Light emitting diode array

A contact layer having a low resistance in an LED is extended from an inside edge of a light take-out region in a light emitting dot to a central position thereof, whereby an input current in the light take-out region expands, while an electrode is extended from an inside edge of the light take-out region to the center of the light take-out region so as to shape like substantially a letter T with a length which is not over the center, whereby increase in an electrode covering ratio in the light take-out region can be suppressed. As a result, a uniform light output in the light take-out region can be attained, so that a light output can be remarkably improved. When electrodes in the light take-out regions in adjacent light emitting dots are staggered relative to the direction perpendicular to an aligned direction of the light take-out region (a reference straight line), the light take-out regions are arranged horizontally in substantially a straight line, so that a light emitting pattern in the aligned direction of the light take-out regions are not so staggered. Thus, an LED array having a high output and a uniform distribution of light intensity is provided.

FIELD OF THE INVENTION
 The present invention relates to a light emitting diode (hereinafter
 referred to simply as "LED") array, and more particularly to an LED array
 providing a high light output and a uniform distribution of light
 intensity.
 BACKGROUND OF THE INVENTION
 There is a GaAlAs LED as one of the LEDs, and since such LED exhibits high
 intensity, they are arrayed to provide an LED array which has been
 utilized as a light source for LED printer.
 FIG. 1 is a plan view showing an example of conventional LEEDs, and FIG. 2
 is a plan view showing another example of conventional LEDs.
 LED array has a fine structure, and it is classified into two types. One of
 them is peripheral electrode type LED array (FIG. 1) wherein an electrode
 (peripheral electrode) 30 is disposed on a side (peripheral part) of a
 light take-out region 20 in a light emitting dot 10, and the other type is
 a central electrode type LED array (FIG. 2) wherein an electrode (central
 electrode) 31 is disposed at an central portion of a light take-out region
 21 in a light emitting dot 11. In either of the LEDs, the electrode 30 or
 31 is disposed through a contact layer 40 or 41 for electrode, and the
 electrode 30 or 31 is connected to a metal interconnection 50 or 51.
 In the peripheral electrode type LED shown in FIG. 1, since the peripheral
 electrode 30 is disposed on a peripheral portion of the light take-out
 region 20, it is very difficult to uniformly expand an input current over
 the whole area of the light take-out region 20. As a result, the light
 which can be taken out from the light take-out region 20 decreases with
 increase in a distance from the peripheral electrode 30, because the light
 output decreases, resulting in unevenness of light output in the light
 take-out region 20, besides, high light output cannot be obtained in this
 case. Furthermore, there is a possibility of variations in a mode of
 expanding an input current depending upon a slight difference in
 crystallizability of each LED, so that variations of each LED in an LED
 array become also remarkable.
 Moreover, since distribution of light intensity is biased toward the
 peripheral electrode 30, there has been such a problem that a light
 emitting region effective for printing of an LED printer is not arranged
 horizontally in a straight line, but staggered, even in the light take-out
 regions 20 of adjacent LEDs are arranged horizontally in a straight line.
 On the other hand, in the central electrode type LED shown in FIG. 2, since
 the central electrode 31 is positioned in a central portion of the light
 take-out region 21, this type of LEDs can overcome the above-mentioned
 disadvantage involved in peripheral electrode type LEDs. In other words,
 the input current can be extended over the whole area of the light
 take-out region 21 in the central electrode type LEDs.
 However, in the case where a finer structure of LED is required as, for
 example, in a high density LED array of 600 DPI or more, when a central
 electrode type structure is used, a size of the central electrode cannot
 be reduced further because of assuring reliability. For this reason, a
 ratio of LED covered by electrode (hereinafter referred to as "electrode
 covering ratio") increases, so that it becomes difficult to obtain an LED
 of high output. In addition, as a result of providing the remarkably
 narrow light take-out region 21, a problem arises also in a shape of light
 emitting spot.
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to provide an LED array
 affording a high light output and a uniform distribution of light
 intensity.
 According to the invention, an LED array, comprises: a light take-out
 region provided on a pn junction structured by a cladding layer of a first
 conductivity type and an active layer of a second conductivity type, the
 light take-out region having a predetermined area; a contact layer
 provided on the cladding layer, the contact layer being extended from an
 inside edge of the light take-out region beyond a central position of the
 light take-out region; and an electrode provided on the contact layer, the
 electrode being extended from an inside edge of the light take-out region
 not to reach the central position thereof.
