Abstract:
A high power stripe-geometry heterojunction laser diode device is provided which may be employed in a radar system designed to measure the distance to a target. The laser diode device has an electric circuit path extending from a first electrode connected to a voltage source to a second electrode connected to ground and features addition of a resistance of 1 mΩ or more to the electric circuit path to provide uniform current distribution in an active layer for emitting a high density laser beam.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to a laser diode, and more particularly to an improved structure of a heterojunction laser diode suitable for radar systems designed to measure the distance to a target. 
     2. Background of Related Art 
     In recent years, automotive radar systems have been proposed which measure the distance between a system-equipped vehicle and a preceding vehicle using a laser diode (i.e., a semiconductor diode) and output an alarm signal and/or apply brakes when the system-equipped vehicle is too close to the preceding vehicle. Such systems are designed to detect an object 100 m ahead, requiring laser diodes capable of being excited by pulse voltage to output a beam of light of several tens of watts (W). Specifically, unlike a low power laser diode, as shown in FIG. 26, which emits a laser beam of several milliwatts (mW), a high power laser diode, as shown in FIG. 27, has a wider light-emitting portion (i.e., a stripe width) of an active layer, which causes difficulty in producing a laser beam approximating a circular shape. 
     In order to alleviate the above problem, U.S. Pat. No. 5,559,819 filed on Apr. 18, 1995, assigned to the same assignee as that of this application teaches a semiconductor laser in which the sum of thicknesses of an active layer and an optical guide layer is set to 1.5 μm or more for decreasing an ellipticity of a laser beam (i.e., a ratio of a major axis to a minor axis of a cross section of the laser beam). The decrease in ellipticity of the laser beam is achieved by reducing the diffraction of light in a thicknesswise direction of an optical guide layer to narrow the laser beam in that direction without changing the diffraction of light in a widthwise direction of the optical guide layer to approximate the cross sectional shape of the laser beam to a circle. Thus, there is also need for narrowing laser beam in the widthwise direction of the optical guide layer. 
     Specifically, a high power laser diode operates in a multimode, and the width of a light-emitting portion (i.e., a stripe width) of an active layer is great, therefore, the current flowing through the active layer has, as shown in FIG. 30, the Gaussian distribution which causes an equiphase surface of light in the laser diode to be deformed, resulting in deterioration in coherence. This will cause an output laser beam to be widened undesirably so that a far-field pattern (FFP) becomes wide, the density of an output laser beam at a remote location thereby being decreased. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved structure of a high power laser diode capable of emitting a high density laser beam. 
     In in-depth analysis of internal structure of a high power laser diode to achieve the above object, the inventors of this application have found that addition of a resistor to a current path of the high power laser diode causes current distribution in an active layer (usually, Gaussian distribution) to be uniform or flat, as shown by a solid line in FIG.  31 . 
     Specifically, a high power laser diode having a wider stripe width can be viewed, as shown in FIG. 32, as a circuit having a plurality of diodes D 1 , D 2 , . . . D n  arranged in parallel. In this circuit, it is conceivable that a large amount of current flows through each of the diodes D 1 , D 2 , . . . D n  when energized, and current values I 1 , I 2 , . . . I n  all exhibit the Gaussian distribution. In contrast, it has been observed that when resistors R are disposed, as shown in FIG. 33, between the diodes D 1 , D 2 , . . . D n  and the ground, it will cause the current flows to be slowed down so that they show a constant value over a wide range within a stripe width. The present invention was made based on this fact. 
     According to a first aspect of the present invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 μm or more which comprises: (a) a laser diode chip including (1) a first electrode layer connected to a voltage source, (2) a second electrode layer connected to ground, and (3) an active layer disposed between the upper and the lower electrode layers to emit a laser beam when energized; and (b) a resistance layer having a resistance of 1 mΩ or more, the resistance layer being disposed within an electric path extending from the first electrode layer to the second electrode layer through the active layer. 
     In the preferred mode of the invention, the resistance layer is disposed between the second electrode layer and the ground. 
     The resistance layer may alternatively be disposed between the first electrode layer and the voltage source. 
