Blocked impurity band detectors are of interest for various astronomy applications such as the Infrared Astronomical Satellite and Cosmic Background Explorer. The fabrication details and characterization of Si:P blocked impurity band detectors have been reported. The bottom contact was epitaxially grown instead of utilizing ion-implantation to improve the quality of epitaxial stack. The photocurrent spectra and dark current of the detectors have been measured. The spectral response extends from 2.5 μm to 40 μm. Several additional sharp peaks superimposed on the broadband response are designated to associate with impurity transitions of phosphorus in the blocking layer. The origin of dark current at small bias has been studied at low temperature. It was found that the shield of cryostat is the source of the background infrared light which hits the detectors to produced the dominant dark current at this regime. The detectors attain a peak responsivity of 20.1 A/W (at 28.8) and peak detectivity of 3.7×1013 cm·Hz1/2/W (below a flux of 1013 ph/cm2·s ), which exhibits highly competitive figure of merits.