Degradation of Si1-xGex epitaxial heterojunction bipolar transistors by 1-MeV fast neutrons

Irradiation damage in n⁺-Si/p⁺-Si_1-x Ge_x/n-Si epitaxial heterojunction bipolar transistors (HBTs) by 1-MeV fast neutrons is studied as a function of fluence and germanium content for the first time. The degradation of the electrical performance of HBTs by irradiation increases with increasing fluence, while it decreases with increasing germanium content. The induced lattice defects in the base and the collector regions are studied by DLTS methods. In the base region, electron capture levels associated with interstitial boron are induced by irradiation, while two electron capture levels corresponding to the E centers and the divacancy are formed in the collector region. The degradation of device performance is then correlated with simulations of numbers of knock-on atoms. In order to examine the recovery behavior, isochronal thermal annealing is carried out for temperatures ranging from 75 to 300°C. Based on the recovery of electrical performance, it is pointed out that the electron capture levels induced in the base and collector regions are mainly responsible for the increase of base current and the decrease of collector current