referring to fig1 of the drawing , a substrate 10 of a semi - insulating material has a first surface 12 and a second surface 14 . the substrate 10 is preferably formed of gaas , but the invention is not so limited , and may be practiced using other semi - insulating substrates such as indium phosphide ( inp ) or indium gallium arsenide ( ingaas ). the present method may further be practiced using semi - insulating layers or areas formed on insulating substrates , or silicon - on - insulator ( soi ) structures such as silicon - on - sapphire ( sos ) wafers , although not specifically illustrated . in the first step of the process illustrated in fig1 first microelectronic devices 16 are formed in the first surface 12 of the substrate 10 using selective ion implantation . the first devices 16 are laterally spaced from each other , may be similar or different as desired , and may be selected from a wide range of field effect transistor , optoelectronic , or other devices depending on the particular application . the devices 16 are buried under the first surface 12 , thereby maintaining the surface 12 in a planar state which is desirable for subsequent processing . selective ion implantation technology suitable for forming the devices is known in the art per se , such as described in u . s . pat . no . 4 , 396 , 437 , entitled &# 34 ; selective encapsulation , controlled atmosphere annealing for iii - v semiconductor device fabrication &# 34 ;, issued aug . 2 , 1983 , to s . kwok et al , and assigned to the hughes aircraft company , the assignee of the present invention . typically , the substrate 10 will be about 250 micrometers thick . the ion species used is preferably silicon , implanted to a concentration of on the order of 1 to 4 × 10 17 ions / cm 3 and a depth of about 0 . 07 to 0 . 12 micrometers , at a power level of about 70 to 120 kev . subsequent to implantation , the devices 16 are activated by annealing . the annealing may be performed at a temperature higher than the dissociation temperature of gaas under an arsine overpressure as described in the above referenced patent to kwok et al , and also in u . s . pat . no . 4 , 473 , 929 , entitled &# 34 ; process for fabricating gaas fet with ion implanted channel layer &# 34 ;, issued oct . 2 , 1984 , to m . feng et al , and assigned to the hughes . aircraft company , the assignee of the present invention . as illustrated in fig2 a first epitaxial layer structure 18 and a second epitaxial layer structure 20 are formed on the second surface 14 of the substrate 10 , preferably by selective low pressure omcvd . the layer structures 18 and 20 may include one or more layers of gaas , algaas , or other suitable materials , with individual second microelectronic devices 22 and 24 formed therein respectively . where the devices 16 are optoelectronic devices such as phototransistors , the devices 22 formed in the first layer structure 18 are preferably low noise signal processing devices , and the devices 24 formed in the second layer structure 20 are power devices for processing signals associated with the optoelectronic devices . the devices 22 and / or 24 may be hemts or hbts , which require epitaxial layer structures and cannot be fabricated by ion implantation . typically , the layer structures 18 and 20 will be on the order of 0 . 5 to 2 micrometers thick , although the invention is not so limited . selective epitaxy deposition suitable for forming the layer structures 18 and 20 is known in the art per se , such as described in an article entitled &# 34 ; selective epitaxy in the conventional metalorganic phase epitaxy of gaas &# 34 ;, by t . kuech et al , in applied physics letters , vol . 54 , no . 10 , mar . 6 , 1989 , pp . 910 to 912 . a first etch stop layer 26 may be formed on the surface 14 under the first layer structure 18 , and a second etch stop layer 28 may be formed between the layer structures 18 and 20 . the etch stop layers 26 and 28 are formed of a material such as algaas which resists etching in subsequent processing steps , and are typically on the order of 500 to 2 , 000 angstroms thick . in the next step of the method as illustrated in fig3 the second layer structure 20 is etched away from the first layer structure 18 over a selected area 18a of the layer structure 18 . it will be understood that although only one area 18a is shown in the drawing , an actual structure fabricated using the present method will typically include a large number of such areas 18a . the preferred technology for performing the etching illustrated in fig3 is reactive ion etching such as described in an article entitled &# 34 ; use of thin algaas and ingaas stop - etch layers for reactive ion etch processing of iii - iv compound semiconductor devices &# 34 ;, by c . cooper et al , in applied physics letters , vol . 