     The resistance layer is made of one of a GaAs-based material, a Si-based material, and an InP-based material. 
     A base material of the resistance layer may be identical with a base material of the laser diode chip. 
     The resistance layer has at least one side surface. The laser diode chip has at least one side surface. The resistance layer is soldered to a bottom of the laser diode chip with the side surface of the resistance layer shifted from the side surface of the laser diode chip. 
     According to a second aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 μm or more which comprises: (a) a laser diode chip emitting a laser beam when energized; (b) a base having disposed thereon the laser diode chip; and (c) a junction layer disposed between the laser diode chip and the base to join the laser diode chip to the base, the junction layer having a thickness which provides a resistance of 1 mΩ or more. 
     In the preferred mode of the invention, the thickness of the junction layer is 20 μm or more. 
     According to a third aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 μm or more which comprises: (a) a laser diode chip emitting a laser beam when energized; (b) a base having disposed thereon the laser diode chip; and (c) a junction layer disposed between the laser diode chip and the base to join the laser diode chip to the base, the junction layer having formed therein a non-junction portion so as to produce a resistance of 1 mΩ or more. 
     In the preferred mode of the invention, the non-junction portion occupies in area 90% or more of the junction layer. 
     According to a fourth aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 μm or more which comprises a laser diode chip emitting a laser beam when energized. The laser diode chip includes a substrate which is made of a base material of the laser diode chip and which has a resistance of 1 Ω or more. 
     The substrate has impurities which provides the resistance of 1 mΩ or more to the substrate. 
     The substrate may alternatively have a thickness which provides the resistance of 1 mΩ or more to the substrate. 
     According to a fifth aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 μm or more which comprises: (a) a laser diode chip including (1) a first electrode layer connected to a voltage source, (2) a second electrode layer connected to ground, and (3) an active layer disposed between the upper and the lower electrode layers to emit a laser beam when energized; (b) an electric path extending from the first electrode layer to the second electrode layer through the active layer; and (c) a base having disposed thereon the laser diode chip, the base having a resistance of 1 mΩ or more and constituting a portion of the electric path between the second electrode layer and the ground. 
     In the preferred mode of the invention, the base is made of a material other than copper, silver, and gold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a perspective view which shows a heterojunction laser diode according to the first embodiment of the invention; 
     FIG. 2 is a longitudinal sectional view of FIG. 1; 
     FIG. 3 is a longitudinal sectional view for explaining a manufacturing process of the laser diode in FIG. 1; 
     FIG. 4 is a graph which shows test results of measurement of the beam divergence angle in terms of electric resistance added by the n-GaAs substrate  14  of the laser diode in FIG. 1; 
     FIG. 5 is a longitudinal sectional view which shows the first modification of the laser diode in FIG. 1; 
     FIG. 6 is a longitudinal sectional view which shows the second modification of the laser diode in FIG. 1; 
     FIG. 7 is a perspective view which shows the third modification of the laser diode in FIG. 1; 
     FIG. 8 is a longitudinal sectional view which shows the fourth modification of the laser diode in FIG. 1; 
     FIG. 9 is a longitudinal sectional view which shows the second embodiment of the invention; 
     FIG. 10 is a longitudinal sectional view for explaining a manufacturing process of the laser diode in FIG. 9; 
     FIG. 11 is a longitudinal sectional view which shows the third embodiment of the invention; 
     FIG. 12 is a longitudinal sectional view for explaining a manufacturing process of the laser diode in FIG. 11; 
     FIG. 13 is a longitudinal sectional view which shows the fourth an embodiment of the invention; 
     FIG. 14 is a longitudinal sectional view which shows a modification of the laser diode in FIG. 13; 
