8.1 Total Ionizing Dose (TID) Basics
The most common radiation requirement in the aerospace design community is Total Ionizing Dose (TID), also known as total dose. Total dose effects are caused when electrons and protons create excess charge in the dielectric layers used for insulation in electronic devices. Total dose effects are cumulative and require chronic exposure to numerous radiation events before device degradation becomes obvious. Electronics in a satellite or spacecraft thus accumulate TID damage over time as they operate under continuous levels of radiation. While electrons are mobile in insulators, the holes (positively-charged atoms) must move by breaking bonds and can become trapped in defects. The result of this accumulated positive charge in a device’s insulators leads to degradation and/or device failure. The oxide charge buildup affects the current-voltage characteristics of transistors used in semiconductor circuits. Proper operation of a transistor relies on the ability to switch it from a low-conductance (off) state to a high-conductance (on) state as the gate voltage passes through a threshold. Extended exposure to TID radiation can shift the threshold voltages, making transistors easier or harder to switch. Radiation may also increase the leakage current, causing the on and off states of the transistors to become less distinguishable. Either effect can ultimately cause circuit failure. For our space products, these effects have been characterized and summarized in our TID Radiation Reports.