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The performances of thermal infrared light detector based on a model system of resonant semiconductor microcavities are theoretically investigated. An original transfer matrix formalism of cavity enhanced absorption is presented which makes use of the small thickness of the absorbing layer compared to the light wavelengths. This formalism yields exact expressions which take standing wave effects into account in a built-in way. Approximations lead to tractable expressions which allow deriving asymptotic behaviors and general trends. The tradeoff between large cavity absorption enhancement and reduction of the detector bandwidth is particularly studied, leading to a gain-bandwidth product analysis. Approximated expressions for detectors based on resonant (i.e type I quantum dots) and nonresonant (bulk or type II quantum wells) optical transitions are also derived, which are physically meaningful and may be conveniently used for engineering purposes. It is found that the limitations due to the gain-bandwidth product conservation can be overcome. However, these cavity enhancement effects are only important for very small quantum efficiency for which the finesse of the microcavity is not seriously deteriorated.