. Also, the coupler may be a part of a larger hybrid such as a DC-DC converter, which includes other radiation sensitive components [Reed98]. Third, as described in the previous section, IV-92 limits in our current understanding of the energy dependence of the NIEL for III-V (and ternary) materials do not permit accurate on-orbit performance predictions, thereby necessitating significant radiation design margins. It is also worthwhile to note that little is known about possible connections between reliability issues such as lifetime and temperature and radiation-induced degradation. Currently, these effects are assumed to be independent. However, we do know that some optocouplers have exhibited a significant CTR temperature dependence, which is an important consideration since operation well above room temperature is not uncommon on spacecraft [Rax96]. The importance of proton testing to evaluate the on-orbit response of an optocoupler is clearly demonstrated in figure 24 which shows greatly decreased CTR for proton as compared to Co-60 irradiation. Rax et al. disassembled two types of optocouplers to investigate the modes of CTR degradation [Rax96]. They found that the CTR performance was primarily determined by the response of the LED to displacement damage, and that the amphoterically doped AlGaAs LED was significantly more susceptible than the GaAIP LED. As is often the case, a higher performance device fabricated with more pristine material is also more radiation sensitive (e.g., CCDs, solar cells, etc.), and an engineering trade of initial performance versus the radiation sensitivity of the device on-orbit must be performed. Displacement damage also affected phototransistors in each optocoupler studied by Rax et al., with the reduction photoresponse being a more significant than gain degradation. Optocouplers that use photodiode (as opposed to a phototransistor) have been observed to have the best performance to date [Reed98]. Texas Instruments 4N49 If=1.1 Vce=5.0V Co-60 195 MeV Protons Total Ionizing Dose 