Silicide Schottky-barrier midwave infrared detectors for the 3 to waveband typically use a 2 to 8 nm metal-silicide layer grown on silicon substrates as an absorption layer. We demonstrate the use of an -film-coated subwavelength grating as an antireflection incident plane in such detectors to enhance absorption at the 3 to waveband in the metal-silicide layer for improving the external quantum efficiency (EQE). Taking a structure as an example, we fabricate samples and build a test platform to characterize the EQE of a Schottky-barrier detector. Simulation results show that low reflection efficiency () for the subwavelength-grating incident plane and factors of 1.5 to 1.8 enhancement in absorption of the PtSi layer are achieved at normal incidence across the 3 to waveband. Measurement results show that this increase translates into factors of 1.3 to 1.7 enhancement in EQE. This improvement in EQE results from absorption enhancement due to antireflection effects and the forward scattering of incident infrared radiation in the subwavelength grating, which increases the effective optical path in the detector. The results also suggest that fabricating a subwavelength-grating incident plane is a general, low-cost method to enhance EQE in various infrared material platforms used for back-illuminated detectors.