Ultra-high gain 1550 nm Ge photodetector enabled by doping-engineered carrier multiplication

High-responsivity photodetectors at 1550 nm are crucial for silicon photonic circuits, time-of-flight (ToF) LiDAR receivers, and low-light optical sensing systems. We present a Ge vertical p–i–n photodetector with internal gain achieved through a doping-engineered band structure. The device uses an n + -Si/Ge/p + -Si heterostructure created by ion implantation and epitaxial growth, resulting in a graded doping profile and a carrier-confining potential well in Ge. Scanning capacitance microscopy (SCM) reveals overlapping phosphorus and boron diffusion profiles, confirming our band-engineering approach. The photodetector reaches a responsivity of 370 A/W and a detectivity of 1.59 × 10 11 cm·Hz 1/2 ·W −1 at −0.5 V under 1550 nm light. Simulations (TCAD and optical) suggest an internal gain over 2000, indicating this low-voltage platform is promising for silicon photonics and optical receivers. Small-signal S21 measurements yield a 3-dB bandwidth of approximately 148.8 MHz, demonstrating the expected gain–speed trade-off associated with the carrier-confinement mechanism.