A dose rate independent pMOS dosimeter for space applications

A dual-dielectric pMOS dosimeter (RADFET) has been recently designed at Sandia. The RADFET consists of a thermally grown oxide and a CVD deposited nitride. With a negatively applied bias, holes are generated in the SiO₂ transport and are trapped at the SiO ₂/Si₃N₄ interface producing a measurable threshold-voltage shift. Because holes are trapped away from the Si/SiO₂ interface, hole neutralization by tunneling and interface-trap buildup are minimized resulting in little fade or annealing of the RADFET output response. RADFETs were irradiated at dose rates from 0.002 to 50 rad(Si)/s with biases from -5 to -20 V. RADFETs were also annealed for times up to 10⁷ s at temperatures up to 100°C. Within experimental uncertainty, no difference in RADFET output response at a given bias was observed over the dose rate range examined and for 25°C anneals. At an anneal temperature of 100°C only a 20% decrease in RADFET output response was observed. These results show that Sandia´s RADFETs exhibit little or no fade of their output characteristics and are ideal for low dose rate space applications