     FIG. 15 is a longitudinal sectional view which shows a radar system equipped with a laser diode according to the fifth embodiment of the invention; 
     FIG. 16 is a longitudinal sectional view which shows the laser diode used in the radar system of FIG. 15; 
     FIG. 17 is a longitudinal sectional view which shows the sixth embodiment of the invention; 
     FIG. 18 is a plan view which shows a laser diode in FIG. 17; 
     FIG. 19 is a longitudinal sectional view for explaining a manufacturing process of a laser diode in FIG. 17; 
     FIG. 20 is a partially sectional view which shows a junction of a laser diode chip and a substrate in the sixth embodiment; 
     FIG. 21 is a view for explaining a junction of a laser diode chip and a substrate when side walls of the laser diode chip are arranged in alignment with side walls of the substrate; 
     FIG. 22 is a view for explaining a short-circuit produced in a structure of FIG. 21; 
     FIG. 23 is a perspective view which shows a modification of the sixth embodiment; 
     FIG. 24 is a view for explaining soldering of a laser diode chip and a substrate in the sixth embodiment; 
     FIG. 25 is another modification of the sixth embodiment; 
     FIG. 26 is a longitudinal sectional view which shows a conventional lower power laser diode; 
     FIG. 27 is a longitudinal sectional view which shows a conventional high power laser diode; 
     FIG. 28 is a perspective view which shows the high power laser diode of FIG. 27; 
     FIG. 29 is a sectional view which shows a laser beam emitted by the laser diode of FIG. 27; 
     FIG. 30 is a graph which shows refractive index distribution and current distribution in a conventional high power laser diode, 
     FIG. 31 is a graph which shows refractive index distribution and current distribution in a high power laser diode of the invention; 
     FIG. 32 is a diagram which shows an equivalent circuit of a conventional high power laser diode; and 
     FIG. 33 is a diagram which shows an equivalent circuit of a high power laser diode of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein like reference numbers refer to like parts, particularly to FIGS. 1 and 2, there is shown a high power oxide stripe-geometry heterojunction laser diode, i.e., a gain-guided heterojunction laser diode according to the present invention. 
     The laser diode includes a diode body  1  (i.e., a laser diode chip  22 ), as shown in FIG. 2, which consists of an n-GaAs substrate  2 , and n-GaAs layer  3 , an n-Al 0.4 Ga 0.6 As clad layer  4 , an n-Al 0.2 Ga 0.8 As optical guide layer  5 , an active layer  6  made of multi-quantum well structure of Al 0.2 Ga 0.8 As/GaAs, a p-Al 0.2 Ga 0.8 As optical guide layer  7 , a p-Al 0.4 Ga 0.6 As clad layer  8 , and a p-GaAs contact layer  9 . 
     The active layer  6  consists of five Al 0.2 Ga 0.8 As layers and six GaAs layers which are alternately laminated. The clad layer  4 , the optical layer  5 , the active layer  6 , the optical layer  7 , the clad layer  8 , and the contact layer  9  form a mesa. The active layer  6  is coated on a front end surface, as viewed in FIG. 1, with a low reflective film and on a rear end surface with a high reflective film. 
     The n-GaAs substrate  2  has a thickness t 1  of 120 μm and contains impurities of 2×10 18  atms/cm 3 . The n-GaAs layer  3  has a thickness of 500 nm (0.5 μm). The n-Al 0.4 Ga 0.6 As clad layer  4  has a thickness of 1 μm. The n-Al 0.2 Ga 0.8 As optical guide layer  5  has a thickness of 0 to 2.25 μm. In the active layer  6 , each of the Al 0.2 Ga 0.8 As layers has a thickness of 7.5 nm (0.0075 μm), and the total thickness of the five Al 0.2 Ga 0.8 As layers comes to 37.5 nm (=7.5 nm×5). Each of the GaAs layers has a thickness of 15 nm (0.015 μm), and the total thickness of the six GaAs layers comes to 90 nm (=15 nm×6). The thickness of the active layer  6 , thus, comes to 127.5 nm (0.1275 μm). 
     The p-Al 0.2 Ga 0.8 As optical guide layer  7  has a thickness of 0 to 2.25 μm. The p-Al 0.4 Ga 0.6 As clad layer  8  has a thickness of 1 μm. The p-GaAs contact layer  9  has a thickness of 0.8 μm. 
     In this embodiment, the thickness of the active layer  6  is, as described above, 127.5 nm, and the total thickness of the active layer  6  and the optical guide layers  5  and  7  is 1.5 μm or more, thereby decreasing an ellipticity (an H-W ratio in FIG. 29) of a cross section of a laser beam. 
