MUTC photodiodes with integrated stub-lines for band-selective flat response in millimeter-wave applications

Millimeter-wave photonic links increasingly demand photodetectors with frequency responses tailored to specific operating bands. Conventional modified uni-traveling carrier photodiodes (MUTC-PDs) exhibit low-pass frequency responses, and maintaining a flat in-band response becomes increasingly challenging as the operating band approaches the intrinsic device bandwidth. Here, we demonstrate a waveguide MUTC-PD integrated with an on-chip stub-line matching network for band-selective response shaping. Baseline devices with active areas of 4 × 10, 5 × 10, and 5×12 μm² were fabricated and characterized. The 4×10 μm² device exhibited a 3-dB bandwidth of 104 GHz, and all device sizes showed an external responsivity of approximately 0.4 A/W. A 5×10 μm² active area was then used for the stub-line-integrated devices to improve fabrication tolerance. The integrated matching network introduces a resonant peak that compensates the RC roll-off and forms a flat passband. For a fixed photodiode geometry and electrode configuration, the passband center frequency is mainly adjusted by the stub-line length. An 80-µm-stub device achieved a measured in-band flatness within ±1 dB from 75 to 110 GHz. A 60-µm-stub device shifted the response toward the F-band; its measured response agreed with the circuit model up to the 110-GHz instrumentation limit, and the model predicted comparable flatness over the intended F-band range. These results indicate that stub-line-integrated MUTC-PDs can provide band-tailored responses for millimeter-wave photonic links.