Physics / mechanism
Germanium photodiodes exploit Ge’s bandgap (~0.66 eV) to absorb photons out to ~1600 nm, covering the full O- through L-band telecom window where silicon is blind. Incident photons generate electron-hole pairs; reverse bias sweeps carriers across a thin intrinsic region (p-i-n architecture) producing photocurrent. Key figures of merit: responsivity 0.6–0.9 A/W at 1310/1550 nm, dark current 1–100 nA (waveguide-integrated devices tighter), bandwidth >50 GHz in advanced implementations, and operating voltage typically −1 to −3 V. State of the art: monolithic Ge-on-Si grown by CVD epitaxy, threading dislocation density reduced to ~10⁷ cm⁻² via graded buffers or cyclic annealing. Leading edge devices (Intel, imec, GlobalFoundries 45RFSOI/45CLO) are fully CMOS-back-end integrated for silicon photonics PICs.
Competitive landscape
InGaAs p-i-n and APDs are the primary competition—superior dark current and noise floor, but III-V fab incompatible with CMOS, expensive, and harder to integrate monolithically. Si APDs dominate <900 nm. Extended-InGaAs and HgCdTe push to mid-IR but at significant cost and complexity penalties.
Companies using
Connected ideas
Sources
Frontier (open questions)
- To be added.