P-i-n photodiodes with ND as low as 1−2×1015 cm−3 in the I-region were fabricated from InGaAsP (λ=1.26 μm). At −20 V, the 2.5 μm I-region of the 150 μm diameter mesas was completely swept out, the junction capacitance was below 1 pF, the dark current was less than 0.2 nA and the external quantum efficiency was 63% without a.r. coatings.