Bulk damage in DMILL npn bipolar transistors caused by thermal neutrons versus protons and fast neutrons

DMILL bipolar transistors (npn) were exposed to 24 GeV protons and fast and thermal neutrons to fluences up to 6/spl middot/10/sup 14/ n/cm/sup 2/. Transistor common emitter current gain (/spl beta/=I/sub collector//I/sub base/) was measured after irradiations. It was found that /spl beta/ degradation scales as /spl Delta/(1//spl beta/)=k/sub T//spl middot//spl Phi//sub T/ where /spl Phi//sub T/ is the fluence of thermal neutrons and as /spl Delta/(1//spl beta/)=k/sub eq//spl middot//spl Phi//sub eq/, with /spl Phi//sub eq/ 1-MeV equivalent fluence, if transistors are irradiated with protons or fast neutrons. Large damage factor k/sub T//spl sim/3/spl middot/k/sub eq/ was measured. Thermal neutrons do not have sufficient energy to displace a Si atom. Their damage mechanism is, therefore, identified with /sup 10/B(n,/spl alpha/)/sup 7/Li reaction from which energetic /spl alpha/ and Li particles produce bulk damage in the base of the device. Boron is used as the base dopant in these transistors having also highly doped regions below base contacts. Irradiations with neutrons with energies distributed from thermal to fast show that gain degradation adds up as /spl Delta/(1//spl beta/)=k/sub T//spl middot//spl Phi//sub T/+k/sub eq//spl middot//spl Phi//sub eq/.