embodiments of the invention will now be described with reference to the accompanying drawings . in fig1 the constant current circuit according to the first embodiment comprises a light receiving or photovoltaic device 12 , a constant current circuit 14 , and a control circuit 16 . the resistor 20 ′ of the constant current circuit , however , is different from the conventional one in fig9 as follows . that is , it has a resistance characteristic that the current value increases with the light amount received . in fig2 the resistor 20 ′ has an opening 37 in the light shielding film for receiving light that strikes the photovoltaic device 12 , too . transistor circuits and photodiodes are represented by tr and rounded squares , respectively . the resistor 20 ′, which has such a pattern as shown in fig2 is made by a process such as shown in fig6 ( a ) through ( c ). the semiconductor resistor element is provided on one of the n - type monolithic islands 34 formed on a dielectric partition board 35 . a p - type semiconductor layer 36 and an n - type semiconductor layer 33 having a density higher than that of the board are formed by diffusion process on the monolithic island 34 , respectively . then , an insulation film 32 is coated and a light shielding film 31 is formed thereon with aluminum . the p - type semiconductor 36 constitutes the resistor 20 ′. the n - type semiconductor layer 33 is provided to not only prevent current leak through the surface of the n - type monolithic island 34 but also conduct current to the p - type semiconductor layer 36 . in this resistor 20 ′, the n - p - n junction can form a parasitic phototransistor . to prevent it , the light shielding film 31 is provided over the entire surface of the resistor 20 ′ to prevent incident light to the p - n - p junction . the p - type semiconductor layer 36 is formed in a zigzag fashion to increase the distance between the terminals and thus the resistance . according to the invention , the opening 37 is provided in the light shielding film 31 and the insulation film 32 to expose the p - type semiconductor layer 36 so that it can receive light . when light strikes the opening 37 , the junction between the p - type semiconductor layer 36 and the n - type monolithic island 34 operates as a parasitic phototransistor . this diffusion resistance allows the current flow to increase with the amount of light received in the normal range of usage . the operation of mosfets 18 - 1 and 18 - 2 that are connected to the semiconductor resistor element will be described below . in fig1 when light strikes the light receiving device 12 , each light receiving diode 12 produces an electromotive force of approximately 0 . 6 v so that a voltage of about 5 v is established across the terminals 2 and 2 ′. the electric charges produced by this voltage are accumulated in the gates of the driven transistors mosfets 18 - 1 and 18 - 2 to turn them on . when the p - type semiconductor layer 36 receives the light , it works as not only a resistor but also a parasitic phototransistor in cooperation with the n - type semiconductors 33 and 34 . when there is no light illumination , there is no electromotive force so that the electric charges stored in the gates of mosfets 18 - 1 and 18 - 2 are discharged through the p - type semiconductor layer 36 to turn them off . by providing the opening 37 in the light shielding film 31 it is possible to form a parasitic phototransistor in the region of the opening 37 so that the number of parasitic phototransistors is increased so as to change the resistance characteristic . the input current vs . turn - on time characteristic of the photocoupler semiconductor device according to the invention is shown in fig3 . when the input current increases , the turn - on time decreases more than the conventional device that is represented by dotted line . in other words , the turn - on time can be selected in a range wider than that of the conventional one with respect to the input current . in operation , when electric current flows through the light emitting element 10 , such as a light emitting diode , the light emitting element 10 gives off light and the light receiving device 12 , such as photodiodes , generates an electromotive force to apply a voltage to the gates of mosfets 18 - 1 and 18 - 2 to turn them on . the light from the light emitting element 10 strikes the resistor 20 ′, too . when the light is weak , the resistance value of the resistor 20 ′ is little changed but as the light increases , the parasitic phototransistor operates to reduce the resistance value . the constant current of the constant current circuit 14 is determined by this resistor value so that the larger the received light amount , the higher the electric current . consequently , at a time of small light amounts , the resistance is high to provide a low level of current , thus lengthening the turn - on time . as the received light amount increases , the current value increases to shorten the turn - on time . the turn - on time dependency on the input power is shown in fig3 in comparison with the conventional one . as shown in fig3 the turn - on time varies widely with the input current so that it is possible to control the turn - on time to be approximately 10 ms at approximately 1 ma of the led current and below 2 ms at 10 ma or more of the led current regardless of the led power . according to the second embodiment , two or more openings are provided so as to increase the led current dependency or the received light amount . the circuit pattern according to the second embodiment is shown in fig4 wherein two openings 37 are provided in the light shielding film 31 that covers the p - type semiconductor layer 36 . alternatively , three or more openings 37 may be provided . in operation , electric current makes the led 10 emit light , the photodiodes 12 generate the output corresponding to the received light amount and the resistor 20 ′ has the resistant value corresponding to the light amount to the led 10 . as described above , the constant current circuit 14 with the resistor 20 ′ supplies a constant current to mosfets 18 - 1 and 18 - 2 to turn them on . since there are two openings 37 in the light shielding film 31 , such parasitic phototransistor effects as described above are produced at two places to quickly shorten the turn - on time with the increased light amount received . the input current vs . turn - on time characteristic of the second embodiment is shown in fig5 . as the input current increases , the turn - on time decreases to a large extent . the turn - on time is approx . 10 ms at 1 ma of the led current and 1 ms or lower at 10 ma or more of the led current . alternatively , the opening 37 may take any shape as well as square . the constant current circuit 14 may be any circuit capable of providing a constant current by means of the resistor that its resistant value increases with the received light amount . as has been described above , according to the invention , it is possible to shorten the turn - on time of a photocoupler semiconductor device to a large extent as the input current increases . thus , it is possible to provide the characteristic that the turn - on time varies widely with the input current .