Abstract: An avalanche photodiode (APD) includes a first electrode, a substrate layer, a buffer layer, a gain layer, a gradient layer, an absorption layer, a diffusion barrier layer, a contact layer, and a second electrode. The gain layer, the gradient layer, and the absorption layer are arranged vertically in sequence. The gain layer, the gradient layer, and the absorption layer are located between the buffer layer and the diffusion barrier layer. The gain layer includes at least two gain units, and the gain units are arranged in a stacked manner. Each of the gain units includes a multiplication layer and a charge layer that are arranged vertically. A distance between the charge layer and the gradient layer is less than a distance between the multiplication layer and the gradient layer.