In one form, a power converter for a power detector or the like includes first and third transistors of a first conductivity type, and second and fourth transistors of a second conductivity type. The light emitting device includes a package body having a cavity defined by a sidewall and a bottom surface, a light emitting device disposed in the cavity, a radiator inserted into the package body and disposed below the light emitting device, and a second electrode pattern disposed around the radiator and electrically connected to the light emitting device via wire bonding. A method for producing a ultrafine fiber substrate, which includes obtaining a substrate from the ultrafine fiber from the above-mentioned method, dissolving and removing the sea polymer in the substrate to obtain the ultrafine fiber substrate.