VO2 based Mott transition field effect transistor is a new element to be incorporated between microbolometer sensitive pixels and on-chip read-out circuitry to fully realize the planar integrated infrared focal plane array. Based on their dielectric dispersions in Lorentz model, its infrared absorption was studied by the theory of reflection from Al surface plasma and absorption by optical phonons in VO2 and especially in α-SiN: H films. The present paper reports on the properties of the absorption in different wavelength regions and the dependence on the thickness of the nitride passivation layer, the absorbance spectra for the layer with thickness of λ/4n after the phase delay is corrected, and the changes in the spectra caused by the transition in the VO2 film.