51 , no . 26 , dec . 28 , 1987 , pp 2225 - 2226 . however , it is within the scope of the invention to perform the etching illustrated in fig3 using other methods , such as conventional masking and chemical etching . fig4 illustrates the results of additional steps performed to complete a microelectronic device structure 30 embodying the present invention . the etch stop layer 28 is stripped away from the first layer structure 18 over the area 18a , and a protective dielectric overlay layer 32 of silicon dioxide ( sio 2 ) or silicon nitride ( si 3 n 4 ) is formed over the exposed areas of the layer structures 18 and 20 . vias 34 are formed through the substrate 10 from the first surface 12 to the second surface 14 thereof using reactive ion etching to form holes , followed by electroplating to fill or at least coat the walls of the holes with an electrically and thermally conductive metal . the vias 34 may extend perpendicular to the surface 12 , or at angles thereto as illustrated . the etch stop layer 26 prevents the via etching to affect the epitaxial layer structures 18 and 20 . due to the vertical integration of devices in the structure 30 , the substrate 10 may be made thicker than with conventional gaas technology , and the vias 34 may also be larger to improve power dissipation without excessively weakening the substrate 10 . the vias 34 may typically have a cross - sectional area on the order of 3 , 600 to 37 , 500 micrometers 2 . as further illustrated in fig4 backside metallizations 36 are formed of an electrically conductive metal such as gold ( au ), platinum ( pt ), aluminum ( al ), etc . on the first surface 12 of the substrate 10 to provide a ground plane and heat sink for heat generated during operation of the devices 16 , 22 and 24 . the vias 34 aid in conducting heat from the devices 22 and 24 through the substrate 10 to the metallizations 36 . where the devices 16 are optoelectronic devices such as an array of phototransistors , the devices 16 may be exposed to an optical beam or signal as indicated by arrows 38 . electrical signals generated by the devices 16 may be transmitted to the devices 22 through the material of the substrate 10 , through vias ( not shown ) which interconnect the devices 16 and 22 , or through metallizations ( not shown ) which wrap around the edges of the substrate 10 . fig5 and 6 illustrate the fabrication of a second microelectronic device structure according to the present invention . subsequent to fabrication of the first devices 16 the surface 12 by ion implantation as illustrated in fig1 second microelectronic devices 38 are fabricated on the surface 14 by selective ion implantation in the same manner as the first devices 16 . the devices 16 and 38 are preferably annealed simultaneously in a unitary process step . then , third microelectronic devices 40 are formed on the surface 14 using selective epitaxy . the devices 40 typically have an epitaxial layer structure similar to the layer structures 18 and 20 illustrated in fig2 to 4 . where the first devices 16 are optoelectronic devices , the second devices 38 will typically be low noise and power mesfets , whereas the devices 40 will be bipolar devices such as hemts or hbts . a complete microelectronic device structure 42 as illustrated in fig6 is provided by forming a dielectric overlay layer 44 over the surface 14 , vias 46 through the substrate 10 , and metallizations 48 on the surface 12 . the structure 42 features three microelectronic device structures formed in three vertically stacked layers respectively , which are spaced from each other in a direction perpendicular to the surface 12 . both surfaces 12 and 14 of the substrate and the devices 16 and 38 formed therein respectively are coplanar . fig7 to 10 illustrate the fabrication of a third microelectronic device structure according to the present invention . subsequent to the fabrication of the first devices 16 on the first surface 12 of the substrate 10 as illustrated in fig1 a dielectric layer 50 and a metallization layer 52 are formed on selected areas of the surface 12 as illustrated in fig7 . it will be noted that the substrate 10 is illustrated in an inverted position in fig7 relative to fig1 to 6 , with the surface 14 being the front or light - receiving surface . although both the dielectric layer 50 and metallization layer are illustrated in fig7 it is within the scope of the invention to provide only the dielectric layer 50 or