Degradation mechanisms of current gain in NPN transistors

An investigation of ionization and displacement damage in silicon NPN bipolar junction transistors (BJTs) is presented. The transistors were irradiated separately with 90-keV electrons, 3-MeV protons and 40-MeV Br ions. Key parameters were measured {\em in-situ} and the change in current gain of the NPN BJTS was obtained at a fixed collector current ($I_{\rm c}=1$~mA). To characterise the radiation damage of NPN BJTs, the ionizing dose $D_{\i}$ and displacement dose $D_{\d}$ as functions of chip depth in the NPN BJTs were calculated using the SRIM and Geant4 code for protons, electrons and Br ions, respectively. Based on the discussion of the radiation damage equation for current gain, it is clear that the current gain degradation of the NPN BJTs is sensitive to both ionization and displacement damage. The degradation mechanism of the current gain is related to the ratio of $D_{\rm d}$/($D_{\rm d}+D_{\rm i})$ in the sensitive region given by charged particles. The irradiation particles leading to lower $D_{\rm d}$/($D_{\rm d}+D_{\rm i})$ within the same chip depth at a given total dose would mainly produce ionization damage to the NPN BJTs. On the other hand, the charged particles causing larger $D_{\rm d}$/($D_{\rm d}+D_{\rm i})$ at a given total dose would tend to generate displacement damage to the NPN BJTs. The Messenger--Spratt equation could be used to describe the experimental data for the latter case.