     The refractive index (mean refractive index) of the active layer  6  is 3.6. The refractive indexes of the n-Al O.2 Ga 0.8 As optical guide layer  5  and the p-Al 0.2 Ga 0.8 As optical guide layer  7  are 3.5. The refractive indexes of the n-Al 0.4 Ga 0.6 As clad layer  4  and the p-Al 0.4 Ga 0.6 As clad layer  8  are 3.3. 
     The diode body  1  also includes insulating films  10  made of SiO 2  and an upper electrode layer  12  made of Cr/Pt/Au laminates. The insulating films  10  formed on both side portions of the n-GaAs layer  3  and extend to upper side portions of the contact layer  9  through side surfaces of the layers  4  to  9  (i.e., on the mesa) so as to define a window  11  on an upper surface of the mesa. The upper electrode layer  12  is formed on the insulating films  10  and the window  11  in ohmic contact with the p-GaAs contact layer  9 . The width of the window  11 , or stripe width is 100 μm or more, e.g., 400 μm in this embodiment. The upper electrode layer  12  has a sheet resistance of, for example, 30 to 70 mΩ/m 2 . The diode body  1  measures 500 μm×600 μm. 
     On the bottom of the n-GaAs substrate  2 , an n-GaAs substrate  14  serving as a resistance layer is formed through an alloy layer  13  serving as a lower electrode layer and a junction layer. The n-GaAs substrate  14  has a thickness of 240 μm. On the bottom of the n-GaAs substrate  14 , a Cu-made block  16  is disposed through an alloy layer  15 . The Cu-made block  16  serves as a heatsink and has an upper peripheral surface plated with gold  17  which is in electric connection to ground and the alloy layer  15 . 
     Au-made wires  12   a  and  12   b  are connected to the upper surface of the upper electrode layer  12 . Note that the resistance of the upper electrode layer  12  is low, and the current density does not depend upon the number of the Au-made wires. 
     With the above arrangements, the alloy layer  13  is connected to ground through the n-GaAs substrate  14 , the alloy layer  15 , and the gold-plated layer  17 . The n-GaAs-substrate  14  has a resistance of 1 mΩ or more. 
     Next, a manufacturing method of the laser diode will be discussed below with reference to FIG.  3 . 
     First, on the n-GaAs substrate  2 , the n-GaAs layer  3 , the n-Al 0.4 Ga 0.6 As clad layer  4 , the n-Al 0.2 Ga 0.8 As optical guide layer  5 , the Al 0.2 Ga 0.8 As/GaAs active layer  6 , the p-Al 0.2 Ga 0.8 As optical guide layer  7 , the p-Al 0.4 Ga 0.6 As clad layer  8 , and the p-GaAs contact layer  9  are formed, in sequence, using MOCVD (Metal, Organic Chemical Vapor Deposition) techniques, after which the mesa is formed by etching. 
     On upper surfaces of the n-GaAs layer  3  and the mesa, a layer of SiO 2  (i.e., the insulating layers  10 ) is deposited by plasma CVD techniques, and then a central portion thereof is chemically etched away to form the window  11 . On the insulating layers  10 , the upper electrode layer  12  made of Cr/Pt/Au laminates is formed. On the bottom of the n-GaAs substrate  2 , an electrode layer  20  is formed with Au-Ge/Ni/Au laminates. These layers are then subjected to an alloy process to form ohmic contacts with the electrode layers  12  and  20 . On the electrode layer  20 , a soldering layer  21  made of Au-Sn is vapor deposited. End walls of the thus formed block are polished or cleaved to complete the laser diode chip  22 . 
     Besides the laser diode chip  22 , an electrode layer  23  is formed with Cr/Pt/Au laminates on an upper surface of the n-GaAs substrate  14  having a thickness of 240 μm, while on a lower surface of the n-GaAs substrate  14 , an electrode layer  24  is formed with Au-Ge/Ni/Au laminates. These layers are then subjected to an alloy process at 350° C. for two minutes to form ohmic contacts with the electrode layers  23  and  24 . Subsequently, a soldering layer  25  made of Au-Sn is deposited on the electrode layer  24  to complete a resistance layer  26 . 
