embodiments of the present invention are described below with reference to the drawings . fig3 is a perspective view showing the construction of the first embodiment of an optical modulator of the present invention , and fig4 a to 4d are sectional views showing different steps of production of the first embodiment of the present invention . the optical modulator shown in fig3 includes n - type inp substrate 11 , a pair of sio 2 stripe masks 12 , n - type inp clad layer 14 formed from n - type inp clad layer 141 and n - type inp clad layer 142 , undoped ingaasp light absorbing layer 15 , undoped ingaasp spacer layer 16 , p - type inp clad layer 17 , p - type inp embedded layer 19 , p - type ingaas cap layer 20 , p electrode 21 and n electrode 22 . it is to be noted that reference numeral 18 in fig3 shows an opening area . in the optical modulator of the present embodiment , the carrier concentration of n - type inp clad layer 141 exhibits a continuous variation . the structure of the optical modulator of the present embodiment is described in more detail together with the method of producing the optical modulator . first , a pair of sio 2 stripe masks 12 of sio 2 of 3 , 500 angstrom thick having opening area 13 of 1 . 5 μm wide therebetween are formed on n - type inp substrate 11 ( carrier concentration : 2 × 10 17 cm - 3 ) as shown in fig4 a by a thermal cvd method . then , n - type inp clad layer 141 of 700 angstrom thick whose carrier concentration continuously varies from 2 × 10 17 cm - 3 to 5 × 10 17 cm - 3 , n - type inp clad layer 142 of 300 angstrom thick whose carrier concentration is 5 × 10 17 cm - 3 , light absorbing layer 15 ( wavelength composition : 1 . 46 μm ) of undoped ingaasp of 2 , 500 angstrom thick , spacer layer 16 of undoped ingaasp of 500 angstrom thick and clad layer 17 of p - type inp of 1 , 000 angstrom thick whose carrier concentration is 5 × 10 17 cm - 3 are selectively and successively grown in opening area 13 as shown in fig4 b by a movpe method . clad layer 141 and clad layer 142 constitute n - type inp clad layer 14 . thereafter , sio 2 stripe masks 12 on the opposite sides of the selective growth area are removed by about 2 μm to expand the selective growth area by chemical etching to form a pair of opening areas 18 as shown in fig4 c . then , p - type inp embedded layer 19 ( carrier concentration : 5 × 10 17 cm - 3 ) is selectively grown to the thickness of 1 . 2 μm ( thickness from the top of p - type inp clad layer 17 ) and p - type ingaas cap layer 20 is selectively grown to 0 . 2 μm as shown in fig4 d both by a movpe method . the condition of a depletion layer in the proximity of a growth interface in the optical modulator having the construction described above when a reverse voltage is applied is schematically shown in fig5 . broken lines in fig5 indicate ends of the depletion layer in the individual areas . in the present embodiment , the carrier concentration of n - type inp clad layer 14 continuously increases from n - type inp substrate 11 side to ingaasp light absorbing layer 15 side . when a reverse bias voltage is applied , width r of the depletion layer at a lowermost portion of n - type inp clad layer 14 is substantially equal to width r of the depletion layer at an upper portion of n - type inp substrate 11 . therefore , the distribution of the extent of the depletion layer can be made smooth , and consequently , an otherwise possible breakdown at the growth interface between area 51 and area 54 ( refer to fig2 ) described in the prior art can be prevented and a high voltage resistance can be obtained . as a result , while the voltage resistance of the conventional optical modulator is 5 to 6 v , a high voltage resistance higher than 10 v was obtained with an optical modulator of a prototype produced in accordance with the present embodiment . meanwhile , since n - type inp clad layer 14 has a sufficiently high carrier concentration in an area thereof adjacent light absorbing layer 15 , the electric field intensity of the absorbing layer exhibited a very small amount of reduction and the extinction characteristic was not degraded . fig6 is a sectional view showing the construction of the second embodiment of an optical modulator of the present invention . the present embodiment is different from the first embodiment in that the carrier concentration of n - type inp clad layer 241 exhibits three stages of variation . for example , n - type inp clad layer 241 is formed from three stages of n - type inp clad layer 251 of a carrier concentration of 2 × 10 17 cm - 3 ( 300 angstrom thick ), n - type inp clad layer 252 of another carrier concentration of 3 × 10 17 cm - 3 ( 200 angstrom thick ) and n - type inp clad layer 253 of a further carrier concentration of 4 × 10 17 cm - 3 ( 200 angstrom thick ) so as to vary the carrier concentration stepwise . consequently , similarly to the first embodiment , the edge