d collection of carriers are much more sensitive to displacement damage than those which do not, such as a fully depleted p-i-n detector [Soda75]. For example, phototransistors are sometimes used as optical detectors (e.g., in optocouplers) since they provide internal gain. However, the transistor gain is dependent on the minority carrier lifetime and therefore sensitive to displacement damage. In contrast, metal-semiconductor-metal (MSM) photodiodes are majority carrier devices and quite robust in a proton environment. As expected, the robustness of a photo-detector also can be very application dependent. Consider an APD that can operate successfully in an analog communications link to the equivalent of 100 krad(Si) in a proton environment, whereas its performance as a photon counting device (in the Geiger mode) significantly degrades for a proton exposure below 100 rad(Si) [Sun97]. Photodiodes are available which are optimized for many applications, and the radiation response is design dependent. For example, one may harden a Si photodiode to displacement damage by minimizing the diffusion limited carrier collection (governed by the minority carrier lifetime), and maximizing collection in the depletion region using lightly doped material. Although such diodes can be quite robust in a proton environment, the lightly doped regions are subject to carrier removal and mobility degradation at relatively low fluences [Kord89]. Note also that sometimes a detector that may be relatively robust to displacement damage, like a fully depleted Si p-i-n, may be relatively more susceptible to ionization induced photo-currents which can produce bit errors in a fiber optic data link. (See Part IVA of the course for a description of bit error effects in fiber links.) Obviously, the radiation response of a particular detector to both permanent and transient proton effects must be considered for a real application. In recent years, InGaAs p-i-n detectors have proven useful in fiber optic data link applications because of their supe