A high performance AlxGa1−xSb 1.3‐&mgr;m p‐i‐n photodiode, operating at a very low reach‐through voltage (−1.5 V), has been demonstrated for the first time. The low punch‐through voltage was achieved by growing extremely low doped liquid phase epitaxial layer (ND−NA≃4×1014/cm3). The capacitance is ≃0.4 pF and the dark current ≃20 nA at −1.5 V for a 10−4‐cm2device area. The zero bias external quantum efficiency is ?55% at 1.3 &mgr;m; the rise time is 100 ps and the full width at half‐maximum (FWHM) is 200 ps. These results indicate that p‐i‐n photodiodes comparable in performance to homojunction InGaAs p‐i‐n detectors can be fabricated with the AlGaSb alloy system.