breakdown was successfully suppressed and a high voltage resistance was obtained . while the carrier concentration of area 141 in the first embodiment is varied continuously , similar effects were obtained also by the stepwise variation of the carrier concentration in this manner . it is to be noted that , in fig6 reference numeral 242 denotes an n - type inp clad layer whose carrier concentration is fixed , and clad layer 241 and clad layer 242 constitute n - type inp clad layer 24 . while the two embodiments described above employ an n - type substrate , similar effects can be anticipated also with devices which employ a p - type substrate . fig7 is a perspective view showing the construction of the third embodiment of an optical modulator of the present invention , and fig8 a to 8d are sectional views illustrating different steps of producing the third embodiment of the present invention . the optical modulator shown in fig7 includes n - type inp substrate 511 , a pair of sio 2 stripe masks 512 , n - type inp clad layer 514 , undoped ingaasp light absorbing layer 515 , undoped ingaasp spacer layer 516 , p - type inp clad layer 517 , p - type inp embedded layer 519 formed from p - type inp embedded layer 5191 and p - type inp embedded layer 5192 , p - type ingaas cap layer 520 , p electrode 521 and n electrode 522 . in the optical modulator of the present embodiment , the carrier concentration of p - type inp embedded layer 5191 is lower than the carrier concentration of p - type inp embedded layer 5192 . the structure of the optical modulator of the present embodiment is described in more detail with reference to fig8 a to 8d together with the method of producing the optical modulator . first , a pair of sio 2 stripe masks 512 of sio 2 of 3 , 500 angstrom thick having opening area 513 of 1 . 5 μm wide therebetween are formed on n - type inp substrate 511 ( carrier concentration : 2 × 10 17 cm - 3 ) as shown in fig8 a by a thermal cvd method . then , n - type inp clad layer 514 of 1 , 000 angstrom thick whose carrier concentration is 5 × 10 17 cm - 3 , light absorbing layer 515 ( wavelength composition : 1 . 47 μm ) of undoped ingaasp of 2 , 500 angstrom thick , spacer layer 516 of undoped ingaasp of 500 angstrom thick and p - type inp clad layer 517 of 1 , 000 angstrom thick whose carrier concentration is 5 × 10 17 cm - 3 are selectively and successively grown in opening area 513 as shown in fig8 b by a movpe method . thereafter , sio 2 stripe masks 512 on the opposite sides of the selective growth area are removed by about 2 μm by chemical etching to form a pair of opening areas 518 as shown in fig8 c . then , embedded layer 5191 ( carrier concentration : 1 × 10 17 cm - 3 ) of p - type inp of 0 . 3 μm thick ( thickness from the top of p - type inp clad layer 517 ), embedded layer 5192 ( carrier concentration : 1 × 10 18 cm - 3 ) of p - type inp of 0 . 9 μm thick and cap layer 520 of p - type ingaas of 0 . 2 μm thick are selectively and successively grown as shown in fig4 d by a movpe method . the principle of operation of the present embodiment having the construction described above is described below with reference to fig9 a and 9b . fig9 a is a sectional view showing the construction of the present embodiment , and fig9 b is a circuit diagram showing an equivalent circuit to the present embodiment . when a reverse bias voltage is applied to the entire device , photo - current i l flows through the entire circuit . in the present embodiment , the carrier concentration of p - type inp embedded layer 519 is set so that it is as low as 1 × 10 17 cm - 3 in a portion of p - type inp embedded layer 519 having a thickness within which a depletion layer extends from any of homozygous portions 523 ( embedded layer 5191 ) and is as high as 1 × 10 18 cm - 3 in an upper portion of embedded layer 519 ( embedded layer 5192 ). due to the construction , the resistance of embedded layer 519 can be reduced to reduce a voltage drop there , and consequently , the electric field applied to light absorbing layer o can be increased . as a result , significant improvement in extinction characteristic can be anticipated . in fact , while the prior art optical modulator has a extinction ratio of 7 to 8 db , a high extinction ratio of 15 db or more was obtained with an optical modulator of a prototype produced in accordance with the present embodiment . meanwhile , since the carrier concentration in the proximity of any of homozygous portions 523 is comparatively low , the extent of the depletion layer can be comparatively smooth . from this , a good result on the voltage resisting property can be anticipated . further , also capacitance c of homozygous portions 523 on the opposite sides of the double heterostructure portion can be reduced ( reduced by about 30 % compared with that of the prior art optical modulator ). consequently , the cr time constant of the entire device can