having generally described the present invention , a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for purposes of illustration only and are not intended to limit the scope of the appended claims . fig1 illustrates a schematic cross sectional view of a construction of a thin film el element of a preferred embodiment according to the present invention . the thin film el element comprises a glass substrate 1 , an ito film formed and stacked on a surface of the glass substrate 1 as a transparent electrode 2 , an insulator layer 3 formed and stacked on a surface of the transparent electrode 2 , a luminous layer 4 formed and stacked on a surface of the insulator layer 3 , an insulator layer 5 formed and stacked on a surface of the luminous layer 4 , and an aluminum electrode 6 formed and stacked on a surface of the insulator layer 5 so as to be a topmost surface thereof . the insulator layers 3 and 4 are formed of a composite oxide insulator film including ta 2 o 5 and either y 2 o 3 or wo 3 . the luminous layer 4 is formed of zinc sulfide ( zns ) doped with either terbium ( tb ) emitting green light or praseodymium ( pr ) emitting white light . first of all , the insulator layers of the preferred embodiments according to the present invention were examined for their electric characteristics , e . g ., dielectric breakdown fields , relative dielectric constants and figures of merit . the following six insulator films were formed on si substrates , and al was further vacuum - deposited as an electrode on the following six insulator films : a simple ta 2 o 5 insulator film including y 2 o 3 in an amount of 0 atomic %, four types of the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 in yttrium amounts of 8 . 2 atomic %, 13 . 7 atomic %, 19 . 9 atomic % and 32 . 0 atomic % with respect to the sum of tantalum and yttrium therein taken as 100 atomic %, and a simple y 2 o 3 insulator film including y 2 o 3 in an amount of 100 atomic %. the insulator films were formed with a sputtering apparatus , and the sputtering was carried out under the following conditions : each of y 2 o 3 and ta 2 o 5 oxide sintered targets was employed . the sputtering atmosphere was argon including oxygen in an amount of 30 %. the si substrates were kept at a temperature of 300 ° c . during sputtering . the insulator films were formed in a thickness of 200 angstroms ( i . e ., 0 . 02 micrometers ). the contents of y 2 o 3 in the resulting insulator films were analyzed quantitatively by rutherford back scattering analysis . fig2 is a scatter diagram which illustrates the relationships between the yttrium amounts of the simple ta 2 o 5 insulator film free from y 2 o 3 , the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 , and simple the y 2 o 3 insulator film free from ta 2 o 5 , and the dielectric breakdown fields e bd thereof . in fig2 the ordinate expresses the dielectric breakdown field e bd ( in mv / cm ) of the insulator films , and the abscissa expresses the atomic % of yttrium in the insulator films . hereinafter , an atomic % of yttrium in an insulator film means the atomic % of yttrium elements in the insulator film when the sum of tantalum elements and yttrium elements therein is taken as 100 atomic %. as can be appreciated from fig2 the simple ta 2 o 5 insulator film including tantalum elements in the amount of 100 atomic % exhibited a dielectric breakdown field in a range of 1 . 5 to 1 . 9 mv / cm . on the other hand , the present composite oxide insulator films , including yttrium in the amount of 8 to 30 atomic % with respect to the sum of tantalum and yttrium therein taken as 100 atomic %, exhibited the dielectric breakdown fields higher than 1 . 9 mv / cm which is illustrated with a horizontal broken line in fig2 and which was exhibited by the simple ta 2 o 5 insulator film . specifically , the dielectric breakdown fields exhibited by the present composite oxide insulator films , including yttrium in the above - described amount with respect to the sum of tantalum and yttrium therein taken as 100 atomic % were approximately twice that of the simple ta 2 o 5 insulator film . further , fig4 is a scatter diagram which illustrates the relationships between the compositions of the same insulator films ( i . e ., the simple ta 2 o 5 insulator film free from y 2 o 3 , the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 , and the simple y 2 o 3 insulator film free from ta 2 o 5 ) and the relative dielectric constants &# 34 ; ξ r &# 34 ; thereof . in fig4 the ordinate expresses the relative dielectric constant &# 34 ; ξ r &# 34 ; of the insulator films , and the abscissa expresses the atomic % of yttrium in the insulator films . as can be noted from fig4 the relative dielectric constant &# 34 ; ξ r &# 34 ; tended to decrease gradually as the yttrium amount increased , but the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 exhibited relative dielectric constants &# 34 ; ξ r &# 34 ; of approximately at least 16 . furthermore , fig6 is a scatter diagram which illustrates the relationships between the compositions of the