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

Abstract

DMILL bipolar transistors (npn) were exposed to 24 GeV protons and fast and thermal neutrons to fluences up to 6 10 14 n/cm 2 . Transistor common emitter current gain ( = I collector I base ) was measured after irradiations. It was found that degradation scales as 1(1 ) = k T 8 T where 8 T is the fluence of thermal neutrons and as 1(1 ) = k eq 8 eq , with 8 eq 1-MeV equivalent fluence, if transistors are irradiated with protons or fast neutrons. Large damage factor k T 3 k eq was measured. Thermal neutrons do not have sufficient energy to displace a Si atom. Their damage mechanism is, therefore, identified with 10 B(n ) 7 Li reaction from which energetic 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 1(1 ) = k T 8 T + k eq 8 eq .

Combined Bulk and Surface Radiation Damage Effects at Very High Fluences in Silicon Detectors: Measurements and TCAD Simulations

“…In case of proton irradiation, this saturation value is reached at relatively low fluences of the order of 1×10 14 1-MeV n/cm 2 , the exact value depending on the proton energy [26]. In case of neutrons, the total ionizing dose is due to the gamma-ray background, and oxide charge saturation is reached at fluences of the order of 1x10 16 1-MeV n/cm 2 [27]. Since surface damage has an effect on the charge collection efficiency only for very large fluences higher than 1x10 16 1-MeV n/cm 2 , the oxide charge density can be assumed to be saturated for both protons and neutrons.…”

Planar sensors for future Vertex and Tracking Detectors

“…Neutron irradiated sensors showed a reduced RGH rate compared to sensors at similar proton fluences. This indicates a relation to surface damage, especially charge up of the silicon oxide due to ionizing radiation, which is much smaller for neutron irradiation (still there is some gamma radiation component in the radiation field of the used reactor at JSI 7 [17,18]). It is not easy to spot this behaviour without systematic scan of the phase space.…”

Combined effect of bias and annealing in gamma and neutron radiation assurance tests of SiGe bipolar transistors for HEP applications

“…Background gamma doses received by samples when irradiated in the F19 channel have been measured with MOS transistors (radfets). It has been estimated that a dose of about 100 krad(Si) is delivered per every 1 MeV equivalent fluence of 1 Â 10 14 n/cm 2 without the Cd shield [14], therefore, this is an upper limit of the background dose for our case.…”

“…In addition, samples were irradiated with 1 mmthick Cadmium shield which absorbs thermal neutrons but does not influence the fast part of the spectrum. Thermal neutrons can enhance excessively the damage on the transistors [14], also making more difficult the dosimetry, therefore we prefer to avoid them for this bias effects study. In these conditions, the neutron flux can be expressed in terms of 1 MeV neutrons NIEL equivalent as U eq = 0.9 Â U fast [15].…”