 This invention is concerned with an LED array containing an electrode
 disposed on a part of each of a plurality of light emitting dots, and an
 interconnection drawn out alternatively with respect to the direction
 perpendicular to the aligned direction of the light emitting dots
 (referred also as "reference straight line"), wherein the electrodes in
 adjacent light take-out regions are staggered relative to the reference
 straight line.
 In addition to the above described constitution, a light emitting pattern
 formed by light take-out regions may be arranged in a straight line in the
 present invention.
 According the present invention, a contact layer having a low resistance
 and made of GaAs or a GaAlAs layer having a low Al mixed crystal ratio is
 extended from an inside edge of a light take-out region in a light
 emitting dot to a central position thereof, whereby an input current in
 the light take-out region expands, and on the other hand, an electrode is
 extended from an inside edge of a light take-out region to the center of
 the light take-out region so as to shape like substantially a letter T
 with a length which is not over the center, whereby increase in an
 electrode covering ratio in the light take-out region can be suppressed.
 In other words, the electrode and the contact layer contribute
 significantly for expanding an input current in the light take-out region.
 Thus, a uniform light output in the light take-out region can be attained,
 and the light output can be remarkably improved.
 Furthermore, when electrodes in the light take-out regions in adjacent
 light emitting dots are staggered relative to the direction perpendicular
 to an aligned direction of the light take-out regions (reference straight
 line), the light take-out regions are arranged horizontally in
 substantially a straight line, so that a light emitting pattern in the
 aligned direction of the light take-out regions are not so staggered, even
 if light emitting regions effective for printing of an LED printer deviate
 to a peripheral electrode side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An embodiment of the present invention will be described hereinafter in
 conjunction with the accompanying drawings wherein FIG. 3 is a plan view
 showing an LED array according to the embodiment of the invention. In FIG.
 3, two adjacent LEDs in a peripheral electrode type GaAlAs LED array are
 shown. This LED is a GaAlAs LED having a mesa isolated type single hetero
 structure.
 In an LED D1 shown on the left side of FIG. 3, a contact layer 42 is
 disposed on one side (the lower side in the figure) of a light take-out
 region 22 in a light emitting dot 12, and a peripheral electrode 32 is
 formed on the contact layer 42. The contact layer 42 is extended from an
 inside edge of the light take-out region 22 to a central position thereof,
 while the peripheral electrode 32 is extended from an inside edge of the
 light take-out region 22 to the center of the light take-out region 22 so
 as to shape like substantially a letter T with a length which is not over
 the center.
 In an LED D2 shown on the right side of FIG. 3, a contact layer 42 is
 disposed on one side (the upper side in the figure) of a light take-out
 region 22, and a peripheral electrode 32 is formed on the contact layer
 42. The contact layer 42 is extended from an inside edge of the light
 take-out region 22 to a central position thereof, while the peripheral
 electrode 32 is extended from an inside edge of the light take-out region
 22 to the center of the light take-out region 22 so as to shape like
 substantially a letter T with a length which is not over the center.
 Namely, the LED D2 is an inverted state with respect to the LED D1.
 A horizontal length X and a vertical length Y of the light take-out region
 22 of the LED D1 are 25 .mu.m and 30 .mu.m, respectively, so that the LED
 D1 has been very finely designed. The peripheral electrode (n-electrode)
 32 is disposed on a side of the light take-out region 22, and connected to
 a metal interconnection 52 made of Au or the like.
 The contact layer 42 is formed under the n-electrode 32 in order to take
 good ohmic junction between the n-electrode 32 and the light take-out
 region 22. This contact layer 42 is made from n-type GaAs having a low
 resistance or a GaAlAs layer having a very small mixed crystal ratio, and
 it is extended further from a contact portion with the n-electrode 32
 along a central portion of the light take-out region 22 with a width of 9
 .mu.m. Further, the n-electrode 32 is extended from a peripheral portion
 of the light emitting dot 12 to a central portion of the light take-out
 region 22 in a shape like substantially letter T with a length of about 10
 .mu.m. Moreover, the light take-out region 22 of the light emitting diode
 D1 and the light take-out region 22 of the light emitting diode D2
 adjacent to each other are staggered in such that the respective
 n-electrodes 32, 32 approach in the Y-direction with each 5 .mu.m
 displacement in up and down directions.
 FIG. 4 is a sectional view of an LED array taken along the line A-A of FIG.
 3, and FIG. 5 is a sectional view of the LED array taken along the line
 B-B of FIG. 3.
