Light emitting diode

A light emitting diode constituted by a pair of semiconductor layers having a planar P-N junction therebetween, the semiconductor layers having at least one side surface extending transversely across the P-N junction and from which light is emitted in a direction parallel with the plane of the P-N junction, a pair of electrodes on the respective outer surfaces of the pair of semiconductor layers parallel with the plane of the P-N junction plane, at least one of the pair of electrodes covering an area less than the total area of the P-N junction and being located adjacent to the one side surface from which the light is emitted, the remainder of the area of the outer surface of the semiconductor layer not being covered by the electrode, and a light shielding member covering the remainder of the area of the outer surface of the semiconductor layer which is not covered by the one electrode. The light shield member is made of a material identical with that of the one electrode, and is insulated from the one electrode, and the light shielding member and the one electrode are spaced from each other to define a gap therebetween for insulating them from each other.

BACKGROUND OF THE INVENTION AND PRIOR ART 
The present invention relates to a light emitting diode utilizable as a 
light source for a triangulation type distance measuring device such as is 
used in an automatic focus control camera. 
A light emitting diode is known which includes a pair of semiconductor 
layers with a planar P-N junction therebetween, and which is provided with 
at least one surface formed by cutting the semiconductor layers across the 
P-N junction, the edge of the junction emitting light in a direction 
parallel with the P-N junction, such as is disclosed in Japanese Patent 
Application Laid Open No. 54-40662 and in copending U.S. patent 
application Ser. No. 940,135, filed Sept. 6, 1978, and assigned to the 
same assignee as the present application. 
FIG. 1 is a schematic illustration of such a prior art light emitting 
diode. As shown in FIG. 1, the light emitting diode comprises a P-type 
semiconductor layer P.sub.1 and an N-type semiconductor layer N.sub.1 with 
a planar P-N junction 2 formed therebetween. The entire upper surface of 
the P-type semiconductor layer P.sub.1 is coated with AuZn deposited by an 
evaporation deposition technique to form an electrode A.sub.1 to which a 
wire W.sub.1 is bonded to supply power to the diode. The N-type 
semiconductor layer N.sub.1 has firmly mounted on the lower surface 
thereof an electrically conductive substrate (not shown), which serves as 
the counter electrode to the electrode A.sub.1. 
Four side surfaces such as 4 and 6 are formed by cutting the semiconductor 
layers P.sub.1 and N.sub.1 across the P-N junction so that light beams a, 
b, c and d are emitted from the edges of the junction in a direction 
parallel with the plane of the junction 2. Any one of the light beams a, 
b, c and d can be utilized as a source of light to be projected toward an 
object in a triangulation type distance measuring device. 
However, the above-described ordinary light emitting diode has an 
undesirable characteristic that the light emitting efficiency decreases 
when the current supply is increased for the purpose of increasing the 
absolute intensity of the emitted light due to an increase in the 
temperature of the P-N junction. In other words, a greater part of the 
increase in the current supplied does not effect a desirable increase in 
intensity, but rather is consumed by the increased temperature of the P-N 
junction. 
OBJECT AND BRIEF DESCRIPTION OF THE INVENTION 
An object of the present invention is to provide a light emitting diode 
with an improved light emitting efficiency. 
To this end, the present invention provides a light emitting diode which 
comprises a pair of semiconductor layers having a planar P-N junction 
therebetween, said semiconductor layers having at least one side surface 
extending transversely across said P-N junction and from which a light is 
emitted in a direction parallel with the plane of said P-N junction; and a 
pair of electrodes on the respective outer surfaces of said pair of 
semiconductor layers parallel to the plane of said P-N junction plane, at 
least one of said pair of electrodes covering an area less than the total 
area of said P-N junction and being located adjacent to said one side 
surface from which the light is emitted, the remainder of the area of the 
outer surface of said semiconductor layer not being covered by said 
electrode. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The invention will now be described in greater detail in connection with 
the accompanying drawings, in which: 
FIG. 1 is a perspective view of a prior art light emitting diode; 
FIG. 2 is a perspective view of an embodiment of a light emitting diode 
according to the present invention; 
FIG. 3 is a cross-sectional view of a practical use of the embodiment; 
FIG. 4 is a graph of the function of the light emitting diode of the 
present invention; 
FIG. 5 is a perspective view of a second embodiment of a light emitting 
diode according to the present invention; 
FIG. 6 is a perspective view of a third embodiment of the light emitting 
diode according to the present invention; and 
FIG. 7 is a perspective of a fourth embodiment of a light emitting diode of 
the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the light emitting diode LED according to the 
invention and shown in FIG. 2 comprises a P-type semiconductor layer 
P.sub.2 and an N-type semiconductor layer N.sub.2 with a planar P-N 
junction 8 formed therebetween. The semiconductor layers P.sub.2 and 
N.sub.2 have four side surfaces, only two of which, i.e. surfaces 10 and 
12, are shown, which are formed by cutting the semiconductor layers 
P.sub.2 and N.sub.2 transversely across the plane of the P-N junction 8. 
