Radiation damage in bipolar transistors caused by thermal neutrons

DMILL bipolar transistors (npn) were exposed to thermal and fast 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 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 if transistors are irradiated with thermal neutrons and as /spl Delta/1//spl beta/ = K/sub eq//spl middot//spl Phi//sub eq/, where /spl Phi//sub eq/ is 1 MeV equivalent fluence if transistors are irradiated with fast neutrons behind Cd shield. Large damage factor K/sub T/ /spl sim/ 3/spl middot/K/sub eq/ was measured. Thermal neutrons don't 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 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.