Single package electro-optic transmitter-receiver

Abstract: An improved electro-optic transmitter-receiver (40,60) is provided by arranging a variable light source (54,74) and a photosensitive device (52,72) on one or more parts of a substantially flat leadframe (42,44,46;62,64,66) . Both the variable light source (54,74) and photosensitive device (52,72) face outwardly in the same direction approximately perpendicular to the plane of the package leads (42,44,46;­ 62,64,66). A transparent cover (48,68) extends over the variable light (54,74) source and the photosensitive device (52,72). A lense (49,69) is provided over the photosensitive device (52,72) that does not extend over the variable light source (54,74). When an N-substrate LED (54,74) is used it is conveniently located on another lead (42,62) to one side of the lense (49,69). It is desirable to have a recess (49′,69′) extending into the cover (48,68) between the LED (54,74) and the phototransistor (52,72) to reduce transmission of internally back-scattered light therebetween.

Description: Field of the Invention This invention concerns electro-optic transmitter-receivers and, more particularly, a single package, a series connected semiconductor electro-optic transmitter-receiver in which both emitter and receiver face outwardly through a transparent cover. Background of the Invention It is known to package semiconductor optical emitter and receiver devices in a single housing or package for use, among other things, as a combined transmitter-receiver. In a typical application, the emitter is a light emitting semiconductor diode (LED) which emits light when energised. The light from the LED travels through the transparent cover and exits the device package where it may encounter a separate receiver device or an object that returns part of the emitted light back through the transparent cover to the receiver device in the same housing. Phototransistors, photodiodes and photoresistors are well known receiver devices. It is important that the wavelength sensitivity of the receiver device be such that it can reasonably detect the light wavelength emitted by the LED or other optical emitter within the housing. Various electro-optic emitter-receivers are described in US Patents 4,309,605 to Okabe, 4,614,958 to Mikami et al., 4,521,681 to Inaba et al., and Japanese Patent Application no. JP-A-56-110273. Prior art devices suffer from a number of disadvantages, such as for example, use of complex and expensive semiconductor devices and/or housings which increase manufacturing cost and/or fail to provide the desired performance. US Patent No. 3,842,263 discloses an opto-electronic transducer comprising a molded body of material having a pair of depressions in which a light source and photoresponsive device are respectively located. The depressions are closely spaced to avoid the need for elaborate lens systems for focusing emitted or received light. Accordingly, there is an ongoing need for improved single package, semiconductor emitter-receiver devices, and it is an object of the present invention to provide such. It is a further object of the present invention to provide an improved single package, semiconductor emitter-receiver device wherein the emitter and receiver are both outward facing and adapted to be mounted on a simple lead frame well suited for automated assembly and low cost. Brief Summary of the Invention The foregoing and other objects and advantages are provided by a single package, electro-optic transmitter-receiver device for transmitting and receiving light through an outer surface thereof comprising: lead means having thereon a variable intensity light source oriented to project light toward the outer surface and a photosensitive device oriented to receive light entering the outer surface; and transparent cover means for allowing passage of light from the variable intensity light source and to the photosensitive device, the transmitter-receiver device being characterised in that the lead means comprises a support surface facing and separated from the outer surface by a space, the variable intensity light source being mounted on a first portion of the support surface and the photosensitive device being mounted on another portion of the support surface, and in that the transparent cover means is one piece and is located over the photosensitive device and the variable intensity light source, and has a curved lens means substantially centrally located over the photosensitive device for collecting light, a substantially flat or concave portion located over the variable intensity light source for transmitting light and a recess inwardly extending from the outer surface toward the support surface between the variable intensity light source and the photosensitive device, said recess being arranged so that light from the variable intensity light source backscattered within the packages is insufficient to turn on the photosensitive device. In a first embodiment, the photosensitive device is mounted on a first portion of the lead means and the variable intensity light source is mounted on a second, spaced apart portion of the lead means. It is convenient if the lead means comprises a first portion having thereon the photosensitive device, a second portion having thereon the variable intensity light source and a third portion, wherein the first, second and third portions are spaced apart in a substantially common plane. A first electrical interconnection means desirably extends between the variable intensity light source and the first portion of the lead means and a second electrical interconnection means desirably extends between the third portion of the lead means and the photosensitive device. In a preferred embodiment, the photosensitive device is a phototransistor having its collector bonded to the first portion of the lead means and its emitter coupled to the third portion of the lead means and the variable intensity light source is an LED having one terminal bonded to the second portion of the lead means and another terminal coupled to the first portion of the lead means. It is desirable that the curved lens means is recessed within the transparent cover means so as to not protrude therefrom. In a second embodiment, the photosensitive device is mounted on a first portion of the lead means and the variable intensity light source is mounted on the same portion of the lead means, but spaced-apart from the photosensitive device. The foregoing summary of the invention and the objects and advantages thereof will be better understood in view of the accompanying drawings and description that follows. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified top view of a single package semiconductor emitter-receiver according to the prior art; FIG. 2 is a side view of the device of FIG. 1; FIG. 3 is a schematic diagram of the electrical circuit associated with a pair of devices according to FIG. 1, arranged face-to-face; FIG. 4 is a simplified top view of a single package semiconductor emitter-receiver according to a first embodiment of the present invention; FIG. 5 is a side view of the device of FIG. 4; FIG. 6 is a front view of the device of FIG. 4; FIG. 7 is a schematic diagram of the electrical circuit associated with the device of FIG. 4; FIG. 8 is a simplified top view of a single package semiconductor emitter-receiver according to a further embodiment of the present invention; FIG. 9 is a side view of the device of FIG. 8; and FIG. 10 is a schematic diagram of the electrical circuit associated with the device of FIG. 9. DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified top view and FIG. 2 a simplified side view of single package semiconductor emitter-receiver 10 according to the prior art. Emitter-receiver 10 comprises leads 12, 14, 16 enclosed within cover 18. Central lead 14 has die bond portion 20 on which are mounted phototransistor 22 and LED 24. Wirebond 23 connects phototransistor 22 to lead 16 and wirebond 25 connects LED 24 to lead 12. Cover 18 has protruding transparent convex lens 19 located over both phototransistor 22 and LED 24. In this fashion, light 26 emitted by LED 24 and light 28 received by phototransistor 22 pass through lens 19. GaAs LED's are suitable for use in connection with silicon receivers, as for example silicon based phototransistors. Such devices are well known in the art and means for assembling such are also well known. FIG. 3 shows a circuit diagram in which a pair of identical prior art devices 10, 10′ of FIGS. 1-2 comprising transistors 22, 22′ and LED's 24, 24′, are arranged emitter to receiver across gap 30. LED current limiting resistors 32, 32′ and output resistors 34, 34′ are provided. Power is applied between leads 16, 16′ and ground. There need not be any common electrical connection between circuits 10, 10′, only the optical connection across gap 30. A difficulty associated with the arrangement of FIGS. 1-2, is that both the LED and phototransistor are located in close proximity so as to be under the same lens. This increases the likelihood of crosstalk, i.e., light from LED 24 unintentionally reaching phototransistor 22 through internal backscattering within the package. When this is the case, the signaling arrangement shown schematically in FIG. 3 cannot run in a duplex mode, i.e., devices 10 and 10′ transmitting and receiving simultaneously, but must run only in a simplex mode, i.e., device 10 transmitting and device 10′ receiving or vice versa, but not both at the same time. A further difficulty with the device of FIGS. 1-2 is that, where an NPN phototransistor is desired to be used, then a P-substrate LED must be provided so that the LED can be arranged in the correct electrical direction on common bonding portion 20. P-substrate LED's are often less desirable than N-substrate LED's because of their higher cost. The foregoing and other problems are overcome by the invented structure illustrated in a first embodiment in FIGS. 4-6. FIGS. 4-6 show, respectively, a simplified top view, side view, and front view of single package semiconductor emitter-receiver 40 according to the present invention. Emitter-receiver 40 comprises leads 42, 44, 46 enclosed within cover 48. Central lead 44 has die bond portion 50 on which is mounted phototransistor 52 so that, for example, the collector thereof is in electrical contact with lead 44. LED 54 is mounted on lead 42 so that, for example, the anode of LED 54 is in electrical contact with lead 42. Wirebond 53 connects, for example, the emitter of phototransistor 52 to lead 46 and wirebond 55 connects, for example, the anode of LED 54 to lead 44. Transparent cover 48 has convex lens 49 centrally located over phototransistor 52 but not over LED 54. Light 58 received by phototransistor 52 passes centrally through lens 49, but light 56 emitted by LED 54 does not. This provides improved light collection efficiency for phototransistor 52 as compared to the prior art arrangement of FIGS. 1-2 in which phototransistor 22 is not centrally located under lens 19, but is located to one side in order to leave space for LED 24 which shares lens 19. Lens 49 may protrude in a fashion analogous to lens 19 of FIGS. 1-2. However, the recessed arrangement of lens 49 shown in FIGS. 4-6 is preferred since it makes lens 49 less susceptible to surface damage which adversely affects device performance. For example, surface scratches on protruding lens 19 of FIGS. 1-2 can significantly increase the amount of internal light backscattering. While a convex lens may also be provided over LED 54, in a preferred embodiment, light 56 from LED 54 passes through substantially flat or concave (dotted) surface portion 59 of transparent cover 48. The preferred structure is particularly simple to manufacture and has been found to give improved results compared to the prior art device of FIGS. 1-2. With surface portion 59 of cover 48 above LED 54 substantially flat, broader external light dispersion was obtained. Also, the arrangement of FIGS. 4-6 is less susceptible to internal light backscatter from LED 54 affecting transistor 52. This is because, among other things, surface portion 49′ of lens 49 extending into body 48 acts as a built-in optical separator that impedes transmission of internally scattered light from LED 42 to transistor 52, such as for example, light that is back-scattered from the body-air interface at surface portion 