same insulator films ( i . e ., the simple ta 2 o 5 insulator film free from y 2 o 3 , the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 , and the simple y 2 o 3 insulator film free from ta 2 o 5 ) and the figures of merit ( i . e ., the products of the relative dielectric constants and the dielectric breakdown fields ) thereof . in fig6 the ordinate expresses the figure of merit , and the abscissa expresses the atomic % of yttrium in the insulator films . as can be understood from fig6 the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 exhibited figures of merit which increased as the yttrium amount increased . hence , when taking both of the evaluation results illustrated in fig4 and 6 into consideration , it is preferable to add y 2 o 3 into the simple ta 2 o 5 insulator film in 8 to 30 atomic % yttrium with respect to the sum of tantalum and yttrium therein taken as 100 atomic %. in addition , instead of the present composite oxide insulator films including ta 2 o 5 and y 2 o 3 , four other types of present composite oxide insulator films including ta 2 o 5 and wo 3 in tungsten amounts of 2 . 3 atomic %, 3 . 6 atomic %, 15 . 8 atomic % and 25 . 1 atomic % with respect to the sum of tantalum and tungsten therein taken as 100 atomic % on the same si substrates under the same conditions , and al was further vacuum - deposited as an electrode on the insulator films . the insulator films thus sandwiched between the si substrates and the al electrodes were also evaluated for their dielectric breakdown electric fields , relative dielectric constants and figures of merit . the results of the evaluations were illustrated in fig3 and 7 , respectively . fig3 is also a scatter diagram which illustrates the relationships between the tungsten amounts of the simple ta 2 o 5 insulator film free from wo 3 and the present composite oxide insulator films including ta 2 o 5 and wo 3 , and the dielectric breakdown fields e bd thereof . also , in fig3 the ordinate expresses the dielectric breakdown field e bd ( in mv / cm ) of the insulator films , and the abscissa expresses the atomic % of tungsten in the dielectric films . as can be appreciated from fig3 when wo 3 was added into the simple ta 2 o 5 insulator film in an amount of 2 to 4 atomic % tungsten with respect to the sum of tantalum and tungsten therein taken as 100 atomic %, the present composite oxide insulator films including ta 2 o 5 and wo 3 exhibited dielectric breakdown fields which were improved to approximately twice or more that of the simple ta 2 o 5 insulator film free from wo 3 . however , when wo 3 was added into the simple ta 2 o 5 film in an amount of tungsten more than 4 atomic %, the present composite oxide insulator films including ta 2 o 5 and wo 3 exhibited the dielectric breakdown fields which were deteriorated sharply . for instance , when wo 3 was added into the simple ta 2 o 5 film in an amount of 25 . 1 atomic % tungsten , the present composite oxide insulator film including ta 2 o 5 and wo 3 exhibited dielectric breakdown fields which were deteriorated below that of the simple ta 2 o 5 insulator film and which is illustrated with a horizontal broken line in fig3 . contrary to the evaluation results on the relative dielectric constants of the present composite oxide insulator films of ta 2 o 5 and y 2 o 3 illustrated in fig4 the present composite oxide insulator films of ta 2 o 5 and wo 3 exhibited higher relative dielectric constants than the simple ta 2 o 5 insulator film did as illustrated in fig5 . also , in fig5 the ordinate expresses the relative dielectric constant &# 34 ; ξ r &# 34 ; of the insulator films , and the abscissa expresses the atomic % of tungsten in the insulator films . as can be seen from fig7 which illustrates the figures of merit of the present composite oxide insulator films of ta 2 o 5 and wo 3 , when wo 3 was added into the simple ta 2 o 5 insulator film in an amount of 2 to 4 atomic % tungsten with respect to the sum of tantalum and tungsten therein taken as 100 atomic %, the present composite oxide insulator films including ta 2 o 5 and wo 3 exhibited figures of merit which were twice that of the simple ta 2 o 5 insulator film free from wo 3 . also , in fig7 the ordinate expresses the figure of merit , and the abscissa expresses the atomic % of yttrium in the insulator films . hence , when taking all of the evaluation results illustrated in fig3 and 7 into consideration , it is preferable to add wo 3 into the simple ta 2 o 5 insulator film in amount of 2 to 4 atomic % tungsten with respect to the sum of tantalum and tungsten therein taken as 100 atomic %. moreover , the following five insulator films , e . g ., the present composite oxide insulator film including ta 2 o 5 and y 2 o 3 with yttrium in an amount of 19 . 9 atomic % with respect to the sum of tantalum and yttrium therein taken as 100 atomic %, the present composite oxide insulator film including ta 2 o 5 and wo 3 with tungsten in an amount of 3 . 