 that results in proton sensitivity [Rax98]. Figure 13 shows test results for an operational amplifier that fails its specification limit at an equivalent ionization level that is only 60% of that observed during gamma irradiation [Rax98]. The authors found that the output stage of the circuit is asymmetrical; it sources up to 10 mA, but is only guaranteed to sink 1 mA. It relies on the gain of a single substrate pnp transistor to sink current from an external load, making it quite susceptible to displacement damage as a direct result of circuit design. Cobalt-60 gamma rays 52-MeV protons Specification limit Device falls below specification limit at 60% of cobalt-60 test result Equivalent Total Dose [krad(Si)] Figure 13 The degradation in the output sink current of an OP221 operational amplifier is significantly more pronounced for proton (as compared to gamma) irradiation. The sensitivity to displacement damage is a direct result of a specific design technique. [After Rax98]. As expected, a precision, high performance linear application using a device with demanding electrical specifications will be significantly more sensitive to displacement damage induced degradation. For example, an application may require a very low input offset voltage, and/or input offset and bias current, or low noise. Recently Rax et al. found significant differences between proton and gamma ray results, which showed that displacement damage could be important, even in precision reference circuits that primarily on high fT npn transistors [Rax97]. As described in the last section, laboratory IV-78 test results must be carefully interpreted since the actual failure mode and sensitivity to displacements can depend on the specific environment (e.g., relative amounts of TID and displacement damage dose), and the device circuit and operating conditions. In contrast, heterojunction bipolar transistor (HBT) technology is known to be robust in a proton environment. Ohyama et al. have investigated the proton induced degradation of SiGe HBTs as a