We report absorption measurements on two types of long-wave infrared detector structures. Both types were grown by ultrahigh vacuum chemical vapor deposition, and were characterized by multiple analytic techniques. In both multiple quantum well (MQW) and heterojunction internal photoemission (HIP) structures, it is found that free-carrier absorption is dominant for normally incident radiation. The measured absorption is fit well by the classical expression for free-carrier absorption, with scattering times of about10−14 s(MQW) and5×10−15 s(HIP). The measured absorption is used to evaluate the responsivity that results when all carriers energetically able to surmount the barrier are collected. Based on this analysis, higher responsivity is predicted for HIP detectors, largely because of the greater density of initial states. The responsivity obtained in practice depends upon the photoconductive gain (MQW detectors) or the escape probability (HIP detectors). The escape probability for HIP detectors is measured in Part II. ©1997 American Institute of Physics.