Improvement of CMOS-Integrated Vertical APDs by Applying Lateral Well Modulation

A method for the improvement of CMOS-compatible reach-through avalanche photodiodes is presented, which works without any process modification. Special fill patterns are used in order to achieve an arbitrary modulation doping factor of the used well. Due to lateral diffusion, the dopant concentration is smoothed and a well results which is in average less doped than the original well. This gives additional degrees of freedom for the design of CMOS-integrated APDs without needing process modifications. The method is verified by the characterization of six avalanche photodiodes with different modulation doping factors for the multiplication zone fabricated in a 0.35 μm CMOS process. The breakdown voltage is adjusted within the range from 15.4 to 182 V while the property of avalanche multiplication of photo-generated carriers is kept. The CMOS process used allowed the isolation of APDs and circuits on the same chip for modulation doping factors between 100% and 50%, which resulted in an increase of the APDs bandwidth from 8 MHz (100% doping) to 386 MHz (50% doping).