Protons as a Screen for Displacement Damage in Bipolar Junction Transistors

Abstract

Protons provide a useful alternative to neutrons for displacement damage testing. However, results can be complicated due to protons causing ionization damage in addition to displacement damage. Displacement damage leads to increased bulk recombination due to traps in silicon. Simultaneously, ionization damage causes the creation of electron-hole pairs in both silicon and silicon dioxide, leading to states that increase surface recombination. The interaction of these mechanisms causes the calculation of the exact amount of displacement damage due to proton irradiation to be difficult without explicit knowledge of a device’s geometry and doping. Previous research shows that X-rays approximate the ionization damage due to protons. This allows proton irradiation to be compared with X-ray irradiation of the same device to generate a rough estimate of displacement damage caused by protons. In bipolar junction transistors (BJTs), both displacement and ionization damage effects are manifested as decreases in current gain. Using inverse current gain, it is possible to find the damage factors due to both protons (KP) and X-rays (KX) and calculate a ratio, KX/KP, to describe the relationship. Multiplying this ratio with a charge yield ratio and the proton-induced gain degradation gives a rough estimate of the amount of displacement damage caused. NPN and PNP BJTs of different sizes were irradiated with 4 MeV protons and 10 keV X-rays to evaluate the accuracy of proton-plus-X-ray data in predicting displacement damage. The results show that proton irradiation can provide a useful first-order approximation of sensitivity to displacement damage.