     A base  27  consisting of the Cu-made block  16  and the gold-plated layer  17  is prepared. To the gold-plated layer  17 , the resistance layer  26  and the laser diode chip  22  are attached in sequence by die-bonding. This causes the lower electrode layer  20  of the laser diode chip  22  and the upper electrode layer  23  of the resistance layer  26  to be alloyed with the soldering layer  21  to form the alloy layer  13 , as shown in FIG. 2, and also causes the lower electrode layer  24  of the resistance layer  26  and a portion of the gold-plated layer  17  on the base  27  to be alloyed with the soldering layer  25  to form the alloy layer  15 , as shown in FIG.  2 . 
     In operation, application of the pulse voltage across the upper electrode layer  12  and the gold-plated layer  17  of the block  16  causes the pulse current to flow between the upper electrode layer  12  and the alloy layer  13 , positive holes and electrons being thereby injected into the active layer  6  from the p-Al 0.4 Ga 0.6 As clad layer  8  and the n-Al 0.4 Ga 0.6 As clad layer  4 , respectively, so that they are recombined to emit light. The light repeats reflection on the cleaved front and rear surfaces of the laser diode chip  22  and is amplified so that a laser beam is emitted from the front surface. 
     The n-GaAs substrate  14  exhibits a small resistance (1 mΩ or more) between the diode body  1  (i.e., the laser diode chip  22 ) and the Cu-made block  16 , thereby causing the current distribution, as shown by the solid line in FIG. 31, to be uniform or flat as compared with the Gaussian distribution shown by a broken line. This results in flat refractive index distribution of the active layer  6  so that an equiphase surface of light transmitted through the active layer  6  is oriented to the direction in which a beam of light is outputted, thereby improving the coherence and decreasing the divergence angle of the output beam to enhance the directivity thereof greatly. 
     FIG. 4 illustrates test results of measurement of the beam divergence angle in terms of electric resistance added by the nGaAs substrate  14 . The addition of resistance causes, as clearly shown in the graph, the beam divergence angle to be narrowed and almost kept constant in a resistance range of more than about 4 mΩ. 
     Note that the measurement in FIG. 4 was performed using a laser diode having a stripe width of 360 μm. 
     FIG. 5 shows a first modification of the heterojunction laser diode as described above. 
     In this modification, an n-GaAs substrate  30  serving as a resistance layer is formed on an upper electrode layer  31  of the laser diode chip  22 . The n-GaAs substrate  30  has a resistance of 1 mΩ or more. 
     An alloy layer  31  is disposed between the n-GaAs substrate  30  and the laser diode chip  22 . An electrode layer  32  is disposed on the n-GaAs substrate  30 . 
     It is found that the structure in FIG. 1 is preferred to that in FIG. 5 for good FFP (Far-Field Pattern). 
     FIG. 6 shows a second modification of the heterojunction laser diode in FIG.  1 . 
     An n-GaAs substrate  14  serving as a resistance layer is disposed on the bottom of the lower electrode layer  13  of the laser diode chip  22 . A resistance layer  33  is disposed on an upper surface of the upper electrode layer  12 . The total resistance of the n-GaAs substrate  14  and the resistance layer  33  is 1 mΩ or more. 
     FIG. 7 shows a third modification of the heterojunction laser diode in FIG.  1 . 
     Grooves  34  are formed in parallel in the n-GaAs substrate  14  for increasing the resistance of the n-GaAs substrate  14 . Higher resistance rod members may be inserted into the grooves  34 . 
     The resistance layer (i.e., the substrate  14 ) may be formed with laminates. 
     The laser diode substrate  2  is made of GaAs-AlGaAs based materials, but may alternatively be made of InP-InGaAsP based materials. 
     The upper electrode layer  12  may alternatively be made of Cr/Au laminates or Ti/Pt/Au laminates. The lower electrode layer  20  may alternatively be made of Au-Ge/Au laminates. 
     The resistance layers are not limited in material to n-GaAs and may be made of p-type conductive materials or another material such as a Si or an InP-based material or may be formed with an insulating member which is made of diamond or aluminum nitride and which is plated with a high-resistance material. 
