ensed at the critical mid-bit decision when a "0" is being transmitted. Though the photodiode must be large enough to capture the optical signal, it obviously should be no larger. The analysis indicates better SEE characteristics for III-V direct bandgap detectors since a depletion depth of only about 2-3 microns can result in > 80% quantum efficiency. This is in contrast with indirect bandgap detectors, such as Si for 830 nm applications, in which depletion depths are about twenty times larger. Specifically, the thinner InGaAs structure minimizes both the "target" size for ion strikes as well as the ion IV-31 pathlength when hit. Also, the III-V device is characteristic of the design choices being considered for high bandwidth data busses since the thin junction offers minimal capacitance. To take advantage of these benefits, most design efforts use III-V InGaAs detectors for 1300 nm lightwave detection. More recently, 850 nm photodetectors with thin depletion regions and favorable SEE performance have been identified [Mars-98]. 4.2.2.1 Proton SEE Measurements on Fiber Optic Receiver Circuits: Proton testing of operating links can be done in situ using subsystem hardware or on components using a commercial bit error rate test set. For the purpose of understanding the test approach and underlying mechanisms and their effects, we present the latter approach here. The subsystem hardware level effects can in turn be inferred from this material, and the impact on the subsystem will differ according to the particular protocol and architecture. Examples of the relation between hardware level effects from "generic" device testing and the impact on specific subsystems can be found in [Carts-97, Dale-97, Mars-96, and references therein]. The SEE response of all associated circuitry must be considered, but we focus on bit errors in the photodetector receiver since it is primary importance in many cases. Figure 14 illustrates a typical test setup for measuring link bit-error-ratio (BER) performance at the component level. T