SiGe Waveguide Photodetector

Tools Used: RSoft FullWAVE & Sentaurus TCAD

For photodetector incorporation into the silicon photonic infrastructure, a CMOS-compatible material with efficient near-infrared absorption is essential. Germanium, available in standard front-end CMOS processes, is suitable for making photodetectors due to its high absorption at telecom (1.3-1.5) wavelengths. As such, waveguide-integrated Ge photodetectors amenable for integration within the CMOS device layer stack have become a topic of intense R&D.


SiGe Waveguide photodetector


  • For optimum receiver performance, the waveguide-integrated photodetector has to achieve high bandwidth, low capacitance, high responsivity, and ideally single-mode operation at a wavelength of 1.5um. This requirements are mostly in conflict with each other, therefore the photodetector has to be designed by evaluating and minimizing the tradeoffs.
  • Of critical importance is to maximize the responsivity for the 100nm thick Ge layer without large increases in length (which would increase the total light absorption, but would also increase the capacitance and the RC bandwidth limitation).
  • Photodetector design involves electronic, process, and optical physics. A complete photodetector simulation must incorporate all these effects.


  • A mixed-level simulation solution combining Synopsys’ Sentaurus TCAD and RSoft tools offers a complete and rigorous SOI photodetector design solution.
    • RSoft is used to compute the optical properties & absorption profile
    • TCAD inputs the absorption profile computed by RSoft, and continues the calculation by computing the electrical response of the system.

SWB flow. Step 1: structure drawn. Step 2: RSoft optical calculation.
Step 3: TCAD SDevice electrical simulation

Ex and Hy mode profiles

Propagating power and resulting absorption in photodetector, as computed by BeamPROP™


  • RSoft’s mode solvers are used to confirm the single-mode operation of this device.
  • The steady state-optical power and absorption is computed via RSoft’s BeamPROP
    BPM propagation tool. BPM offers a fast power & absorption calculation (~50x faster than FDTD) with comparable results to FDTD.
  • The computed power distribution shows the coupling from the silicon layer to the germanium layer, which it quickly decays due to material absorption. Note that the “dark spots” represent absorption at the base of the tungsten pillars.
  • Synopsys Sentaurus TCAD, using the absorption profile computed by BeamPROP , computes the I-V photodetector electrical response. 



Assefa, Solomon, et al. "CMOS-integrated high-speed MSM germanium waveguide photodetector." Optics Express 18.5 (2010): 4986-4999.