59. The above-described features combine to give improved performance in a variety of applications. For example, when a pair of devices of the present invention are arranged as in FIG. 3, they are less sensitive to the emitter-receiver alignment on either side of gap 30. When used singley, as is illustrated for example by the schematic diagram of FIG. 7, light 56 from LED 54 is directed toward object 60, and light 58 is reflected from or scattered by object 60 back toward phototransistor 52. The wider dispersion angle of emitted light combined with the improved light collection efficiency of lens 49 above phototransistor 52 provides higher performance in this situation as well. In particular, the placement of object 60 with respect to emitter-receiver 40 is less critical and a greater on-off signal ratio is obtained from transistor 52 for the same exitation level on LED 54, as object 60 is inserted or removed from the path of emitted light 56. Among other things, this is because the invented arrangement of FIGS. 4-6 has lower levels of internal light backscattering than the prior art arrangement of FIGS. 1-2. Other things being equal, this means that the dark signal level from transistor 52 (no external reflection or scattering) is lower than is obtained with the prior art arrangement. A further embodiment of the present invention is illustrated in FIGS. 8-10. FIGS. 8-9 show, respectively, a simplified top view and side view of single package semiconductor emitter-receiver 60 and FIG. 10 shows a simplified schematic diagram applicable to the device of FIGS. 8-9. Emitter-receiver 60 comprises leads 62, 64, 66 enclosed within cover 68. Central lead 64 has die bond portion 70 on which is mounted phototransistor 72 so that, for example, the collector thereof is in electrical contact with lead 64. LED 74 is mounted elsewhere on die bond portion 70 or elsewhere on lead 64 so that, for example, the cathode of LED 74 is in electrical contact with lead 64. Wirebond 73 connects, for example, the emitter of phototransistor 72 to lead 66 and wirebond 75 connects, for example, the anode of LED 74 to lead 62. Transparent cover 68 has convex lens 69 centrally located over phototransistor 72 but not over LED 74. Light 78 received by phototransistor 72 passes centrally through lens 69, but light 76 emitted by LED 74 does not. While a convex lens may also be provided over LED 74, in a preferred embodiment, light 76 from LED 74 passes through substantially flat or concave (dotted) portion 79 of transparent cover 68. This arrangement has the advantages already described in connection with FIGS. 4-6, including recessed surface portion 69′ between LED 74 and photo-transistor 72. FIG. 10 illustrates how the device of FIGS. 8-9 may be connected to resistors 82, 84. Light 76 from LED 74 is reflected or scattered as light 78 from object 80 back to phototransistor 72 when object 80 is in the path of light 76. Otherwise there is substantially no optical path between LED 74 and transistor 72. A feature of the arrangement of FIG. 10 is that it is regenerative, that is, as the impedance of phototransistor 72 decreases in response to receiving reflected or scattered light 58, the current through LED 74 also increases causing an increase in emitted light 76. Other things being equal, this increase in emitted light 76 increases received light 78, further reducing the impedance of transistor 72. This regenerative action continues until phototransistor 72 is in saturation where it will remain until object 80 is removed or the light path otherwise interrupted so that the reflected or scattered light drops below the level necessary to sustain conduction in transistor 72. The circuit then shuts off. This circuit has the advantage of a snap-action due to the regeneration and a larger on-off signal ratio than would be obtainable without the regenerative feedback. Those of skill in the art will appreciate that the substantially flat leadframe employed in the preferred implementation of the present invention is easy to manufacture by means well known in the art and is easy to handle in assembly, as compared to other types of leadframes commonly employed with optoelectronic components. Both the LED and the phototransistor are mounted on flat portions of the leadframe which lie substantially in the same plane. Although various parts of the leadframe might be down-formed or up-formed, this is not necessary. Further the LED and phototransistor are conveniently in the form of die having one contact which is made to the surface on which they are mounted and which are attached to the leadframe by means well known in the art. It is not necessary to use insulators on the leadframe to isolate either the LED or the phototransistor therefrom. Additionally, the simple, back-contact die and planar leadframe of the preferred embodiment of the present invention is especially well suited for low cost automated assembly. Hence the invented product may be produced particularly economically. Having thus described the invention, those of skill in the art will appreciate that it provides an improved single package, semiconductor emitter-receiver device, and in particular, one in which the emitter and receiver are both outward facing and adapted to be mounted on a simple planar lead frame, and which is well suited for assembly at low cost. Those of skill in the art will understand, based on the description herein, that many variations may be made in choice of receiver and emitter components without departing from the present invention. For example, while the present invention has been described in terms of silicon phototransistors, those of skill in the art will appreciate that photodiodes and/or photoresistor and/or photothyristors or other photosensitive components made from many different materials could equally well be used. Similarly, while the invention has been described in terms of light emitting diodes, as for example GaAs LED's, other emitters and other emitting materials could equally well be used. Those of skill in the art will understand, based on the description herein, how to choose appropriate components for emitters and receivers for use in connection with the present invention.

