Operation and characteristics of the impurity doped infrared sensing MOSFET (IRFET) are described. Responsivities of over 1,000 amps/joule have been observed with only small channel width to length ratios and large area devices. Much higher responsivities would be possible with smaller area devices. In the case of indium- and gallium-doped devices it has been shown that quantum efficiencies in the range 1.0 to 10.0% can be achieved by using substrates with low boron dopings. The infrared sensing MOSFET (IRFET) is a detector, integrating element, and amplifier all combined in one device structure. As such it is distinctly different than other types of infrared detectors and possesses many very unique characteristics. When compared to CCD scanned photoconductors two of these are: (i) the responsivity of the IRFET and uniformity of responsivity do not depend upon carrier life-time and residual impurity concentrations (ii) the IRFET has a nondestructive D.C. or static memory type readout as opposed to the A.C. or dynamic memory type of CCD's. This report describes in detail the experimental results obtained on gold-, indium-, and gallium-doped devices for use in the near, middle, and far infrared wavelength regions respectively.