An amorphous Avalanche Photo-Diode (APD) with a heterojunction of a-SiC : H and a-SiGe : H was formed. The bandgaps of a-SiC : H and a-SiGe : H are 3.5eV and 1.55eV, respectively. The heterojunction has larger conduction band discontinuity than the bandgap of a-SiGe : H. In this amorphous APD, the photo-current multiplication is observed under low electric field. The quantum efficiency strarts to exceed unity when the conduction band discontinuity becomes larger than the bandgap of lower-gap material, and it is likely to saturate at two. The slope of photo-electric conversion characteristics is 1.00. This multiplication is explained by the impact ionization process at the band edge discontinuity region.
