An amorphous avalanche photodiode (APD) with a heterojunction of hydro
genated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous
silicon germanium (a-SiGe:H) was formed. The band gaps of a-SiC:H and
a-SiGe:H are 3.5 eV and 1.55 eV, respectively The discontinuity of co
nduction bands at the heterojunction is larger than the band gap of a-
SiGe:H. In this amorphous APD, photocurrent multiplication is observed
under low electric field. The quantum efficiency starts to exceed uni
ty when the conduction band discontinuity becomes larger than the band
gap of a lower-gap material, and it is likely to saturate at 2. The s
lope of photoelectric conversion characteristics is 1.00. The multipli
cation is explained by the impact ionization process at the Land edge
discontinuity region.