     The soldering layer  25  used to join the laser diode chip  22  to the base  27  may alternatively be made of Sn-Pb based material, In-based material, or silver paste. 
     The resistance layer  14  may be greater or smaller in size than the laser diode chip  22 . 
     The laser diode may have a junction-down (upside-down) structure, as shown in FIG. 8, wherein the laser diode chip  22  is joined to the block  16  with the mesa thereof facing thereto. 
     FIG. 9 shows a laser diode of the second embodiment. 
     An alloy layer  40  is disposed on the bottom of the n-GaAs substrate  2  to serve as a junction layer to join the substrate  2  to a Cu-made block  16 . The alloy layer  40  has a thickness t 2  of 20 μm or more and a resistance of 1 mΩ or more and is formed, as shown in FIG. 10, by joining the laser diode chip  22  through a 25 μm-thick Au-Sn soldering film  41  onto a base  27  consisting of the Cu-made block  16  and a gold-plated layer  17 . The interposition of the Au-Sn soldering film  41  between the laser diode chip  22  and the base  27  provides a resistance of 1 mΩ or more to the alloy layer  40 . 
     Other arrangements are identical with those of the first embodiment and the modifications thereof, and explanation thereof in detail will be omitted there. 
     FIG. 11 shows a laser diode of the third embodiment. 
     An alloy layer  50  which has formed therein non-junctions  51   a,    51   b,  and  51   c  is disposed on the bottom of the n-GaAs substrate  2 . The total area (S 1 +S 2 +S 3 ) of the non-junctions  51   a,    51   b,  and  51   c  occupies 90% or more of the area S of the bottom of the n-GaAs substrate  2  ((S 1 +S 2 +S 3 )/S≧90%). The alloy layer  50  is, as shown in FIG. 12, formed by joining the laser diode chip  22  through the AuSn soldering film  21  to the base  27  consisting of the Cu-made block  16  and the gold-plated layer  17 . The joining is performed at 300° C. in order to leave the non-junctions  51   a  to  51   c  in the alloy layer  50 . If the laser diode chip  22  and the gold-plated layer  17  of the base  27  are joined through the soldering film  21  at about 350° C., then they are alloyed entirely. 
     The presence of the non-junctions  51   a  to  51   c  decreases a joined area of the laser diode chip  22  and the base  27  in the alloy layer  50 , thereby resulting in an increased resistance of the alloy layer  50  greater than or equal to 1 mΩ. 
     FIG. 13 shows a laser diode of the fourth embodiment which is different from the first embodiment in the concentration of impurities in the n-GaAs substrate  2  made of a base material of the laser diode chip  22  and the absence of the alloy layer  13  and the substrate  14 . 
     Specifically, while the n-GaAs substrate  2  in the first embodiment contains impurities of 2×10 18  atms/cm 3 , the n-GaAs substrate  2  in this embodiment contains impurities of 6.67×10 17  atms/cm 3  to have a high resistance of 1 mΩ or more. This provides the same effect as that in the first embodiment. 
     The increase in resistance of the substrate  2  may alternatively be achieved by increasing the thickness t 1 t hereof to 360 μm. 
     FIG. 14 shows a modification of the fourth embodiment which has vertically piled laser diode chips  55  each identical with the one shown in FIG.  13 . In this modification, the lower one of the laser diode chips  55  serves as a resistance layer like the n-GaAs substrate  14  in the first embodiment. 
     FIG. 15 shows a radar system equipped with a laser diode of the fifth embodiment as shown in FIG.  16 . 
     The radar system includes the laser diode chip  66  mounted on a Cu-made stem  60  through a base  65 . The stem  60  has disposed therein lead pins  61  and  63 . The lead pin  63  is inserted into a hole  62  in the stem  60  and hermetically sealed with glass  64 . The lead pin  63  and the laser diode chip  66  are connected through a wire  67 . The base  65  and the laser diode chip  66  are covered with an aluminum cap  68 . The lead pin  61  is connected to ground. The lead pin  63  is supplied with the pulse voltage. The lead pin  61 , the stem  60 , the base  65 , the laser diode chip  66 , the wire  67 , and the lead pin  63  form an electric circuit. When the pulse voltage is applied to the lead pin  60 , the laser diode chip  66  emits a laser beam out of a window  69  formed in the cap  68 . 