The side surfaces are here shown as substantially perpendicular to the 
plane of the P-N junction. Light beam e emitted from the edge of P-N 
junction 8 and appearing on surface 10 can be utilized as a source of 
light to be projected toward an object, the distance of which is to be 
measured by a triangulation type distance measuring device. An electrode 
A.sub.2 is formed on the portion of the upper surface of the P-type 
semiconductor layer adjacent to the surface 10 from which the light beam e 
is emitted. The total area of the electrode A.sub.2 is less than the total 
area of the planar P-N junction 8 so that the area 14 on the upper surface 
of the P-type semiconductor layer P.sub.2 is left uncovered by the 
electrode A.sub.2. A lead wire W.sub.2 is bonded on the electrode A.sub.2. 
An example of a practical size of the diode is as follows: 
##EQU1## 
In other words, the area of the electrode A.sub.2 is approximately equal 
to the area of the portion 14 not covered by the electrode. The overall 
dimensions of the diode shown in FIG. 2 are identical with those of the 
prior art diode shown in FIG. 1. However, the dimensions of the light 
emitting diode and the electrode A.sub.2 or its shape are not necessarily 
limited to the above values or shape, but can be widely modified. 
The embodiment shown in FIG. 2 can be used as a light source unit in the 
manner illustrated in FIG. 3. In FIG. 3, an electrically conductive 
substrate 16 has the lowest surface of the N-type semiconductor layer 
N.sub.2 of the diode firmly mounted thereon. An electrically conductive 
member 18, which is similar to the electrically conductive substrate 16 in 
shape and material is positioned so as to be insulated from the 
electrically conductive substrate 16. The wire W.sub.2 connects the 
electrically conductive member 18 and the electrode A.sub.2. All the above 
elements are contained within a package 20 formed by molding a transparent 
plastic over the diode, the wire W.sub.2 and the opposed edges of the 
substrate 16 and the member 18 with the free end of the conductive 
substrate 16 and the free end of the conductive member 18 both projecting 
out of the package 20, as shown in FIG. 3, as a pair of terminals for 
external connection to the diode. The direction of the light beam e as 
shown in FIG. 2 is perpendicular to the plane of FIG. 3. 
In the operation of the diode according to the abovedescribed embodiment 
shown in FIGS. 2 and 3, a current which is fed from the conductive member 
18 through wire W.sub.2, mainly flows through the area of the planar P-N 
junction 8 corresponding to the area of the electrode A.sub.2 adjacent to 
the side surface 10 from which the light beam e is emitted. The part of 
the semiconductor layers which are not covered by the electrode A.sub.2 
serve as a heat absorber since the current flow through the area of the 
P-N junction 8 corresponding to the area 14 not covered by the electrode 
A.sub.2 is considerably less, resulting in a smaller increase in the 
temperature. In other words, the portion of the planar P-N junction 8 
corresponding to the area of the electrode A.sub.2 is cooled by the 
absorption of heat by the lower temperature part of the semiconductor 
layers corresponding to the area 14 not covered by the electrode A.sub.2. 
In this manner, the area of the P-N junction adjacent the side surface 10 
from which the light beam e is emitted is supplied with the electric 
current yet the increase of the temperature is limited so that it 
effectively emits light. Further, the intensity of the light beams emitted 
in other directions, such as directions corresponding to beams b, c and d 
in FIG. 1, is reduced since the current flowing through the areas of the 
P-N junction adjacent to the other side surfaces, such as surface 12, is 
reduced in the embodiment in FIG. 2 due to the reduced area of electrode 
A.sub.2 adjacent thereto. This means that emitted light which is not used 
is reduced since only a light beam extending in one direction, such as e 
in FIG. 2, is normally required in a light source of a triangulation type 
distance measuring device. 
