LED light emitting devices are devices that emit light when a forward current is passed through a pn junction of a semiconductor, produced using a III-V semiconductor crystal such as GaAs or GaN. In recent years, advances in semiconductor epitaxial growth technologies and light-emitting device process technologies have led to development of LED light-emitting devices excelling in conversion efficiency, used widely in various fields.
LED chips are composed of a p-type and an n-type layer formed by epitaxial growth of a III-V semiconductor crystal on a growth substrate, and a photoactive layer sandwiched therebetween. In general, a III-V semiconductor crystal is epitaxially grown on a growth substrate such as monocrystalline sapphire, then electrodes or the like are formed to yield an LED chip.
Since the thermal conductivity of monocrystalline sapphire is about 40 W/(m·k), it cannot adequately dissipate the heat generated by the III-V semiconductor devices. In particular, with high-output LEDs taking high currents, the temperature of the device can rise, causing reduced light emission efficiency and reduced device lifetime. In order to solve this problem, a method of epitaxially growing a III-V semiconductor crystal on a growth substrate, then bonding to a package substrate (retaining substrate) through a metal layer, followed by removing the growth substrate has been proposed (Patent Document 1), but was not entirely satisfactory. In other words, metal package substrates (retaining substrates) are conductive, and therefore must have a non-insulated structure at the time of mounting. For example, when soldering to a mounting substrate such as a circuit board, an insulation layer of low thermal conductivity such as a resin or the like must be provided immediately under the bonded portion, but this insulation layer inhibits the satisfactory dissipation of heat.
On the other hand, in high-output LED light emitting devices in which problems due to heat generation by the LED chip must be reduced as much as possible, a method of mounting an LED chip to a circuit board or the like via a heat dissipating plate such as a copper (Cu) plate has been proposed (Patent Document 2). However, the coefficient of linear thermal expansion of Cu is about 17×10−6/K, which differs largely from the about 5×10−6/K of LED chips, so cracks can appear in the bonded portion during use, thereby lowering the heat dissipating ability.    Patent Document 1: JP-A 2006-128710    Patent Document 2: JP-T 2008-544488