Continuous-wave terahertz photomixing in a silicon-compatible Ge-on-Si nano-air-channel photodiode

We demonstrate a silicon-compatible Ge-on-Si nano-air-channel photodiode for continuous-wave terahertz photomixing at 1550 nm. While nano-air-channel devices offer a promising route to ultrafast carrier transport, their implementation in a telecom-band, silicon-compatible photomixer has remained largely unexplored. The device is fabricated by a two-step buffered oxide-etch release process that forms a 50 nm air channel between the Ge surface and suspended Au/Ti electrodes. Under 1550 nm illumination, it exhibits an estimated internal quantum efficiency of 473% at 10 V, indicating that this is due primarily to localized high-field-assisted processes in the near-surface Ge region. Bias-dependent analysis reveals a transport transition near 2.5 V from leakage-dominated collection to nano-air-channel-assisted extraction, and dual-laser excitation produces continuous-wave heterodyne signals from 170 to 260 GHz. These results establish Ge-on-Si nano-air-channel photodiodes as a compact platform for silicon-compatible terahertz photomixers and photonic-electronic interfaces.