Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/0201—Separation of the wafer into individual elements, e.g. by dicing, cleaving, etching or directly during growth
  • H01S5/0202—Cleaving
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/32—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
  • H01S5/323—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
  • H01S5/32308—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm
  • H01S5/32341—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm blue laser based on GaN or GaP
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/01—Manufacture or treatment
  • H10H20/032—Manufacture or treatment of electrodes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/822—Materials of the light-emitting regions
  • H10H20/824—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP
  • H10H20/825—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP containing nitrogen, e.g. GaN
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/831—Electrodes characterised by their shape
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
  • H10H20/835—Reflective materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/85—Packages
  • H10H20/858—Means for heat extraction or cooling
  • H10H20/8581—Means for heat extraction or cooling characterised by their material

Definitions

• This invention relates to the fabrication of semiconductor devices for light emission and refers particularly, though not exclusively, to the fabrication of such semiconductor devices on a sapphire substrate.
• GaN semiconductor devices such as, for example, light emitting diodes (“LEDs”), laser diodes, photodetectors, transistors, switches, and so forth, are widely used in many applications.
• Well known applications include, but are not limited to, traffic signals, mobile telephone display backlighting, liquid crystal display (“LCD”) back lighting, flash lights for cameras, and so forth.
• a semiconductor device for light emission comprising:
• a plurality of epitaxial layers comprising an active layer for light generation, an n-type layer for light transmission and a p-type layer for light reflection;
• the p-type layer having thereon at least one seed layer for an outer layer of a conductive metal;
• At least one seed layer comprising a material for providing a buffer for differential thermal expansion of the outer layer and the light reflecting layer.
• the at least one seed layer may also comprises a diffusion barrier for providing a barrier to diffusion of a layer applied to it from diffusing into at least one of the p- type layer, the active layer and the n-type layer.
• a semiconductor device for light emission comprising:
• At least one seed layer comprising a diffusion barrier for providing a barrier to diffusion of a layer applied to it from diffusing into at least one of the p- type layer, the active layer and the n-type layer.
• the at least one seed layer may also comprise a material for providing a buffer for differential thermal expansion of the outer layer and the light reflecting layer.
• the at least one seed layer may comprise a plurality of seed layers, the plurality of seed layers comprising a first seed layer of reflective material, and having a first co-efficient of thermal expansion, and a second seed layer of a second material having a second co-efficient of thermal expansion.
• the second co- efficient of thermal expansion may be greater than the first co-efficient of thermal expansion.
• the n-type layer may comprise an array of an n-type metal.
• the outer layer and the array of n-type metal may comprise the terminals of the semiconductor device.
• a semiconductor device for light emission comprising a plurality of epitaxial layers, the plurality of epitaxial layers comprising:
• an n-type metal on the n-type layer (e) an n-type metal on the n-type layer; (f) an active layer between the n-type layer and the p-type layer; the n-type metal being arranged in an array at the centre of the n-type layer for , minimizing its. effect on light output; the n ⁇ type metal and the outer layer being the terminals for the semiconductor device.
• the plurality of seed layers may comprise a first seed layer of reflective material and having a first co-efficient of thermal expansion, and a second seed layer of a second material having a second co-efficient of thermal expansion; the second coefficient of thermal expansion being greater than the first co-efficient of thermal expansion.
• first seed layer and the second seed layer there may be at least one intermediate seed layer of at least one intermediate material having a intermediate co-efficient of thermal expansion, the intermediate coefficient of thermal expansion being greater than the first co-efficient of thermal expansion and less that the second co-efficient of thermal expansion.
• the outer layer may be of the second material.
• the reflective material, the second material and the intermediate material may all be different.
• the intermediate material may be the diffusion barrier for preventing the second material diffusing into the epitaxial layers.
• the outer layer may be relatively thick and may be for at least one of: a structural support, a heat sink, a heat dissipater, a current dissipater, and as a terminal, for the semiconductor device.
• the array may comprise a central portion, an outer portion, and a joining portion connecting the central portion and the outer portion; the outer portion and the joining portion being for current dissipation.
• the semiconductor device may be a gallium nitride semiconductor device; and the outer layer may be of the second material.
• a semiconductor device for light emission comprising:
• One of the plurality of seeds layers may be a diffusion barrier for providing a barrier to diffusion of a layer applied to it from diffusing into the p-type layer.
