beo ), aluminum nitride ( aln ), boron nitride ( bn ) or alumina ( al 2 o 3 ); and the thickness of insulating layer 22 depends on the maximum voltage the wafer is called upon to withstand , bearing in mind the dielectric rigidity of the material ( which , for example , in the case of silicon carbide is roughly 300 v / μm , of silicon nitride is about 1000 v / μm and of single crystal alumina is about 500 v / μm ). bond layer 23 may include of silicon oxide , thermal oxide , teos ( tetraethylorthosilicate ) oxide , cvd ( chemical - vapour - deposited ) oxide or polycrystalline silicon , and is so selected as to ensure bonding to insulating layer 22 . the bond layer presents a thickness ranging preferably between 200 å and 0 . 2 μm . if insulating layer 22 includes silicon carbide , bond layer 23 preferably includes polysilicon or thermal oxide . in addition to the advantages typical of soi wafers , wafer 20 also presents a distinct improvement in thermal dissipation . for example , if layer 22 is made of silicon carbide , which presents a high thermal conductivity σ t of about 250 w /( m ° k . ), of beryllium oxide ( σ t = 218 w /( m ° k . )), of aluminium nitride ( σ t = 200 w /( m ° k . )), insertion of layer 22 within wafer 20 provides for improving the overall thermal conductivity . in any case , also the use of alumina ( σ t = 17 w /( m ° k .)) or silicon nitride ( σ t = 19 w /( m ° k .) as a single crystal , σ t ≧ 3 - 5 w /( m ° k .) if obtained from high temperature cvd ) improves the overall thermal conductivity with respect to prior art soi wafers . in particular , in case of an insulating material with high conductivity , the deficiency induced by bond layer 23 may be well compensating , since the bond layer 23 , being thin , does not present a high thermal resistance anyway . adhesion of layer 22 to monocrystalline silicon layer 21 is good and , in case of an insulating layer having a coefficient of linear thermal expansion equal or similar to that of monocrystalline silicon ( roughly 2 . 5 × 10 − 6 /° c . ), low stress is generated between the two layers . in particular , this is the case for silicon carbide with a coefficient of 3 . 3 × 10 − 6 /° c . and of silicon nitride with a coefficient of 2 . 8 × 10 − 6 /° c . ( single crystal ) and of 4 × 10 − 6 /° c . ( cvd deposited ). a process for fabricating wafer 20 with a layer 22 including silicon carbide and deposited oxide will be described with reference to fig4 . the process commences with two wafers 30 , 31 of monocrystalline silicon ; wafer 30 being the one eventually constituting the bottom portion of the substrate ; and wafer 31 being the one eventually constituting the thinner top portion . then , a layer 32 of silicon carbide is grown on wafer 30 in a reaction chamber . in particular , silicon carbide layer 32 may be grown as a single crystal using the process disclosed in the article “ chemical vapor deposition of single crystalline β - sic films on silicon substrate with sputtered sic intermediate layer , s . nishina , y . hazuki , h . matsunami , t . tanaka , j . electrochem . soc . : solid state science and technology , december 1980 . in this case , the sputtered intermediate layer does not prejudice the properties of the substrate , having a small thickness . in the alternative , a silicon carbide insulating layer 22 may be grown in amorphous phase , e . g . using an epitaxial growing method , and then recrystallized to obtaining a monocrystalline layer , as disclosed for example in the article “ complete recrystallization of amorphous silicon carbide layers by ion irradiation ” v . heera , r . kögleer , w . skorupa , appl . phys . lett . 67 ( 14 ), oct . 2 , 1995 . finally , monocrystalline silicon wafer 31 is bonded to the silicon carbide layer either directly or by means of a bond layer 33 . if an oxide layer 33 is used for bonding , this is preferably in the form of a very thin layer , less than 500 å , of thermal silicon oxide . bonding of monocrystalline wafer 31 is carried out using a process similar to the one described with reference to fig2 exploiting reaction ( 1 ), and wafer 35 is obtained . alternatively , as a bond layer , a polycrystalline silicon layer may be used . in this case , a polycrystalline silicon layer 33 of a few micrometers in thickness is first deposited ; the bond layer 