6 atomic % with respect to the sum of tantalum and tungsten therein taken as 100 atomic %, the simple ta 2 o 5 insulator film , the simple y 2 o 3 insulator film , and a simple sio 2 insulator film , were evaluated for their maximum accumulable electric charges . the results of this evaluation are set forth in table 1 below together with their dielectric electric breakdown fields and relative dielectric constants . the following fact is apparent from table 1 : the present composite oxide insulator film including ta 2 o 5 and y 2 o 3 and the present composite oxide insulator film including ta 2 o 5 and wo 3 exhibited figures of merit of 84 microcoulomb / cm 2 which were higher than those of the other insulator films . thus , the present composite oxide insulator films exhibited remarkably high figures of merit on which the functions of the el element depend . therefore , it is very advantageous to add either y 2 o 3 or wo 3 into the simple ta 2 o 5 insulator film in order to make a high performance el element . table 1______________________________________ figure of dielectric merit relative breakdown ( microcoulomb dielectric fieldcomposition / cm . sup . 2 ) constant ( mv / cm ) ______________________________________ta . sub . 2 o . sub . 5 + y . sub . 2 o . sub . 3 84 21 4ta . sub . 2 o . sub . 5 + wo . sub . 3 84 21 4ta . sub . 2 o . sub . 5 84 23 2y . sub . 2 o . sub . 3 12 12 1sio . sub . 2 32 4 8______________________________________ note : thicknesses of the insulator films were 0 . 08 micrometers ( i . e ., 800 angstroms ). a thin film el element of a preferred embodiment ( hereinafter simply referred to as a &# 34 ; present el element &# 34 ;) was manufactured by using the composite oxide insulator film including ta 2 o 5 and y 2 o 3 with an amount of 19 . 9 atomic % yttrium with respect to the sum of tantalum and yttrium therein taken as 100 atomic %. namely , as illustrated in fig1 an ito transparent electrode 2 was formed on a glass substrate in a thickness of 0 . 2 micrometers ( i . e ., 2000 angstroms ), an insulator layer 3 made of the present composite oxide insulator film including ta 2 o 5 and y 2 o 3 was formed by sputtering in a thickness of about 0 . 3 micrometers ( i . e ., 3000 angstroms ). in the sputtering operation , y 2 o 3 and ta 2 o 5 oxide sintered targets were employed , and the insulator layer 3 was formed with a radio frequency magnetron binary multiple sputtering apparatus . further , wattages applied to the sputtering apparatus were controlled so that a ratio of y 2 o 3 with respect to the sum of ta 2 o 5 and y 2 o 3 is about 20 % in an atom - number ratio in the insulator layer 3 . furthermore , since oxygen is insufficiently taken into the insulator layer 3 made of the composite oxide insulator during the formation thereof , oxygen was added into an argon gas in an amount of 30 % in order to compensate the oxygen insufficiency . moreover , the glass substrate 1 was kept at a temperature of 200 ° c . during the sputtering operation . after completing the formation of the insulator layer 3 , a luminous layer 4 including zns doped with tb emitting green light was formed on the insulator layer 3 in a thickness of about 0 . 5 micrometers ( i . e ., 5000 angstroms ) in an argon gas atmosphere while keeping the temperature of the glass substrate 1 at 200 ° c . as aforementioned , instead of zns doped with tb emitting green light , the luminous layer 4 may include zns doped with pr emitting white light . thereafter , an insulator layer 5 was formed on the luminous layer 4 in the same manner as that of insulator layer 3 . finally , a topmost aluminum electrode 6 was formed on the insulator layer 5 by vacuum - depositing al in a thickness of about 0 . 3 micrometers ( i . e ., 3000 angstroms ). the present el element was thus prepared . for a comparison , a comparative el element was manufactured by forming insulator layers 3 &# 39 ; and 5 &# 39 ; including ta 2 o 5 but being free from y 2 o 3 . other than the compositions of the insulator layers 3 &# 39 ; and 5 &# 39 ;, the comparative el element had the same construction as that of the present el element . the present el element includes luminous layer 4 which is a semiconductor containing a compound including at least a combination of a group ii element and a group vi element of the periodic table . however , the present el element is a transparent thin film which exhibits a broad band gap , and accordingly it can be regarded as an equivalent circuit , to which a capacitor film is connected in series , provided that a voltage of a light - emitting threshold voltage or less is applied in an electric field required for light - emitting or less . further , the present el element emits light when a predetermined electric field , e . g ., an electric field of 1 to 2 mv / cm , which enables the electrons , emitted from the interface levels between the luminous layer 4 and the insulator layers 3 and 5 , to fully excite the luminous centers . in the meantime , an electric current flows in the luminous layer 4 , and accordingly the present el element can be regarded as an equivalent circuit , to which a resistance is connected in parallel . as a result , the electric field applied to the luminous layer 4 is clamped . when the present el element emits light , insulator layers 3 and 5 make the electric current exciting the luminous layer 4 alternately flow stably , thereby regulating the flow direction of the electric current in luminous layer 4 . as a result , insulator layers 3 and 5 stabilize the present el element . under voltage application of the light - emitting threshold voltage or less , let an electrostatic capacity of the luminous layer 4 be &# 34 ; c z ,&# 34 ; an electrostatic capacity of the insulator layers 3 and 5 be &# 34 ; c i ,&# 34 ; and a voltage applied to the entire present el element be &# 34 ; v tot .