referring to fig1 and 2 , an embodiment 8 of the present invention includes a gallium arsenide or gallium arsenide phosphide primary region 9 which may be comprised of both a substrate 10 and an epitaxial layer 12 grown on substrate 10 , or entirely of substrate 10 , as is shown in fig3 . epitaxial layer 12 is grown on substrate 10 in order to create a region having relatively well known composition . typically , substrate 10 is approximately 100 to 200 microns thick , and epitaxial layer 12 is approximately 10 to 50 microns thick , a nominal thickness of epitaxial layer 12 being 20 microns . light detecting zone 13 and light emitting zone 14 extend into primary region 9 from a first surface 15 of primary region 9 , zones 13 and 14 being separated from each other , typically by a distance of 5 to 10 microns . typically , zones 13 and 14 are formed of p type material , with primary region 9 being of an n type material . for n type gallium arsenide , the dopant is typically silicon or tellurium ; for p type gallium arsenide , germanium is typically used . a second surface 18 oppositely facing first surface 15 of primary region 9 is typically covered with a metalization layer such as gold , thus forming a primary region contact 11 . light detecting zone 13 should be of such a nature that whenever a junction 16 between light detecting zone 13 and primary region 9 is reverse biased , incoming radiation is absorbed substantially within an electron diffusion length of junction 16 . this can be accomplished by either making light detecting zone 13 transmissive to incoming radiation such that absorption will occur below junction 16 or by making light detecting zone 13 shallow compared to the electron diffusion length of approximately 5 microns . in the latter case , a typical depth of light detecting zone 13 is on the order of 3 microns . light detecting zone 13 typically has a doping level on the order of 10 16 atoms per cubic centimeter , a level which is generally less than the doping level of light emitting zone 14 . the doping level of light detecting zone 13 is designed to be low enough to allow a wide depletion region between p type zone 13 and n type primary region 9 for absorption of radiation . light emitting zone 14 should be of such a nature that whenever a junction 17 between light emitting zone 14 and primary region 9 is forward biased , radiation generated at junction 17 is emitted substantially at the level of first surface 15 rather than being substantially absorbed within the zone . in order to accomplish this , light emitting zone 14 is typically on the order of 5 microns deep . light emitting zone 14 has a typical doping level on the order of 10 18 atoms per cubic centimeter in order to achieve high conductivity in the zone and to achieve fast response . first surface 15 should generally be at least partially covered with a dielectric layer 20 such as silicon dioxide . a primary purpose of dielectric layer 20 is to provide electrical insulation between primary region 9 and any portion of the zone contacts ( discussed below ) which protrude beyond the area of the zones . in addition , the dielectric layer may help prevent deterioration through contamination and thus may help to prevent leaking of junctions 16 and 17 . dielectric layer 20 need not cover light detecting zone 13 and light emitting zone 14 any more than is necessary to provide appropriate electrical insulation from the primary region for the zone contacts discussed below . thus , fig3 shows a portion of the area above zones 13 and 14 being free of any dielectric layer 20 . if light detecting zone 13 and light emitting zone 14 are substantially covered with dielectric layer 20 as shown in fig2 it is necessary that at least an appropriate portion of dielectric 20 be light transmissive in order to permit incoming light to be received by light detecting zone 13 and to permit light emitting zone 14 to emit light . such a layer over zones 13 and 14 , if of appropriate thickness ( typically a quarter of a wavelength ), can help reduce surface reflection of incoming radiation . in addition , such a layer may help to prevent deterioriation . in fig2 a central portion of dielectric layer 20 is crosshatched as being a transparent material , the remaining portion being crosshatched as an electrical insulator . in order to permit electrical connections to be made to light detecting zone 13 and light emitting zone 14 , a separate contact is typically connected to each zone . a light detecting zone contact 21 typically covers a portion of dielectric layer 20 and extends into light detecting zone 13 with at least one protrusion 22 having a first surface area which is small in comparison to the first surface area of light detecting zone 13 . similarly , a light emitting zone contact 23 typically covers at least a portion of dielectric layer 20 and extends into light emitting zone 14 with at least one