     The base  65 , as shown in FIG. 16, includes a block  70  and a gold-plated layer  17  to which the laser diode chip  66  is joined The block  70  is made of Fe to have an electric resistance of 1 mΩ or more which is higher than that of a Cu-made block like the block  16  in FIG.  2 . This provides the same effect as that in the first embodiment. 
     Moreover, the Fe-made base  65  has substantially the same coefficient of thermal expansion as that of the n-GaAs substrate  2 , thereby reducing the thermal stress acting on the substrate  2  and the base  65  when soldered together. 
     FIG. 17 shows a laser diode of the sixth embodiment. 
     Like the first embodiment, the n-GaAs substrate  14  is joined to the bottom of the n-GaAs substrate  2  through the alloy layer  13 , and the Cu-made block  16  is joined to the bottom of the n-GaAs substrate  14  through the alloy layer  15 . The n-GaAs substrate  14  has a thickness of 240 μm. The alloy layer  13  is connected to ground through the n-GaAs substrate  14  having a resistance of 1 mΩ or more, the alloy layer  15 , and the gold-plated layer  17  on the block  16 . 
     Both the n-GaAs substrate  2  and the n-GaAs substrate  14  are rectangular parallelepipeds. The n-GaAs substrate  14  is greater in area than the laser diode chip  22  and is arranged in alignment of a vertical center line thereof with that of the laser diode chip  22 . Specifically, sides of the n-GaAs substrate  14  extend parallel to those of the laser diode chip  22 . This will be discussed in more detail below. 
     In the following discussion, a wall of the laser diode chip  22  from which a laser beam is outputted is, as shown in FIG. 18, referred to as a front wall F 1 , walls neighboring the front wall F 1  are referred to as side walls F 2  and F 3 , respectively, and a wall opposite the front wall F 1  is referred to as a rear wall F 4 . Similarly, a wall of the n-GaAs substrate  14  oriented to the same direction as that of the front wall F 1  is referred to as a front wall F 11 , walls neighboring the front wall F 11  are referred to as side walls F 12  and F 13 , respectively, and a wall opposite the front wall F 11  is referred to as a rear wall F 14 . 
     The front wall F 11  of the n-GaAs substrate  14  is located outward at a distance L 1  from the front wall F 1  of the laser diode chip  22 . The side wall F 12  of the n-GaAs substrate  14  is located outward at a distance L 2  (=L 1 ) from the side wall F 2  of the laser diode chip  22 . The side wall F 13  of the n-GaAs substrate  14  is located outward at a distance L 3  (=L 1 =F 2 ) from the side wall F 3  of the laser diode chip  22 . The rear wall F 14  of the n-GaAs substrate  14  is located outward at a distance L 4  (=L 1 =L 2 =L 3 ) from the rear wall F 4  of the laser diode chip  22 . 
     Specifically, all the walls F 11  to F 14  of the n-GaAs substrate  14  lie outside the walls F 1  to F 4  of the laser diode chip  22 . In other words, the n-GaAs substrate  14  is longer than the laser diode chip  22  in an optical axis direction and also wider than the laser diode chip  22  in a direction perpendicular to the optical axis. 
     A manufacturing method of the laser diode of this embodiment will be described below with reference to FIG.  19 . 
     First, on the n-GaAs substrate  2 , the n-GaAs layer  3 , the n-Al 0.4 Ga 0.6 As clad layer  4 , the n-Al 0.2 Ga 0.8 As optical guide layer  5 , the Al 0.2 Ga 0.8 As/GaAs active layer  6 , the p-Al 0.2 Ga 0.8 As optical guide layer  7 , the p-Al 0.4 Ga 0.6 As clad layer  8 , and the p-GaAs contact layer  9  are laminated in sequence, after which the mesa is formed by etching. 
