Evaluation of radiation tolerance of FETs used for Astro-E2 hard X-ray detector (HXD-II)

We evaluated the radiation tolerance of three types of metal-can MOS Field Effect Transistors (FETs). They are candidates for flight electronics of the Hard X-ray Detector (HXD-II) experiment which is onboard the cosmic X-ray satellite Astro-E2 scheduled for launch in 2005. We irradiated FETs with a Co 60 γ -ray source under several different experimental conditions, and measured changes in their I – V characteristic curves. After a 10 krad irradiation during which the gate voltage is set at 0 V, all types showed a decrease in the switching voltage by ∼ 0.2 –0.4 V. In addition, the gate conductance increased under some irradiation conditions. These experimental results may be explained in terms of trapped charges and boundary levels in the oxide layer beneath the gate electrode. We have confirmed that at least two types of FETs can be used in our satellite-borne experiment, one as relay-driving FETs and the other in TTL-ECL conversion circuits.