 An active layer (p-GaAlAs layer) 61 and a cladding layer (n-GaAlAs layer
 having a high Al mixed crystal ratio) 62 are formed on a p-GaAs substrate
 60. A pn layer composed of the active layer 61 and the cladding layer 62
 has a single hetero structure, and is a mesa separate type as mentioned
 above. An n-contact layer (which exhibits a low resistance as a result of
 being doped at a high concentration, and is a GaAlAs layer having a very
 low Al mixed crystal ratio) is formed on the n-GaAlAs layer 62. A
 protective glass film 63 and the n-electrode 32 are formed on the surfaces
 of the n-GaAlAs layer 62 and the GaAlAs layer 42, and a metal
 interconnection 52 is formed on the n-electrode 32. A p-electrode 64 is
 attached to the underside of the LED D1.
 When the LED array as described above is energized, a current input from
 the n-electrode 32 expands into the whole area of the light take-out
 region 22 formed in the vicinity of the pn interface through the contact
 layer 42 in spite of having a peripheral electrode structure. This is
 because there is an n-type GaAlAs layer as the contact layer 42 which has
 been extended alone a central portion of the light take-out region 22, and
 the n-electrode 32 as a peripheral electrode has been extended from an
 inside edge of the light emitting dot 12 toward the central direction by
 10 .mu.m.
 Accordingly, uniform light output can be realized in the light take-out
 region 22, and in addition, high light output can be attained. An amount
 of the n-electrode 32 to be extended depends upon a longitudinal length of
 the light take-out region 22. In the present embodiment, since a
 longitudinal length of the light take-out region is 30 .mu.m, an amount to
 be extended is set at 10 .mu.m.
 On one hand, the adjacent light take-out regions 22, 22 are staggered with
 a distance of each about 5 .mu.m in such that the respective n-electrodes
 32, 32 approach in the Y-direction. This is because close areas of
 n-electrodes 32 wherein light intensity becomes the most intensive in the
 light take-out region 22 are allowed to be close to each other in the
 Y-direction, whereby there is an effect of preventing from appearance of a
 staggered distribution of a light emitting pattern of the light take-out
 region 22 effective for printing operation by an LED printer.
 Samples of the conventional LED array shown in FIG. 2 and that of the
 present invention shown in FIG. 3 are prepared with light take-out regions
 each having the same size, and both the light outputs obtained therefrom
 are compared. As a result, about 1.4 times higher light output can be
 obtained in the LED array shown in FIG. 3 as compared with that in the LED
 array shown in FIG. 2. Furthermore, variations in light output of each LED
 contained in a LED array have been heretofore.sub.-- 16%, while they
 become an average.sub.-- 8% in the LED shown in FIG. 3 so that they can be
 remarkably improved. Likewise, about 10% higher light output than that of
 the LED array shown in FIG. 2 was obtained in the LED array shown in FIG.
 3.
 As described above, when the peripheral electrode is somewhat extended from
 a vicinity of the light take-out region toward a central position of the
 light take-out region, and the contact layer having a low resistance in
 the peripheral electrode is extended along the direction toward a central
 position of the light take-out region in the present embodiment, the same
 current distribution effect as that of a central electrode type LED can be
 obtained in a peripheral electrode type LED, besides increase in electrode
 covering ratio in light take-out region which is a disadvantage involved
 in a central electrode type LED array can be avoided. As a consequence, it
 becomes possible to manufacture a high light output and high density LED
 array. Furthermore, since adjacent light take-out regions are staggered in
 such that their electrodes approach to each other in the Y-direction, such
 a disadvantage that a light emitting pattern of a light emitting region
 effective for printing of an LED printer becomes staggered in a
 conventional peripheral electrode type LED array can be solved.
 In brief, the following excellent advantages can be achieved in accordance
 with the present invention.
 (1) An LED array having a high light output and a uniform distribution of
 light intensity can be provided by such an arrangement that a contact
 layer is extended from an inside edge of a light take-out region to a
 central position thereof, while a peripheral electrode is extended from an
 inside edge of the light take-out region to the center of the light
 take-out region so as to shape like substantially a letter T with a length
 which is not over the center.
 (2) An LED array having a high light output and a uniform distribution of
 light intensity can be provided by such an arrangement that electrodes in
 adjacent light take-out regions are staggered so as to approach to each
 other with respect to the direction perpendicular to an aligned direction
 of the light take-out regions (reference straight line).
 It will be appreciated by those of ordinary skill in the art that the
 present invention can be embodied in other specific forms without
 departing from the spirit or essential characteristics thereof.
 The presently disclosed embodiments are therefore considered in all
 respects to be illustrative and not restrictive. The scope of the
 invention is indicated by the appended claims rather than the foregoing
 description, and all changes that come within the meaning and range of
 equivalents thereof are intended to be embraced therein.