• a method for fabrication of a semiconductor device for light emission comprising: (a) on a p-type layer of plurality of epitaxial layers of the semiconductor device, forming a layer of a p-type metal;
• the at least one seed layer may comprise a plurality of seed layers, a first seed layer being of a first material that is light reflective and has a first co-efficient of thermal expansion; and forming on the first seed layer a second seed layer of the plurality of seed layers, the second seed layer being of a second material that has a second co-efficient of thermal expansion, the second co-efficient of thermal expansion being greater than the first co-efficient of thermal expansion.
• the method may further comprise forming an outer layer on the second seed layer, the outer layer being relatively thick and being for at least one selected from the group consisting of: a structural support, a heat sink, a heat dissipater, a current dissipater, and as a terminal, for the semiconductor device.
• An array of an n-type metal may be formed on an n-type metal layer of the plurality of epitaxial layers. The array and the outer layer may be the terminals of the semiconductor device.
• a method of fabricating a semiconductor device for light emission comprising a plurality of epitaxial layers, the method comprising: on a p-type layer of the plurality of epitaxial layers forming a first seed layer of a plurality of seed layers as a reflective layer; on the plurality of seed layers forming an outer layer of a conductive material; on an n-type layer of the plurality of epitaxial layers forming an n-type metal in an array at the centre of the n-type layer for minimizing its effect on light output; the n-type metal and the outer layer__being the terminals for the semiconductor device.
• the first seed layer may be of a reflective material and has a first co-efficient of thermal expansion, the method preferably further comprising forming on the first seed layer a second seed layer of a second material having a second co-efficient of thermal expansion; the second co-efficient of thermal expansion being greater than the first co-efficient of thermal expansion.
• the methods may further comprise forming at least one intermediate seed layer on the first seed layer before the second seed layer is formed, the at least one intermediate seed layer being of at least one material having an intermediate co- efficient of thermal expansion that is greater than the first co-efficient of thermal expansion and less than the second co-efficient of thermal expansion.
• the outer layer may be of the second material.
• the reflective material, the second- material and the intermediate seed layer material may all be different.
• the semiconductor device may be a gallium nitride semiconductor device.
• a method of fabricating a semiconductor device for light emission comprising a plurality of epitaxial layers mounted on a substrate, the method comprising: separating the substrate from the plurality of epitaxial layers while the plurality of epitaxial layers are intact for preserving electrical and mechanical properties of the plurality of epitaxial layers.
• the method may further comprise: prior to separation forming at least one seed layer on the plurality of epitaxial layers, and forming an outer layer on the at least one seed layer, the outer layer being relatively thick and being for at least one selected from the group consisting of: a structural support, a heat sink, a heat dissipater, a current dissipater, and as a terminal, for the semiconductor device.
• the following steps may be performed:
• a pattern of thick resists may be applied to the at least one seed layer, the outer layer being formed between the pattern of thick resists.
• the outer layer may also be formed over the pattern of thick resists.
• the outer layer may be polished subsequent to separation.
• the oxide layer may be silicon dioxide.
• the following steps may be performed: (a) a first stage of isolation of individual devices by trench etching along edges of each mesa;
• Figure 1 is a non-scale schematic, cross-sectional view of a semiconductor at a first stage in the fabrication process
• Figure 2 is a non-scale schematic, cross-sectional view of a semiconductor at a second stage in the fabrication process
• Figure 3 is a non-scale schematic, cross-sectional view of a semiconductor at a third stage in the fabrication process
• Figure 4 is a non-scale schematic, cross-sectional view of a semiconductor at a fourth stage in the fabrication process
• Figure 5 is a non-scale schematic, cross-sectional view of a semiconductor at a fifth stage in the fabrication process
• Figure 6 is a non-scale schematic, cross-sectional view of a semiconductor at a sixth stage in the fabrication process
• Figure 7 is a non-scale schematic, cross-sectional view of a semiconductor at a seventh stage in the fabrication process
• Figure 8 is a non-scale schematic, cross-sectional view of a semiconductor at an eighth stage in the fabrication process
• Figure 9 is a non-scale schematic, cross-sectional view of a semiconductor at a ninth stage in the fabrication process
• Figure 10 is a non-scale schematic, cross-sectional view of a semiconductor at a tenth stage in the fabrication process
• Figure 11 is a non-scale schematic, cross-sectional view of a semiconductor at an eleven stage in the fabrication process
• Figure 12 is a non-scale schematic, cross-sectional view of a semiconductor at a twelfth stage in the fabrication process
• Figure 13(a) is a non-scale schematic, cross-sectional view of a semiconductor at a thirteenth stage in the fabrication process
• Figure 13(b) is a bottom view of the semiconductor of Figure 13(a);
• Figure 14(a) is a non-scale schematic, cross-sectional view of a semiconductor at a fourteenth stage in the fabrication process
• Figure 14(b) is a bottom view of the semiconductor of Figure 14(a);
• Figure 15(a) is a non-scale schematic, cross-sectional view of a semiconductor at a fifteenth stage in the fabrication process;
• Figure 15(b) is a bottom view of the semiconductor of Figure 15(a);
• Figure 16 is a non-scale schematic, cross-sectional view of a semiconductor at a sixteenth stage in the fabrication process
• Figure 17(a) is a non-scale schematic, cross-sectional view of a semiconductor at a seventeenth stage in the fabrication process
• Figure 17(b) is a bottom view of the semiconductor of Figure 17(a).