only the metallization layer 52 . the dielectric layer 50 may be formed of sio 2 or si 3 n 4 to a thickness on the order of 900 to 1 , 300 angstroms , whereas the metallization layer 52 may be formed of au , pt , al , etc . to a thickness of about 1 , 000 to 5 , 000 angstroms . as illustrated in fig8 a buffer layer 54 of a material similar to that of the substrate 10 is formed over selected areas of the first surface 12 . where the substrate 10 is formed of gaas , the buffer layer 54 will also be formed of gaas . the buffer layer 54 is preferably formed using selective epitaxy onto the exposed areas 56 of the surface 12 , followed by lateral epitaxial overgrowth deposition extending laterally over the adjacent surfaces of the dielectric and metallization layers 50 and 52 . the exposed areas 56 of the substrate 10 provide a nucleation seed for monocrystalline growth of gaas from the surface 12 . a technology suitable for fabricating the buffer layer 54 is described in an article entitled &# 34 ; lateral epitaxial overgrowth of gaas by organometallic chemical vapor deposition &# 34 ;, by r . gale et al , in applied physics letters , vol . 41 , no . 6 , sep . 15 , 1982 , pp . 545 - 547 . the buffer layer 54 provides a second semi - insulating layer in which microelectronic devices can be fabricated . as illustrated in fig9 second microelectronic devices 58 , 60 and 62 are formed on the exposed surface of the buffer layer 54 using selective ion implantation . the devices 58 are formed over the dielectric layers 50 , the devices 60 are formed over the exposed areas 56 of the surface 12 , whereas the devices 62 are formed over the metallization layer 52 . the devices 58 , 60 and 62 may be of any desired mesfet type , including low noise and power fets . although fig9 illustrates a structure including all of the devices 58 , 60 , and 62 , it is within the scope of the invention to fabricate only the devices 58 , only the devices 60 , only the devices 62 , or combinations of two thereof . dielectric layer 50 provides isolation between the devices 16 and 58 , whereas the metallization 52 provides interconnection as desired . fig1 illustrates a complete microelectronic device structure 64 including a dielectric overlay layer 66 , electrically conductive vias 68 , and metallizations 70 . in addition , hollow vias 72 may be formed through the substrate 10 to enable an optical signal to be incident on the devices 16 as indicated by arrows 74 . although not illustrated , the substrate 10 may be thinned from the surface 14 thereof to about 100 micrometers subsequent to formation of the overlay layer 66 , and prior to formation of the metallizations 70 . fig1 and 12 illustrate the fabrication of a fourth microelectronic device structure according to the present invention . subsequent to fabrication of the structure as illustrated in fig8 second microelectronic devices 76 , 78 , 80 and 82 are formed on the buffer layer 54 as epitaxial layer structures . the devices 76 are formed over the dielectric layer 50 , the devices 78 are formed over the exposed areas 56 of the surface 12 , the devices are formed over the metallization layer 52 , and the devices 82 span two or all of the layers 50 , 52 and exposed areas 56 . the devices 76 , 78 , 80 and 82 may be of any desired type , including hemts and hbts . although fig1 illustrates a structure including all of the devices 76 , 78 , 80 and 82 , it is within the scope of the invention to fabricate only the devices 76 , only the devices 78 , only the devices 80 , only the devices 82 , or combinations of two or three thereof . fig1 illustrates a complete microelectronic device structure 84 including a dielectric overlay layer 86 , electrically conductive vias 88 , and metallizations 90 . hollow vias 92 may be formed through the substrate 10 to enable an optical pattern to be incident on the devices 16 as indicated by arrows 94 . the substrate 10 may be thinned from the surface 14 thereof to about 100 micrometers subsequent to formation of the overlay layer 86 , and prior to formation of the metallizations 90 , as with the structure 64 of fig1 . while several illustrative embodiments of the invention have been shown and described , numerous variations and alternate embodiments will occur to those skilled in the art , without departing from the spirit and scope of the invention . accordingly , it is intended that the present invention not be limited solely to the specifically described illustrative embodiments . various modifications are contemplated and can be made without departing from the spirit and scope of the invention as defined by the appended claims .