33 is ground and polished ( mirror machined ); monocrystalline silicon wafer 31 is bonded to the polycrystalline silicon layer using a process similar to the one described with respect to fig2 for bonding oxide layer 12 to wafer 10 , for example at a temperature of 1100 ° c . for 2 hours in a o 2 dry environment . since silicon oxide layer 33 contains bonded hydrogen , in the silicon - oxygen matrix , in the form of sioh , it is hydrophilic and , when heat treated , permits the reaction ( 1 ) indicated with reference to fig2 so as to bond monocrystalline silicon layer 31 to carbide layer 32 , and form wafer 35 which may then be ground and polished in the usual way to form wafer 20 in fig3 . wafer 35 presents extremely good thermal characteristics , by virtue of silicon carbide presenting a thermal conductivity of 350 w / m ° c . and polycrystalline silicon presenting a thermal conductivity of 50 - 85 % that of monocrystalline silicon ( roughly 75 - 125 w / m ° k .). for manufacturing a substrate with an insulating layer of silicon nitride , a similar method to the method described with reference to fig6 may be used . in particular , the silicon nitride layer 32 may be obtained by lpcvd ( low pressure cvd ) at a relatively high temperature ( roughly 900 ° c .). in the alternative , the silicon nitride layer may be obtained according to the process shown schematically in fig5 and described hereinbelow . according to the process of fig5 a monocrystalline silicon wafer 60 is thermally oxidized at 900 ° c . in an o 2 dry environment for growing a silicon oxide layer 62 of , e . g ., 280 å thickness . then a silicon nitride layer 63 is grown from a mixture of silane and ammonia , e . g . using the htcvd ( high temperature chemical vapour deposition ) method at 900 ° c . using the reaction tube and cvd system shown in fig1 of the above cited article to nishina , hazuki , matsunami and tanaka . layer 63 may have a thickness of 1400 å . then a silicon oxide ( preferably a teos oxide ) layer 64 is grown with a thickness of 500 å and the monocrystalline silicon wafer 61 , previously oxidized , is bonded . the surface oxide layer 66 covering the wafer 61 is very thin ( e . g . 50 å ), for example the native oxide may be exploited . from indirect evaluations made by the inventors , the thermal conductivity of htlpcvd deposited silicon nitride has been found to be at least 2 - 3 times the thermal conductivity of silicon oxide ; therefore cvd silicon nitride offers already an advantage with respect to prior art soi wafers . a much better improvement may be obtained by using sintered silicon nitride having a σ t of 19 w / m ° k . a fabrication process using sintered silicon nitride is shown in fig6 wherein a first monocrystalline silicon wafer 70 is firstly bonded to a sintered silicon nitride layer 72 . then layer 72 is lapped , obtaining layer 72 a with a thickness of 1 μm ; a teos silicon oxide layer 73 is deposited and monocrystalline silicon layer 71 is thermally bonded according to reaction ( 1 ). finally , layer 71 lapped to obtain layer 71 a . the above process including a sintered insulating layer may be used also in case of insulating layer 72 made of beryllium oxide , aluminium nitride and alumina . clearly , changes may be made to the wafer and fabrication process as described and illustrated herein without , however , departing from the scope of the present invention . in particular , as bond layer 23 provides solely for bonding , it may be eliminated in the event the fabrication process allows bonding monocrystalline silicon layer 21 directly to insulating layer 22 . moreover , the bond layer may be formed from any suitable bonding material , deposited or thermally grown , provided that its nature or selected thickness has substantially no effect on the thermal conductivity of the wafer . having thus described at least one illustrative embodiment of the invention , various alterations , modifications , and improvements will readily occur to those skilled in the art . such alterations , modifications , and improvements are intended to be within the spirit and scope of the invention . accordingly , the foregoing description is by way of example only and is not intended as limiting . the invention is limited only as defined in the following claims and the equivalents thereto .