be reduced to allow a high speed response . from the foregoing , according to the present invention , a device which has a high extinction ratio and operates at a high speed can be realized . fig1 is a sectional view showing the construction of the fourth embodiment of the present invention . the present embodiment is different from the third embodiment in that the carrier concentration of n - type inp clad layer 514 in the third embodiment exhibits a stepwise variation . in particular , n - type inp clad layer 814 formed in opening area 513 ( refer to fig7 ) has a layered structure of n - type inp clad layer 8141 of 700 angstrom thick whose carrier concentration continuously varies from 2 × 10 17 cm - 3 to 5 × 10 17 cm - 3 and n - type inp clad layer 8142 of 300 angstrom thick whose carrier concentration is 5 × 10 17 cm - 3 . the other construction of the present embodiment is similar to that of the third embodiment shown in fig7 and the same elements are denoted by the same reference numerals as those in fig7 and description thereof is omitted herein . the present embodiment is characterized in that the device of the third embodiment which is improved in extinction characteristic has an improved voltage resistance realized by varying the carrier concentration of n - type inp clad layer 814 stepwise . the extent of a depletion layer at a growth interface between n - type inp clad layer 814 and n - type inp substrate 511 is made smooth by the concentration gradient of the carrier concentration and a breakdown at the portion is suppressed . consequently , a device having a high voltage resistance can be realized . in fact , while the prior art optical modulator has a voltage resistance of 6 to 8 v , a high voltage resistance of 10 v or more was obtained with an optical modulator of a prototype produced in accordance with the present embodiment . meanwhile , since the carrier concentration is sufficiently high ( 5 × 10 17 cm - 3 ) in a portion of n - type inp clad layer 814 adjacent light absorbing layer 515 ( n - type inp clad layer 8142 ), the electric field intensity applied to absorbing layer 515 is not reduced . consequently , a high extinction ratio can be maintained . while the carrier concentrations are specified here , they need not be restricted particularly , and similar effects can be anticipated with any structure which provides a stepwise variation in carrier concentration as described above . further , while the embodiment here employs an n - type substrate , similar effects can be anticipated also with devices which employ a p - type substrate . since the present invention is constructed as described above , the following effects are anticipated . by varying the carrier concentration of the n - type inp clad layer continuously or stepwise , there is an effect in that the extent of a depletion layer at a growth interface can be made smooth and occurrence of a breakdown there can be prevented . further , as a result of improvement in voltage resistance , there is another effect in that a device can be realized having a high degree of reliability and in which unexpected occurrence of deterioration of the device is eliminated . by varying the carrier concentration of the p - type inp embedded layer stepwise , there is a further effect in that the voltage drop there can be reduced to increase the electric field applied to the light absorbing layer , improving the extinction characteristic . in fact , while the extinction ratio of the conventional optical modulator was 7 to 8 db , a high extinction ratio of 15 db or more was obtained with the optical modulator of the prototype produced in accordance with the present invention . by varying the carrier concentration of the n - type inp clad layer stepwise , there is a still further effect in that a device can be obtained having such a high voltage resistance that a breakdown at a growth interface between the n - type inp clad layer and the substrate can be prevented . in fact , while the resisting voltage of the conventional optical modulator was 6 to 8 v , a high voltage resistance of 10 v or more was obtained with the optical modulator of the prototype produced in accordance with the present invention . further , by using the various methods , an optical modulator having the effects described above can be produced . as apparent from the foregoing description , according to the present invention , a sufficient margin is provided to the characteristic with respect to the operating voltage and the required extinction ratio . further , according to the present invention , since also reduction of the capacity of a device can be realized , a device which is effective to a response characteristic and is high in reliability can be realized . the contents of the present invention are effective to various devices which employ selective growth technology including integrated devices with a light source laser .