Fig. 1. Schematic figure of the basic operating principles and electric field distribution of detectors: (a) PIN photodetector; (b) SAM-structured avalanche photodetector.

Fig. 2. Schematic diagrams of avalanche processes for different impact ionization coefficient ratios: (a) k=0(β=0) and (b) k=1(α=β).

Fig. 3. Probability distribution functions of impact ionization occurring in the multiplication layer: (a) local equilibrium model; (b) high electric field (solid line) and low electric field (dashed line) in the non-local equilibrium model.

Fig. 4. Parameters of InP. (a) Electron impact ionization coefficient (red solid line), hole impact ionization coefficient (blue solid line), and the impact ionization coefficient ratio β/α (black dashed line) for InP [31]; (b) excess noise factor as a function of gain for multiplication layers of different thicknesses [31].

Fig. 5. Bandwidth as a function of multiplication in a photodiode [34].

Fig. 6. Recent advances in APDs using InAlAs and Si as multiplication materials [35–54" target="_self" style="display: inline;">–54].

Fig. 7. Planar APDs with different structures. (a) Schematic of a back-illuminated planar APD based on SACM structure [55]; (b) schematic of a back-illuminated planar APD with an InGaAs P-contact layer [56]; (c) schematic of a normal-illuminated planar APD based on SACM structure [57]; (d) schematic of an APD integrated with a distributed Bragg reflector [58].

Fig. 8. APD with a hybrid absorption layer structure. (a) Band diagram of the hybrid absorption layer structure [61]; (b) carrier transit time in hybrid absorption layers with different thickness ratios [61]; (c) schematic of a dual-mesa APD based on SACM structure [39]; (d) electric field distribution at 19.6 V bias [39].

Fig. 9. Extended study on APDs with hybrid absorption layer structures. (a) Schematic of an APD structure with a transition layer [41]; (b) schematic of an APD structure with dual charge layers [62]; (c) corresponding band diagram of the APD with dual charge layers [62]; (d) gain-bandwidth characteristics of the APD [62]; (e) band structure schematic of an APD with dual carrier injection [63]; (f) comparison of linearity across different APD absorption layer structures [63].

Fig. 10. InP-based waveguide APDs. (a) Schematic of a waveguide APD with SACM structure [35]; (b) schematic of a structure with waveguides placed on both sides of the APD [38]; (c) 3 dB bandwidth curves corresponding to different device lengths [38]; (d) schematic of an APD with transient coupling structure [37].

Fig. 11. InP-based avalanche photodiodes with a butt-joint structure. (a) Schematic of a butt-joint waveguide APD structure based on SACM [44]; (b) overall schematic of the device integrated with a metal-insulator-metal capacitor [44]; (c) 3 dB bandwidth curves of the device at different gain levels [44]; (d) schematic of the optimized butt-joint APD structure [66].

Fig. 12. Vertically incident Si-Ge APDs. (a) Schematic of a planar Si-Ge APD structure based on SACM [67]; (b) schematic of a normal-illuminated mesa Si-Ge APD structure [68]; (c) schematic of a resonant-cavity-enhanced mesa Si-Ge APD structure [70]; (d) bandwidth curves of the resonant-cavity-enhanced Si-Ge APD at different gain levels [70].

Fig. 13. Vertically incident Si-Ge APDs with a resonance enhancement effect. (a) Schematic of a resonant Si-Ge APD structure based on SACM [46]; (b) bandwidth curves of the device at different bias voltages [46]; (c) variation of device gain with bias voltage under different optical power levels [46]; (d) variation curve of GBP with gain at different optical power levels [46]; (e) schematic of a planar resonant Si-Ge APD structure and its current curves [71]; (f) bandwidth-gain curves of the planar resonant Si-Ge APD. The parameter ϕclear denotes the clear aperture diameter [71].

Fig. 14. Waveguide-type Si-Ge APDs. (a) Schematic representation of the specific structure and doping concentration of a waveguide Si-Ge APD [72]; (b) simulated electric field distribution [72]; (c) schematic of a 56 GHz high-speed waveguide Si-Ge APD structure [48]; (d) frequency response curves corresponding to different gain levels [48]; (e) schematic of a grooved waveguide structure Si-Ge APD [50]; (f) frequency response curves at different gain levels [50]; (g) Si-Ge waveguide avalanche photodiode enhanced by a loop reflector [74].

Fig. 15. Waveguide-type APDs with a lateral structure. (a) Schematic of a lateral waveguide APD structure [49]; (b) schematic of a lateral PIN structure Si-Ge APD [51]; (c) schematic of an SAM structure APD integrated with a grating coupler [76]; (d) schematic of the electric field distribution in the APD [76].

Fig. 16. Si-Ge APDs with a shallow trench. (a) Schematic of the shallow trench Si-Ge APD structure [77]; (b) comparison of the electric field distribution between the shallow trench device and a standard device without a shallow trench [77]; (c) eye diagram of the device at −13.5  V bias and −15  dBm optical power, corresponding to 50 Gbps [77]; (d) schematic of the improved shallow trench structure [52]; (e) bandwidth curves of the improved shallow trench APD at different bias voltages. S21 is used to characterize the frequency response of the device, with the −3  dB point typically defining the bandwidth [52]. (f) Gain-bandwidth curves of the improved shallow trench APD [52].

Fig. 17. Lateral p+-i-p-i-n+ APD (a) structure and (b) gain-bandwidth curves [79].

Fig. 18. APDs with an inductance enhancement effect. (a) Schematic of the lateral SACM structure APD and image of the spiral inductor [54]; (b) bandwidth curves for the device without inductor, with a small inductor, and with a large inductor [54]; (c) overall schematic and cross-sectional view of the high-speed lateral APD device [53]; (d) simulated impact of different inductor sizes on bandwidth [53]; (e) bandwidth curves of the device at different bias voltages [53]; (f) gain-bandwidth curves corresponding to the bandwidth curves [53].

Fig. 19. APDs based on digital alloy materials. (a) I–V and gain curves for random alloy and digital alloy InAlAs [91]; (b) excess noise measurements for random alloy and digital alloy InAlAs devices [91]; (c) schematic of an APD structure with digital alloy InAlAs as the absorption layer [92]; (d) excess noise measurements for the APD with digital alloy InAlAs [92].

Fig. 20. APDs based on advanced materials. (a) Schematic of an APD structure with AlAsSb material used as the absorption and multiplication layers [93]; (b) noise measurements for the APD with AlAsSb material [93]; (c) schematic of an InAs quantum dot waveguide APD structure [95]; (d) schematic of the InAs planar APD [96].