e studies to follow, we will see examples of other useful device damage factors such as the CCD CTE damage factor, the dark current damage factor, the solar cell efficiency damage factor, and SO on. In each case, the device parameter in question changes linearly with fluence, or else is defined in the linear region. Note that parameters such as inverse bipolar transistor gain, detector responsivity, and CCD dark current, which have a well-defined regime with a linear response, may also exhibit a nonlinear response at very low or very high proton fluences. (We will also see examples where a device parameter of interest such as LED light output or optocoupler current transfer ratio does not behave linearly in the proton fluence regime of interest.) Bipolar transistor gain measurements for a variety of incident particles (as a function of energy) have been performed in order to determine whether the NIEL function can be used both to predict the energy dependence of the device damage factor and to correlate the degradation due to different particles [Summ87]. In principle, such correlation also provides the basis for on-orbit performance predictions based on the measurement of a damage factor at a single proton energy. Likewise, if neutron data already exists, the correlation can be used to predict the device response to protons. In this work, the well-known Messenger-Spratt equation is used to describe the radiation response of the common emitter DC gain, hFE, of a bipolar transistor: where 1/hFEO is the initial reciprocal gain, K(E) is the particle and energy dependent displacement damage factor, and is the incident particle fluence. The transistor gain (given by the ratio of the collector to base currents) decreases with increasing proton fluence primarily as a result of the decreased minority carrier lifetime in the base region. A more detailed description may be found in [Mess86]. The damage factor is determined experimentally by performing device gain measurements (for a particular set of device operating condi