Claims: An electro-optic transmitter-receiver device (40) for transmitting and receiving light through an outer surface thereof comprising: lead means (42, 44, 46) having thereon a variable intensity light source (54) oriented to project light toward the outer surface and a photosensitive device (52) oriented to receive light entering the outer surface; and transparent cover means (48) for allowing passage of light (56, 58) from the variable intensity light source (54) and to the photosensitive device (52), the transmitter-receiver device being characterised in that the lead means (42, 44, 46) comprises a support surface facing and separated from the outer surface by a space, the variable intensity light source being mounted on one portion (42) of the support surface and the photosensitive device being mounted on another portion (50) of the support surface, and in that the transparent cover means (48) is one piece and is located over the photosensitive device (52) and the variable intensity light source (54), and has a curved lens means (49) substantially centrally located over the photosensitive device (52) for collecting light, a substantially flat or concave portion (59) located over the variable intensity light source (54) for transmitting light (56) and a recess (49′) inwardly extending from the outer surface toward the support surface between the variable intensity light source (54) and the photosensitive device (52), said recess being arranged so that light from the variable intensity light source (54) backscattered within the package is insufficient to turn on the photosensitive device (52). The device of claim 1 wherein the lead means (42, 44, 46) is substantially flat. The device of claim 1 or 2 wherein the support surface of the lead means (42, 44, 46) comprises a first portion (44) having thereon the photosensitive device (52), a second portion (42) having thereon the variable intensity light source (54) and a third portion (46), wherein the first (44), second (42) and third portions (46) are spaced apart in a substantially common plane. The device of claim 3 wherein the photosensitive device (52) is a phototransistor having its collector bonded to the first portion (44) of the lead means and its emitter coupled to the third portion (46) of the lead means and the variable intensity light source (54) is an LED having one terminal bonded to the second portion (42) of the lead means and another terminal coupled to the first portion (44) of the lead means. The device of any preceding claim wherein the curved lens means (49) is convex and recessed (49′) within the transparent cover means (48) so as to not protrude therefrom. A method for forming an electro-optic transmitter-receiver device (40, 60) for transmitting and receiving light through an outer surface thereof, comprising: providing a leadframe (42, 44, 46, 62, 64, 66) having first (44, 64), second (42, 62) and third (46, 66) electrodes; mounting a photosensitive device (52, 72) on the first electrode (44, 64); mounting a light emitting device (54, 74) on the first (44, 64) or second (42, 62) electrode; and providing a transparent cover means (48, 68) for allowing passage of light (56, 58, 76, 78), the method being characterised in that the first providing step comprises providing the leadframe wherein the first, second and third electrodes are separated from the outer surface by a space and in that the second providing step comprises providing a single transparent cover means (48, 68) located over the photosensitive device (52, 72) and the light emitting device (54, 74), and having a first convex lens portion aligned over the photosensitive device (52, 72) but not over the light emitting device (54, 74), a second substantially flat or concave portion (59, 79) over the light emitting device (54, 74) and a recess (49′, 69′) inwardly extending from the outer surface and located between the photosensitive device (52, 72) and the light emitting device (54, 74), said recess being arranged so that light from the variable intensity light source (54) backscattered within the package is insufficient to turn on the photosensitive device (52). The method of claim 6 wherein the first providing step comprises providing a substantially flat leadframe (42, 44, 46, 62, 64, 66).

Inventors: STEVENSON DAVID W, STEVENSON, DAVID W.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H01L31
IPC codes: H01L31, H03K17