Germanium Photodetectors in Silicon Photonic Systems

Germanium photodetectors integrated on silicon platforms have emerged as pivotal components in contemporary silicon photonic architectures, offering seamless compatibility with complementary metal–oxide–semiconductor (CMOS) processes. By harnessing the direct bandgap absorption properties of germanium at telecommunications wavelengths (1.3–1.6 µm), these devices combine high responsivity with low fabrication complexity. The waveguide-coupled architecture, in which germanium is epitaxially grown or bonded onto silicon waveguides, enables efficient evanescent coupling of light and compact device footprints. Progress in device design—ranging from vertical p-i-n geometries to lateral heterojunctions—has yielded substantial gains in bandwidth, responsivity and dark-current performance. Collectively, these advances underpin applications in high-speed data communications, optical interconnects and sensing, supporting data rates beyond 100 Gbit s–1 while preserving the integration density and cost benefits of silicon photonics.

Research from Nature Portfolio

Recent studies have demonstrated ultrafast waveguide-coupled germanium photodiodes achieving optoelectrical 3-dB bandwidths up to 265 GHz at telecommunications wavelengths. This performance is realised through a novel device stack in which a narrow germanium fin is sandwiched between complementary in situ-doped silicon layers, optimising both carrier extraction and optical confinement. Internal responsivities approaching 0.45 A W–1 at 1,550 nm are obtained alongside dark currents below 200 nA, marking a significant leap towards co-integration with silicon electronics for terabit-per-second interconnects.

Germanium Photodetectors in Silicon Photonic Systems publication trend

The graph below shows the total number of articles in germanium photodetectors in silicon photonic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Waveguide-coupled photodetector: A photodiode integrated alongside or on top of an optical waveguide, enabling evanescent absorption of guided light.

p-i-n junction: A semiconductor structure consisting of p-type, intrinsic and n-type layers, used to separate and collect photogenerated carriers.

Responsivity: The ratio of photocurrent generated to incident optical power, expressed in amperes per watt (A W–1).

3-dB bandwidth: The electrical frequency at which the photodetector’s response falls to 70.7% of its low-frequency value, indicating the usable modulation speed.

Dark current: The small leakage current flowing through a photodetector in the absence of light, typically expressed in nanoamperes.

References

1. Ultra-fast germanium photodiode with 3-dB bandwidth of 265 GHz. Nature Photonics (2021).
2. TCAD-Assisted Progress on the Cisco Platform Toward Low-Bias 200 Gbit/s vertical-pin Ge- on-Si Waveguide Photodetectors. Journal of Lightwave Technology (2024).
3. −1 V bias 67 GHz bandwidth Si-contacted germanium waveguide p-i-n photodetector for optical links at 56 Gbps and beyond. Optics Express (2016).
4. Integrated waveguide PIN photodiodes exploiting lateral Si/Ge/Si heterojunction.. Optics Express (2017).