FIG. 4 is a graph showing the change in the intensity of emitted light 
versus the time of application of the current for particular current 
supplies and comparing the embodiment shown in FIG. 1. In FIG. 4, curve 
F.sub.1 represents the change in intensity of the light beam a in FIG. 1 
with the passage of time when the current I.sub.1 is supplied. Curve 
F.sub.2 represents the change in intensity of the light beam e in FIG. 2 
with the passage of time when the current I.sub.2 is supplied. FIG. 4 
shows that in the diode according to the present invention an initial 
intensity equal to the intensity obtained in an ordinary light emitting 
diode by a current I.sub.1 can be attained by a current I.sub.2 which is 
less than the current I.sub.1, and when the current is maintained, the 
intensity decreases over a period of time at a smaller rate than is the 
case with an ordinary light emitting diode. FIG. 4 further shows that the 
initial intensity would be greater for a diode according to the present 
invention than for the ordinary light emitting diode with an identical 
current supplied to both the diodes. Thus, it is seen that the light 
emitting diode according to the present invention is much more efficient 
than the prior art diode. 
FIG. 5 shows a second embodiment of the present invention, in which like 
parts to those in FIG. 2 are represented by like reference numbers and 
characters, so that an explanation of these parts can be omitted. The 
diode shown in FIG. 5 is a type in which a plurality of light beams is 
emitted one in each of the four directions, such as a, b, c and d in FIG. 
1. The electrode A.sub.2 has a shape which covers only the peripheral 
portion of the upper surface of the P-type semiconductor layer P.sub.2 and 
a central opening is left therein which constitutes the area 14 left 
uncovered by the electrode A.sub.2. Thus, electrode A.sub.2 has portions 
adjacent each of the four side surfaces formed by cutting the 
semiconductor layers across the planar P-N junction, respectively, and the 
total area of the electrode A.sub.2 is less than the total area of the 
planar P-N junction. The electrode A.sub.2 has a pair of wires W.sub.3 and 
W.sub.4 bonded thereto. In this embodiment, the part of the semiconductor 
layers which corresponds to the area 14 functions as a heat absorber. 
FIGS. 6 and 7 show third and fourth embodiments of the present invention, 
respectively, in which like parts to the parts shown in FIGS. 2 and 5 have 
like reference numbers and characters, so that an explanation of these 
parts can be omitted. In FIGS. 6 and 7, a light shielding member A.sub.3 
is provided for covering the area 14 of the upper surface of the P-type 
semiconductor layer P.sub.2 which is not covered by the electrode A.sub.2. 
The light shielding member A.sub.3 can be a conductive material identical 
with that of electrode A.sub.2, and can be coated on the semiconductor 
layer P.sub.2 at the same time and by the same method as the electrode 
A.sub.2 by using a suitable mask for forming a very narrow gap g between 
the electrode A.sub.2 and light shielding member A.sub.3 for insulating 
them from each other. Alternatively, the light shielding member A.sub.3 
can be formed by painting or coating the whole of the upper surface or 
part of the upper surface which includes at least the area 14 of the light 
emitting diode as in FIG. 2 or 5 with a non-conductive light shielding 
material. 
The light shielding member A.sub.3 functions in the following 
circumstances. In the embodiment of FIG. 2, for example, in the absence of 
the light shield, light generated at the planar P-N junction 8 is also 
emitted upwardly through the area 14 of the P-type semiconductor layer 
P.sub.2 which is not covered by the electrode A.sub.2. This light 
illuminates the upper surface 21 of package 20 shown in FIG. 3 and makes 
the upper surface 21 a secondary light source. Thus, the light source unit 
as shown in FIG. 3 will emit two light beams, i.e., one directly from the 
edge of the planar P-N junction 8 which corresponds to the light beam e in 
FIG. 2, and the other from the upper surface 21. Two such beams will 
confuse the triangulation type distance measurement. The light shielding 
member A.sub.3 prevents this undesirable emission of light and improves 
the high efficiency light emitting diode of the present invention. 
The light emitting diode according to the present invention can be easily 
manufactured simply by changing the shape of mask used in an evaporation 
deposition coating method for the electrode A.sub.2 in FIG. 2 or 5 or the 
electrode A.sub.2 plus the light shielding member A.sub.3 in FIG. 6 or 7 
from that used for the ordinary light emitting diode as in FIG. 1, and 
greatly improves the efficiency of emitting light for the current 
supplied.