protrusion 24 having a first surface area which is small in comparison to the first surface area of light emitting zone 14 . light detecting zone 13 , junction 16 , primary region 9 and contacts 21 and 11 generally comprise a photodiode 27 . light emitting zone 14 , junction 17 , primary region 9 , and contacts 23 and 11 generally comprise a light emitting diode 28 . optical isolation between light detecting zone 13 and light emitting zone 14 may be provided either physically by a barrier between the zones or by electronic circuitry permitting only non - simultaneous operation of light detecting zone 13 and light emitting zone 14 . if a barrier is desired , it can comprise a groove 25 etched between the two zones and may include a material 26 different from the material of primary region 9 at least partially filling groove 25 . it should perhaps be noted that the double crosshatching used to designate material 26 in fig2 is only designating a material different from primary region 9 and is not meant to indicate any particular material or other characteristic . as previously indicated , fig3 illustrates an embodiment cross section similar to fig2 except that primary region 9 comprises substrate 10 and does not include epitaxial layer 12 . in addition , a portion of the area above zones 13 and 14 is shown free of any dielectric layer 20 , and groove 25 is shown opening to first surface 15 and being free of any material 26 . fig3 also illustrates an optical fiber 29 positioned with a port 30 in proximity to light detecting zone 13 and light emitting zone 14 . also shown is positioning means 31 for positioning optical fiber 29 and port 30 . in operation , light detecting zone 13 generates a current whenever light is received by zone 13 while junction 16 is reversed biased . reverse biasing is accomplished by applying a negative potential to light detecting zone contact 21 with primary region contact 11 connected to a reference potential such as ground . light emitting zone 14 emits light whenever junction 17 is forward biased by applying a positive potential to light emitting zone contact 23 with primary region contact 11 connected to the reference potential . a circuit 39 for operating the device is shown in fig4 . circuit 39 comprises an input terminal 40 adapted to receive two - state logic signals having first and second signal levels . input terminal 40 is connected to the input of a logic inverter 41 for converting logic highs to logic lows and vice versa . an output of inverter 41 is connected to a base 42 of a transistor 43 which acts as a control means . a collector 44 of transistor 43 is connected to a positive voltage terminal 45 through a resistor 46 as well as to light emitting zone contact 23 of light emitting diode 28 . an emitter 48 of transistor 43 is connected to a reference potential 49 . primary region contact 11 of light emitting diode 28 is connected to reference potential 49 as well as to primary region contact 11 of photodiode 27 . light detecting zone contact 21 of photodiode 27 is connected to a negative voltage terminal 47 through a resistor 50 as well as to an input 51 of an amplifier 52 . an output 53 of amplifier 52 is connected to an output terminal 54 . when utilizing the present invention , one may select whether to have light emitting diode 28 on either when data received at input terminal 40 is high or when it is low . if it is desired to have light emitting diode 28 on whenever data received at input terminal 40 is high , inverter 41 should be included in circuit 39 as previously described and shown in fig4 . if , on the other hand , it is desired that light emitting diode 28 be on whenever data received at input terminal 40 is low , inverter 41 should be eliminated from circuit 39 , and input terminal 40 should be connected directly to base 42 of transistor 43 . in operating circuit 39 either with or without inverter 41 , whenever data received at base 42 of transistor 43 is high transistor 43 turns on and acts as a switch to conduct the current derived from positive voltage terminal 45 through emitter 48 to reference potential 49 . thus , when transistor 43 is turned on by a high signal level , no voltage is received by light emitting diode 28 , thus causing light emitting diode 28 to be off . whenever the signal level at base 42 of transistor 43 is low ( and is thereby below a predetermined threshold ) transistor 43 remains off , thus transmitting no current to reference potential 49 and permitting light emitting diode 28 to be on , diode 28 being forward biased by the voltage derived from positive voltage terminal 45 . photodiode 27 is reverse biased by a negative voltage derived from negative voltage terminal 47 . whenever light is received by photodiode .[. 18 .]. . iadd . 27 . iaddend . while it is reversed biased , the change in output from photodiode 27 is amplified by amplifier 52 and detected at output terminal 54 .