• the GaN devices described below are fabricated from epitaxial wafers that consist of a stack of thin semiconductor layers (called epitaxial layers) on a sapphire substrate.
• the composition and thickness of the epitaxial layers depends on the wafer design, and determine the light color (wavelength) of light that will be emitted by the devices that are fabricated from the wafer.
• a thin buffer layer is first deposited on the sapphire substrate with a thickness often in the range 10 to 30 nm, and can be either AIN or GaN. In this specification this layer is not described or illustrated.
• other layers made of GaN, AIGaN, InN, InGaN, AIGaInN, and so forth, are deposited.
• n-type layers are often deposited on the buffer layer, followed by an unintentionally doped active region. Finally, p-type doped layers are deposited.
• the active region is usually a double heterostructure made of a single quantum well, or multiple quantum wells and is for light generation. But it may be in other forms such as, for example, quantum dots.
• the deposition of epitaxial layers is usually by metal organic chemical vapor deposition ("MOCVD") or molecular beam epitaxy (“MBE"). The thickness of the epitaxial layers is in the range from a few nanometers to a few microns.
• the process starts after the sapphire substrate 4 has applied to it the n-type layer 3 of gallium nitride (GaN), the quantum well or active layer 2, and the p-type layer 1 of GaN.
• the p-metal layer 5 is then applied over the p-type layer 1.
• the p-type metal layer 5 may be of nickel-gold (NiAu) or other suitable metal.
• Standard photolithography and etching are then used to pattern layer 5. This is done by applying a thin layer of photoresist (layer 6(a) in Figure 2) on to metal layer 5, followed by resist exposure and development.
• the resist pattern 6(a) serves as an etching mask for etching the metal layer 5.
• the etching may be by wet chemical etching or plasma dry etching (see Figure 2).
• the photoresist 6(a) is then removed.
• the patterned layer 5 that remains on the surface of p-type GaN layer 1 will serve as an ohmic contact layer to the p-type GaN layer 1. Annealing may take place either before or after layer 5 is patterned.
• the p-type Layer 1 is relatively thin - normally no more, but preferably less, than 1 micron.
• a layer 7 of silicon dioxide (SiO 2 ) is deposited over the remaining p-metal layer portions 5 and the P-type GaN layer 1 ( Figure 3) by a standard thin film deposition method. This may be by plasma enhanced chemical vapor deposition ("PECVD"), sputtering, evaporation, or other suitable techniques. As shown in Fig. 4, a second photoresist layer 6(b) is applied over the oxide layer 7. The resist is then patterned and serves as mask for patterning the oxide layer 7. Wet etching or dry etching (plasma etching) of the oxide layer 7 is carried out. The oxide in the areas where there is no photoresist 6(b) is removed, while oxide 7 protected by the resist 6(b) remains after etching.
• PECVD plasma enhanced chemical vapor deposition
• the patterned second resist layer 6(b) is larger in area than the NiAu layer 5 so that the SiO 2 layer 7 remaining extends across the NiAu layer 5 and down the sides of NiAu layer 5 to the p-type GaN layer 1 , as shown in Figure 4.
• the second resist layer 6(b) is removed for mesa etching of the p-type GaN layer 1 , the quantum well layer 2 and the n-type GaN layer 3.
• the etching is for the full depth of the p-type GaN layer 1 , and the quantum well layer 2, but for a small part only of the depth of the n-type GaN layer 3.
