Detailed measurements of the photosensitivity of Si microcavity plasma photodetectors in the visible and near-infrared (420–1100 nm) are reported for input optical intensities to a 100×100 μm2 inverted pyramid device varied over three orders of magnitude (10−5–10−2 W cm−2). By resolving the contribution to the overall device response from the plasma/semiconductor interaction, as opposed to bulk Si photoconductivity, the photosensitivity of the plasma photodetector operating in 500 Torr of Ne was determined to range from (2.2±0.4) A/W for 2 nW of input power (at λ = 780 nm) to (1.3±0.2) A/W at ∼ 0.65 μW. The spectral response profile of the hybrid plasma/semiconductor detector is similar to that of a conventional pn junction photodiode, but is blueshifted by ∼ 60 nm. Also, the peak photosensitivity (3.5 A/W at λ ≃ 900 nm) of a Si microplasma device having a 50×50 μm2 aperture is approximately twice that for its larger (100×100 μm2) counterpart under identical conditions. Analysis of the data suggest that bandbending at the p-Si surface is sufficiently strong for a thin n-type region to form, thereby resembling a metal-oxide-semiconductor capacitor in the inversion mode. Electrons in this thin layer tunnel through the vacuum (Si-plasma) barrier, followed by electron avalanche in the nonequilibrium plasma. These results illustrate the potential for novel optoelectronic devices when interfacing a plasma with a semiconductor and coupling the two media with a strong electric field imposed across the interface.