Fig. 1. WG MUTC PD^[25]. (a) Epitaxial layers of WG PD; (b) schematic diagram of PD; (c) SEM image of PD

Fig. 2. Si based WG PD^[35]. (a) Epitaxial layers of WG PD; (b) bands of the PD under -1 V and -3 V bias voltages; (c) optical mode in the WG; (d) light field distribution in the PD

Fig. 3. All Si PD^[45]. (a) Schematic diagram of PD; (b) measured responsivity and photo to dark current ratio

Fig. 4. Butt coupling Ge PD^[47]. (a) Schematic diagram of PD; (b) cross section of PD

Fig. 5. Vertical PIN Ge PD^[49]. (a) Schematic diagram for the Ge PD integrated with a passive waveguide; (b) cross section of Ge n-i-p WG PD; (c) cross section of SEM image of the Ge WG PD (7.4 μm×50 μm); (d) measured frequency response for PD

Fig. 6. Lateral PIN Ge PD^[54]. (a)-(d) Process flow of the Ge-fin PD; (e) cross section image of Ge PD; (f) EDX cross section image of Ge PD

Fig. 7. PIN Ge PD with inductance gain^[59]. (a) Cross section of Ge WG PD; (b) microscopic image of PD; (c) microscopic image of different inductors