A highly sensitive avalanche photodiode (APD) in 0.35 μm CMOS technology is presented. Due to a thick intrinsic absorption layer, a high responsivity at a low bias voltage, where the avalanche gain is 1, is combined with an excellent avalanche gain at high voltages to achieve a maximum overall responsivity of the APD of more than 10 kA/W. This responsivity exceeds that of other submicrometer CMOS APDs by a factor of more than 700. As a figure of merit the responsivity-bandwidth product is defined, and the achieved value of 23.46 A/W·GHz is 2.4 times higher than the values found in the literature.