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Linearized moments of the Boltzmann transport equation are shown to imply a new equivalent circuit model for partially depleted absorber photodiodes. Application of the proposed model demonstrates excellent quantitative agreement with previously reported frequency-response measurement. The influences of depletion region width, placement, and field strength on modulation bandwidth and charge balancing are explored. Simulations indicate that the design goals of high bandwidth and resistance to optical saturation are mutually compatible for variations in depletion-region placement, though not depletion-region width.