     On upper surfaces of the n-GaAs layer  3  and the mesa, a layer of SiO 2  (i.e., the insulating layers  10 ) is deposited by plasma CVD techniques, and then a central portion thereof is chemically etched away to form the window  11 . On the insulating layers  10 , the upper electrode layer  12  made of Cr/Pt/Au laminates is formed. On the bottom of the n-GaAs substrate  2 , an electrode layer  20  is laminated with Au-Ge/Ni/Au. These layers are then subjected to an alloy process to form ohmic contacts with the electrode layers  12  and  20 . On the electrode layer  20 , a soldering layer  21  made of Au-Sn is vapor deposited. End walls of the thus formed block are cleaved to complete the laser diode chip  22 . 
     The n-GaAs substrate  14  which is greater in size than the nGaAs substrate  2  is prepared. An electrode layer  23  is formed with Cr/Pt/Au laminates on an upper surface of the n-GaAs substrate  14 , while on a lower surface of the n-GaAs substrate  14 , an electrode layer  24  is formed with Au-Ge/Ni/Au laminates. These layers are then subjected to an alloy process at 350° C. for two minutes to form ohmic contacts with the electrode layers  23  and  24 . Subsequently, a soldering layer  25  made of Au-Sn is deposited on the electrode layer  24  to complete a resistance layer  26 . 
     Finally, a base  27  consisting of the Cu-made block  16  and the gold-plated layer  17  is prepared. The laser diode chip  22  is joined to the resistance layer  26  and the gold-plated layer  17  in sequence by the die bonding. 
     The joining of the laser diode chip  22  and the resistance layer  26  is achieved with the soldering layer  21 . When the solder layer  21  is melted, the solder stays, as shown in FIG. 20, on the substrate  14 , without flowing over the side walls of the substrates  2  and  14 . Specifically, the location of the walls F 1  to F 4  of the laser diode chip  22  inside the walls F 11  to F 14  of the n-GaAs substrate  14  prevents a portion of melted solder from flowing out of the soldering layer  21 , as illustrated at numeral  80  in FIG. 21, which may adversely affect the operation of the laser diode chip  22  or from being separated from the soldering layer  21  in the form of a ball, as illustrated at numeral  81 , which may roll in a diode package and collide with the laser diode chip  22  to damage it or which may be stuck to the front wall F 1  of the laser diode chip  22 , causing the intensity of a laser beam to be decreased. Particularly, in the case of a laser diode, as shown in FIG. 22, wherein the laser diode chip  22  is disposed upside down, the portion  80  of the melted solder flowing out the soldering layer  21  would cause a short-circuit between the laser diode chip  22  and the substrate  14 . 
     When the laser diode chip  22  and the resistance layer  26  are joined through the soldering layer  21 , it will cause the lower electrode layer  20  of the laser diode chip  22  and the upper electrode layer  23  of the resistance layer  26  to be alloyed with the soldering layer  21  to form the alloy layer  13 . Similarly, when the resistance layer  26  and the base  27  are joined through the soldering layer  25 , it will cause the lower electrode layer  24  of the resistance layer  26  and the gold-plated layer  17  on the base  27  to be alloyed with the soldering layer  25  to form the alloy layer  15 . 
     In the sixth embodiment, the four walls F 1  to F 4  of the laser diode chip  22  are all located inside the walls F 11  to F 14  of the n-GaAs substrate  14 , however, somewhat similar effects as those offered by this embodiment may be obtained as long as at least one of the walls F 1  to F 4  lies inside corresponding one of the walls F 11  to F 14 . The walls F 1  to F 4  of the laser diode chip  22  may alternatively be, as shown in FIG. 23, located outside the walls F 11  to F 14  of the n-GaAs substrate  14 . In this case, a portion of melted solder flowing out of the soldering layer  21  is, as illustrated at  84  in FIG. 24, held at a corner of a step of the substrate  2  (i.e., the electrode layer  20 ) without being separated from the soldering layer  21 . 
     The n-GaAs substrate  14  may alternatively be, as shown in FIG. 25, shifted an angle θ horizontally from the laser diode chip  22 . FIG. 25 illustrate for the case where the laser diode chip  22  and the substrate  14  are identical in shape and size and arranged so that center lines thereof may be aligned with each other. 
     While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate a better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the present invention may be used with buried heterostructure laser diodes having the stripe-geometry.