For diffusion-limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. We discuss the use of a metallic grating to couple the incident light into laterally propagating surface plasmon polariton (SPP) modes and increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform profile without increasing the dark current. Using this process in conjunction with a 0.5-μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T  =  78 to 240 K. An nBn structure with an InAs_0.8Sb_0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band and can potentially reduce the dark current by another factor of 3 to 8 times in addition to the 5  ×   reduction due to the thinner absorber.