• the patterned oxide 7 in Figure 5 serves as the dry etching mask to define the mesa on which the device is subsequently formed.
• the S1O 2 layer 7 of Figure 5 is removed and replaced by a full coating isolation layer 8 of SiO 2 ( Figure 6) that covers the entire top surface.
• a photoresist layer 6(c) is then applied over the SiO 2 layer 8.
• the photoresist layer 6(c) is then patterned by light exposure and ' subsequent development, so that it covers everywhere except the center part of the mesas - over the P-type metal layer 5.
• the third resist layer 6(c) extends around the periphery and toward the center, with the third resist layer 6(c) being generally annular. As such the third resist layer 6(c) does not cover the central portion 17 of SiO 2 layer 8.
• SiO 2 window etching follows to remove the central portion 17 of the SiO 2 layer 8 above the NiAu layer 5 to thus expose the top of the NiAu layer 5.
• the third resist layer 6(c) is then removed and seed layer deposition follows, as is shown in Figure 8.
• the seed layer is of three different metal layers.
• the first seed layer 11 adheres well to the NiAu layer 5 and may be of chromium or titanium. It is followed by second layer 10 and third layer 9 of tantalum and copper respectively. Other materials may be used.
• the first seed layer 11 preferably has good reflectivity for the reflection of light generated in the light emitting device.
• the second seed layer 10 acts as a diffusion barrier, preventing copper or other materials placed on top of it (such as, for example, the third seed layer 9) from diffusing into the Ohmic contact layers and the semiconductor epitaxial layers.
• the third seed layer 9 acts as a seeding layer for subsequent electroplating.
• the coefficients of thermal expansion of the seed layers may be different from that of GaN which is 3.17. While the thermal expansion coefficients of the Ohmic contact layers (Ni and Au) are also different from that of GaN (they are 14.2 and 13.4 respectively), they are relatively thin (a few nanometers) and do not pose serious stress problems to the underlining GaN epitaxial layers. However, plated copper to be added later may be as thick as hundreds of microns and thus may cause severe stress problems. Thus, the seed layers can be used to buffer the stress. This may be by one or more of: (a) by having sufficient flexibility to absorb the stress,
• the first layer 11 preferably less than that of the second layer 10, and that of the second layer is preferably less than that of the third layer 9.
• the first layer 11 may be chromium with a coefficient of thermal expansion of 4.9
• the second layer 10 may be tantalum with a coefficient of thermal expansion of 6.3
• the third layer 9 may be copper with a coefficient of thermal expansion of 16.5.
• the coefficients of thermal expansion are graded from the SiO 2 layer 8 and GaN layer to the outer, copper layer 9.
• An alternative is to have coefficients of expansion that differ such that at the temperatures concerned, one metal layer expands while another contracts.
• the differences in their thermal expansion rates may cause cracking, separation, and/or failure.
• the stresses of thermal expansion are spread through the layers 11 , 10 and 9 with the resultant lower likelihood of cracking, separation and/or failure.
• the first seed layer 11 should be of a material with a relatively low coefficient of thermal expansion, whereas the final layer 9 may have a higher coefficient of thermal expansion.
• the intermediate layer(s) should have coefficient(s) of expansion between those of layers 11 and 9, and should be graded from that of the first layer 11 to that of the final layer 9. There may be no intermediate layer 10, or there may be any required or desired number of intermediate layers 10 (one, two, three and so forth).
• the seed layers 9, 10 and 11 may be replaced by a single layer of dielectric such as, for example, AIN with vias or holes therethrough to enable the copper layer 9(a) to connect to the p-type metal layer 5.
• a pattern of thick resists 12 is applied to the outer, copper seed layer 9 by standard photolithography ( Figure 9), and the remaining metal 9(a) is plated in the regions defined by the thick resists 12 ( Figure 10), and then plated over the thick resists 12 to form a single metal support layer 9(a).
• the outer, seed copper layer 9 may be partially etched in the center of the street between the mesas for the formation of the thick photoresists 12 ( Figure 9) and plating of the main copper layer 9(a) ( Figure 10). This has the advantage of improved adhesion.
• the removal or lift-off of the sapphire substrate 5 then takes place ( Figure 1 1 ) in accordance with known techniques such as, for example, that described in Kelly [M. K. Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, and M. Stutzmann, phys. stat. sol. (a) 159, R3 (1997)].
