Abstract <p>AlGaN-based avalanche photodiodes <b>(</b>APDs) are widely used in ultraviolet detections. Excess noise is an important component of the noise in APDs. AlGaN APDs with graded Al composition multiplication layer are designed to reduce the excess noises. The device performance of AlGaN APDs with an intrinsic AlGaN layer with Al composition of 0.4, graded from 0.4 to 0.5 and from 0.4 to 0.6 are simulated. The electric field is gradually distributed in the multiplication region of the compositionally graded structures. The ratios of the hole to electron ionization coefficients of graded and conventional APDs exceed 1, indicating the avalanche is initiated by holes. The ionization coefficient ratios of the graded structures are larger than that of the conventional structure, resulting in a lower excess noise factor. Although the graded structure offers a better noise characteristic, it shows a disadvantage in the dark current. The proposed structure provides a potential structure for fabricating AlGaN APDs with low excess noises.</p>