Multiplication noise and depletion layer width of silicon avalanche photodiodes are calculated for several impurity density profiles adjacent to the junction. From calculated results on these models of impurity density profile, a low-high-low structure is shown to be superior in obtaining a low multiplication noise property and wide depletion layer width at a given breakdown voltage. Experimental multiplication noise on a diode with low-high-low structure with 100 V breakdown voltage is k=0.03–0.04, where k is ionization coefficient ratio. Pulse response is improved by incorporating an impurity gradient deep in the light absorption region. The gain-bandwidth product is 150 GHz at the multiplication factor of 100.