&# 34 ; the voltage &# 34 ; v i &# 34 ; applied to the luminous layer 4 can be expressed by the following equation : therefore , it is necessary to employ a material which has a high electrostatic capacity , i . e ., which exhibits a large relative dielectric constant , for insulator layers 3 and 5 in order to apply the predetermined voltage to the luminous layer 4 effectively and in order to avoid applying an ineffective voltage to the insulator layers 3 and 5 . when a voltage of the light - emitting threshold voltage or more is applied to the present el element , the voltage applied to the luminous layer 4 is clamped at a predetermined value . accordingly , all of the remaining voltage comes to be applied to the insulator layers 3 and 5 . hence , it is necessary to employ a material which exhibits a large dielectric breakdown electric field for the insulator layers 3 and 5 . the light - emitting luminance of the present el element depends on the product of the light - emitting efficiency , the number of the luminous centers and the number of the electrons required for the excitation . let the characteristics of the luminous layer 4 be the same , the light - emitting efficiency is equivalent to the number of the luminous centers , and accordingly the light - emitting luminance of the present el element depends on the number of the exciting electrons required . provided that a light - emitting threshold voltage or less is applied in an electric field required for light - emitting or less , the present el element can be regarded as an equivalent circuit , to which a capacitor film is connected in series . consequently , the number of the electrons required for the excitation is equivalent to the maximum accumulable electric charges of the insulator layers 3 and 5 . thus , it is necessary to employ a material which exhibits a large maximum accumulable electric charge for the insulator layers 3 and 5 in order to carry out the el light - emission at a high luminance . as set forth in table 1 above , the present composite oxide insulator films including ta 2 o 5 and either y 2 o 3 or wo 3 has superior characteristics to those of other simple insulator films . fig8 illustrates light - emitting luminance - applied voltage characteristic curves which were exhibited by the present and the comparative el elements . as described above , the present el element employed insulator layers 3 and 5 made of composite oxide insulator films including ta 2 o 5 and y 2 o 3 , and the comparative el element employed the insulator layers 3 &# 39 ; and 5 &# 39 ; made of the simple ta 2 o 5 insulator film free from any additive . this is why the present el element exhibited the threshold value voltages which did not vary as much as the comparative el element did as illustrated in fig8 . although the light - emitting luminance - applied voltage characteristic curves of the other comparative el elements which employed the insulator layers 3 &# 39 ; and 5 &# 39 ; made of the simple sio 2 , si 3 n 4 and y 2 o 3 insulator films free from any additive are not illustrated in fig8 the present el element exhibited lower threshold value voltages than these comparative el elements did . further , since the present el element exhibited a higher element - breakdown voltage as well as a higher maximum luminance than the comparative el elements did , the present el element could carry out el light - emission more stably in a wider voltage application range . furthermore , since the insulator layers 3 and 5 of the present el element were made of the composite oxide insulator film concluding ta 2 o 5 and y 2 o 3 which exhibited the large relative dielectric constant , the large dielectric breakdown electric field and the large maximum accumulable electric charge , the present el element could not only exhibit the high luminance and the low threshold voltages , but also operate in a wide operating voltage range satisfactorily . the luminous layers 4 of the present and the comparative el elements were made of material which is likely to absorb moisture in the atmosphere , and accordingly the breakdown of the el elements often results from the coming - off at the interfaces between the luminous layers 4 and the insulator layers 3 and 5 ( or 3 &# 39 ; and 5 &# 39 ;). hence , the lives and the stabilities of the el elements are believed to depend on the adhesion between the luminous