• the substrate may also be removed by polishing or wet etching. This exposes the lowermost surface 13 of the n-type GaN layer 3. It is preferred for lift-off of the substrate 5 to take place while to epitaxial layers are intact to improve the quality of removal, and for structural strength. By having the epitaxial layers intact at the time of removal the electrical and mechanical properties of the epitaxial layers are preserved.
• the thickly plated metal 9(a) acts as: the new mechanical support; and during operation of the semiconductor device is able to act as one or more of: a heat sink, a heat dissipater, a terminal for the p- type layer 1 , and as a current dissipater.
• the p-type layer 1 is relatively thin, the heat generated in active layer 2 is more easily able to be conducted to the thick layer 9(a).
• the devices are then isolated from each other by trench etching from the newly exposed surface along the edges of the mesa, as shown in Figures 12 to 14, with a photoresist layer 6(d) protecting the regions of the n-type GaN layer 3 during the etching- process.
• the lowermost surface 13 of the n-type layer 3 may be cleaved at locations in alignment with the photoresists 12 and the. dies separated. This is of advantage for laser diodes as the exposed side surfaces of the n-type layer 3 are substantially parallel, thus causing a large amount of total internal reflection. This acts as a light amplification system for improved, and directed, light output.
• Pad etching takes place after applying a fifth resist layer 6(e) over the exposed surfaces of the SiO 2 layer 8, the sides of the n-type GaN layer 3, and the center of the n-type GaN layer 3 ( Figures 13(a) and (b)) thus forming projecting portions 14 and recess portions 15 of n-type GaN layer 3.
• the resist 6(e) is then removed and a sixth resist 6(f) applied over the exposed surfaces of the n-type GaN layer 3 and the outer periphery of the SiO 2 layer 8 to thus leave a gap 16 for die isolation. Etching takes place ( Figure 14) through the gap 16 and the SiO 2 layer 8, and seed layer 11 until the ends' of the thick photoresists 12 are exposed. The resist 6(f) is removed.
• a seventh resist layer 6(g) is applied over all exposed lower-surfaces from the edge of the SiO 2 layer 8 through to adjacent the center of the n-type GaN layer 3, where a central gap 17 remains (Figure 15).
• a layer or layers 18 of n-type metals are then applied over the resist 6(g) with the layer 18 at the gap 17 at the center of the n-type GaN layer 3 being applied directly to the GaN layer 3 ( Figure 16).
• the resist layer 6(g) with the layer 18 attached is removed leaving the layer 18 attached to the center 17 of the n-type GaN layer 3 where gap 17 was previously located.
• the copper layer 9(a) is then polished flat ( Figure 17) and the dies separated.
• the seed layers 11 , 10, 9 and the copper layer 9(a) act as reflectors to iricrease light output, with copper layer 9(a) being one terminal, thus not interfering with light output.
• the second terminal is layer 18 on the n-type layer 3 of GaN and this is as an array at and/or around the center of that layer 3, thus minimizing its effect on light output, and increasing the diffusion of current.
• the array has a central portion 19 to which a bonding part will normally be applied, an outer portion 20 and joining portion 21 connection the central portion 19 and the outer portion 20.
• the outer portion 20 and joining portion 21 are for dispersion of current to maximize light output.
• portions 20, 21 may have a small trench formed in the layer 3 to aid adhesion.
• the dies may be left with several dies being physically interconnected, but being electrically isolated on the n-type layer side by virtue of the silicon oxide layer 8.
• the n-type layer connections will be in accordance with normal practice and will be addressable individually, collectively, or in any desired or required combination or permutation.
• the p-type layers will have a common connection for all dies by means of the copper layer 9(a). In this way the several dies can be operated at the one time for maximum light output, or in any possible combination or sequence, by appropriate control of the n-type layer connections.
• the copper layer 9(a) provides common connectivity on the p-type layer side, physical strength and support, and acts as a common heat sink.
• the presence of the oxide layer 8 provides electrical isolation and prevents leakage. •"
• the layer 18 is shown having a square, cruciform and dot array, it may have any suitable form and shape of array.
• the first 0.5-1.5 micron GaN layer 4 in Figure 1 is undoped, and thus it is electrically nonconductive.
• this layer needs to be removed by etching.
• Fig. 17(a) shows an example contact, where under the circular bonding pad 19 the nonconductive material is retained.

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