The electrical and optoelectronic characteristics of planar InP/InGaAsp‐i‐nphotodiodes grown by metalorganic chemical vapor deposition were extensively investigated. A typical room‐temperature dark current of 10 pA for 100‐&mgr;m‐diam devices was measured at −10‐V bias on several wafers. A low capacitance between 0.42–0.46 pF (for 100‐&mgr;m‐diam devices), responsivities at 1.3 &mgr;m between 0.84–0.94 A/W (corresponding to external quantum efficiencies between 80% and 90%), and rise/fall times of less than 150 ps were measured at −5‐V bias on several wafers. Excellent uniformity in dark current, junction capacitance, and optoelectronic characteristics was found over large wafer areas. The diode dark current was studied as a function of applied voltage, temperature, diode dimensions, and junction depth. The carrier transport mechanisms in different ranges of bias voltage and temperature have been identified and interpreted both in form and magnitude. At low to mid reverse bias, bulk generation‐recombination currents dominate below 320 K. At higher temperatures, diffusion currents become dominant. Band‐to‐band tunneling current near the InP/InGaAs upper heterointerface dominates the dark current at high reverse bias. Under forward bias, the diffusion current dominates over the entire temperature range studied. Theoretical models have been developed to account for the magnitude of the reverse and forward diffusion currents and their dependence on diode dimensions. It is found that the lowest dark current measured at room temperature is very close to the theoretical limit achievable for planar InP/InGaAs/InP diodes withndoping of the InGaAs ∼1×1015cm−3.