Optical Sensor having Opaque Sidewall Coating

An electronic device may include a proximity sensor for detecting whether an external object is in the vicinity of the device. The proximity sensor may be implemented as an optical sensor module having a substrate, a light emitter die mounted on the substrate, a light detector die mounted on the substrate, and a package enclosure housing the light emitter and detector dies within the module. A infrared bandpass filter layer may be formed directly on the light detector die. The light detector die may have sidewalls at least partially covered by an opaque coating. The light detector die may include a highly doped backside reflection absorption layer interposed between an intrinsic absorption layer and an n-type layer within the light detector die. Opaque adhesive material may be used to mount the light detector die onto the surface of the substrate.