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This paper presents the analysis and characterization of partially depleted absorber (PDA) photodiodes. Coupling to these photodiodes is achieved through a planar short multimode waveguide (PSMW) structure. Electric transport in the PDA structure has been investigated and an equivalent electric circuit was developed. Measurements on 5×20 μm2 PSMW PDA photodiodes have shown 0.80 A/W responsivity with a fiber mode diameter as high as 6 μm. The transverse electric/transverse magnetic polarization dependence was <0.5±0.3 dB with -1-dB input coupling tolerances as high as ±2.0 and ±1.3 μm for horizontal and vertical directions. The -3-dB bandwidth was 50 GHz, and the -1-dB compression current at 40 GHz was 17 mA corresponding to +4.5 dBm radio frequency (RF) power. Compared to similar evanescently coupled p-i-n photodiodes, the saturation current has been significantly improved while maintaining comparable bandwidth and high responsivity.