Highly efficientpGaSb‐nGa1−xAlxSb photodiodes were fabricated by liquid‐phase‐epitaxial growth of an undopedp‐type GaSb layer, followed by that of ann‐type Ga1−xAlxSb layer with a composition atx=0.7 on ap‐type (100) GaSb substrate. Spectral photocurrent response of the diodes measured at various bias voltages and room temperature was fairly flat between 1.0 and 1.7 &mgr;m. The external quantum efficiency of the photoresponse was 38% at zero bias and 54% at 1 V reverse bias in the vicinity of 1.2 &mgr;m.