fig5

Figure 5. (A) Schematic structure of GaN p-i-n APDs grown on sapphire substrates; (B) schematic structure of GaN p-i-n APDs grown on bulk GaN substrates; (C) schematic structure of Al_0.05Ga_0.95N p-i-n APDs with step gradients; (D) schematic structure of back-illuminated p-i-n APDs on sapphire substrates; (E) schematic structure of front-illuminated n-i-p APDs on block GaN substrate; (F) schematic structure of GaN-based APDs with SAM structure; (G) schematic structure of front-illuminated GaN p-i-p-i-n APDs with a thin AlGaN window layer. GaN: Gallium nitride; p-i-n: positive-intrinsic-negative; APD: avalanche photodetector; SAM: separated absorption multiplication.