Example of the semiconductor light emitting apparatuses above include a white LED having a solid-state light emitting device and a wavelength converter that converts the primary light emitted by the solid-state light emitting device into loner-wavelength light. Commonly used in such a white LED (hereinafter, “LED chip” or simply “chip”) is a light emitting diode having a light-emitting layer of a InGaN-based compound semiconductor. An example of the wavelength converter is a resin phosphor layer made from a phosphor powder (phosphor particles) dispersed in a transparent resin.
In recent years, there is a growing demand for higher-output semiconductor light emitting apparatuses. Unfortunately, however, to increase the output power of a semiconductor light emitting apparatus means to increase the fluorescent excitation light emitted by the LED chip, which leads to heat generation due to the energy loss (Stokes loss) associated with the wavelength conversion by the phosphor. The heat builds up in the resin phosphor layer to elevate the temperature of the resin phosphor layer, which increases the lattice vibrations in the solid to reduce the photon conversion efficiency.
In addition, due to the temperature rise of the resin phosphor layer and exposure to the strong primary light emitted by the LED chip, the transparent resin of the resin phosphor layer undergoes accelerated chemical reaction with surrounding component members and the atmosphere. The physical properties of the transparent resin are adversely affected to reduce the optical output and reliability (such as the reduction of light transmittance).
In view of the above drawbacks, it is suggested to use, as the wavelength converter, a ceramic compact (for example, a compact of transparent phosphor ceramics, phosphor glass, or optical-functional composite ceramics) having high thermal conductivity and excellent heat dissipation to suppress the temperature rise of the wavelength converter (see patent literatures 1 and 2 listed below).