Neutron Exposure and Electrical Analysis of P-Type Boron-Doped Silicon Resistors and Transistors

Neutron detectors are deployed at ports of entry across the world to monitor people and cargos for smuggled nuclear materials and are often incorporated in nuclear power plant designed to monitor power levels and ensure safe operations. With the supply of Helium-3 rapidly decreasing and due to increase of terrorism threats it is vital to U.S. national security that a viable alternative material be identified, and a new neutron detector design made available, especially for portal monitoring applications. The interest to study Boron-10 as an alternative to helium-3, due to the vast natural supply that the United States possesses and its nontoxic characteristics, is increasing in different research arenas. Our work consists on taking advantage of the near 20% of boron-10 present in naturally occurring boron which (just as phosphorus) is used to dope semiconductors. Boron doped semiconductor wafers were characterized using four-point probe techniques. Resistors and transistors made with various levels of boron concentration were exposed to a thermal flux at various fluencies at Los Alamos National Laboratory. The reaction, ¹⁰B+n → L_i + α, caused by neutron irradiation, introduces impurities in the silicon lattice thus producing measurable differences in electronic parameters. These changes are likely to be proportional to the fluence of the source, and hence to the neutron flux. The results show that for irradiated resistors possessing very high values of boron concentration there is a significant reduction in resistivity. This trend is not seen for medium or low values of boron. Additionally, there was no observation of significant changes in other electronic parameters such as threshold voltage or trans-conductance, for the transistors exposed and tested.