layer 4 and the insulator layers 3 and 5 ( or 3 &# 39 ; and 5 &# 39 ;). if the breakdown of one pixel does not result in the breakdown of other pixels , the reliabilities and the lives of the entire el element can be upgraded . although the film forming processes and the film forming conditions vary the adhesion between the luminous layer 4 and the insulator layers 3 and 5 , it is believed that the adhesion depends on portions of the luminous layer 4 and the insulator layers 3 and 5 in which the atoms are bonded chemically . therefore , it is necessary to employ a material which exhibits good adhesion to the luminous layer 4 for the insulator layers 3 and 5 . table 2 below sets forth the evaluation results on the adhesion between the luminous layer 4 and the insulator layers 3 and 5 of the present el element and those of the comparative el elements . the evaluation was carried out by subjecting the el elements to a so - called peeling test . in the peeling test , a scotch tape ( registered trade mark ) was first applied on the insulator layer 5 or 5 &# 39 ;. then , the tape was peeled off , and the insulator layers 5 or 5 &# 39 ; was examined to determine whether it remained on the luminous layer 4 or not . when the insulator layer 5 &# 39 ; was made of the simple ta 2 o 5 insulator film , the insulator layer 5 &# 39 ; came off with tape easily . on the other hand , the insulator layer 5 of the present el element was made by adding y 2 o 3 , which inherently exhibits a good adhesion and a strong electronic affinity to the luminous layer 4 , into the simple ta 2 o 5 insulator film . thus , it is believed that the bonding was improved at the interface between the luminous layer 4 the insulator layer 5 of the present el element by the yttrium atoms . it goes without saying that the same evaluation results could have been obtained if the insulator layers 3 and 3 &# 39 ; were tested similarly . table 2______________________________________composition adhesion______________________________________ta . sub . 2 o . sub . 5 + y . sub . 2 o . sub . 3 goodta . sub . 2 o . sub . 5 bady . sub . 2 o . sub . 3 good______________________________________ table 3 below sets forth the breakdown modes of the present and the comparative el elements . the comparative el elements , i . e ., conventional el elements , exhibited breakdown modes of the propagation type during breakdown . however , the present el element basically exhibited the breakdown mode of the self - healing type during breakdown . in the present el element , the breakdown mode depends on the state of the interface between the ito transparent electrode 2 and the insulator layer 3 , the state of the interface between the aluminum electrode 6 and the insulator layer 5 , or the states of the interfaces between the luminous layer 4 and the insulator layers 3 and 5 . here , there have been reports that the breakdown mode also depends on insulator materials . the mechanism of the breakdown has not been explained explicitly yet ; however , roughly speaking , it is believed to occur as follows : the microstructural defects in insulator layers 3 and 5 , the vacant oxygen lattices therein and the vacant metal lattices therein trap electrons , and the trapped electrons form passages for electric currents , and breakdown eventually occurs . in the meantime , the electric currents generate joule heat . as a result , the insulator layers 3 and 5 or the electrodes 2 and 6 evaporate locally , they discharge electrically , and thereby mechanical fractures occur . thus , it is believed that the breakdown mode depends on the vapor pressures of the insulator materials , the thermal expansion coefficient differences resulting from the heat generation , etc . table 3______________________________________composition breakdown mode______________________________________ta . sub . 2 o . sub . 5 + y . sub . 2 o . sub . 3 self - healing typeta . sub . 2 o . sub . 5 propagation typey . sub . 2 o . sub . 3 self - healing typesio . sub . 2 self - healing type______________________________________ thus , in the present el element , not only the electrical properties of the simple ta 2 o 5 insulator film , but also the thermal and the mechanical properties thereof were improved by adding y 2 o 3 into the simple ta 2 o 5 insulator film . as a result , it is believed that the y 2 o 3 addition transformed one of the conventional characteristics of the simple ta 2 o 5 insulator film , e . g ., the propagation type breakdown mode , into the self - healing type breakdown mode in the present el element . in addition , there has been a problem associated with the production of conventional el elements that the resistance value of the ito transparent electrode 2 was increased by the formation of insulator layers 3 &# 39 ; and 5 &# 39 ; and the luminous layer 4 or by heat treatment adapted for improving the light - emitting efficiency of the el element after the completion thereof . namely , when the resistance value of ito transparent electrode 2 is increased , the el element emits blurred and uneven light , and the heat generation resulting from the increased resistance value accelerates the breakdown of the el element . it is believed that the resistance value of the ito transparent electrode 2 is increased by the following mechanisms : when an oxide insulator layer adjacent to the ito transparent electrode 2 is instable , the indium ( in ) of the ito transparent electrode 2 diffuses into the oxide insulator layer so that the resistance value of the ito transparent electrode 2 is increased , or the resistance value of the ito transparent electrode 2 is increased by the oxidation or the reduction reaction of the ito transparent electrode 2 . fig9 illustrates the resistance value variations of ito transparent electrodes 2 during the production processes of the present and the comparative el elements . during the production processes , the present el element hardly exhibited resistance value variation when compared with those exhibited by the comparative el elements . it is believed that there arose stable composite oxides by adding y 2 o 3 into ta 2 o 5 , and that the stable composite oxides hardly reacted with the ito transparent electrode 2 even when the present el element was subjected to heat treatments . here , let us consider the insulator film formation process . in the prior art , it is necessary to form the insulator layer , which is made of y 2 o 3 , sio 2 , etc ., and which exhibits the self - healing type breakdown mode characteristic , in the interface between the ito transparent electrode layer 2 and the conventional simple ta 2 o 5 insulator film which exhibits propagation type breakdown mode characteristics , and accordingly the conventional insulator film formation process is complicated . however , the present el element includes the insulator layers 3 and 5 which are made of the composite oxide insulator films , and accordingly the insulator film formation process can be simplified in the production process of the present el element . as has been described so far , the present el element includes insulator layers 3 and 5 which are made of composite oxide insulator film including ta 2 o 5 and y 2 o 3 , and accordingly it provides the following advantageous effects : it is improved in terms of adhesion , it is made to exhibit the self - healing type breakdown mode , and it includes the ito transparent electrode 2 whose resistance value is hardly increased . as a result , the present el element can be operated stably even when it is operated for a long period of time . these advantageous effects were verified as follows . fig1 illustrates the light - emitting variations of the present and the comparative el elements with respect to the time passage . here , the present and comparative el elements were made to include a luminous layer 4 which contained pr as the luminous centers , they were enclosed merely with an epoxy resin , and they were operated for a long period of time in the atmosphere . as can be seen from fig1 , the present thin film el element was confirmed to operate stably for a period of 2000 hours or more , and accordingly it was superior in reliability . this advantageous effect results from the fact that the predetermined voltage was applied to the luminous layer 4 so stably that the insulator layers 3 and 5 holding the luminous layer 4 therebetween were improved in terms of stability . further , one of the initial characteristics of the present el element , e . g ., the light - emitting luminance - applied voltage characteristic illustrated in fig8 depends on the maximum accumulable electric charges of the insulator layers 3 and 5 . as can be seen from fig8 the initial characteristic of the present el element was stabilized , and its life was prolonged . this advantageous effect resulted from the construction of the present el element . namely , the present el element included insulator layers 3 and 5 which satisfied the following key factors for making the present el element stable and long lived : the adhesion between the insulator layer 3 or 5 and luminous layer 4 as well as the adhesion between the insulator layer 3 or 4 and the ito transparent electrode 2 or the aluminum electrode 6 was improved , the overall construction was made to exhibit a self - healing type breakdown mode , and the ito transparent electrode 2 was made to exhibit a resistance value which hardly varied . on the other hand , although the initial characteristic was no problem in comparative el element which included a conventional simple ta 2 o 5 insulator film , the comparative el element underwent breakdown approximately 1000 hours earlier as illustrated in fig1 , and it had a short period of life . the following are believed to be the causes of the short life : the adhesion was inferior to those described above , the overall construction exhibited the propagation type breakdown mode , etc . thus , the comparative el element suffered from factors which adversely affected the reliability and the stability . having now fully described the present invention , it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims .