The operational amplifier (op-amp) with high gamma irradiation capability of over 30 kGy have been fabricated by 4H-SiC MOSFETs for measuring instruments which are installed in nuclear power plants. The chip size was 0.7 mm x 1.0 mm, and they consisted of five n-channel MOSFETs and three p-channel MOSFETs on the same die. The output waveform after having irradiated 50 kGy at a rate of 60 Gy/hr was amplified without distortion. On the other hand, the offset voltage became unstable when gamma integral dose was beyond 30 kGy and it at 50 kGy increased to +7.2 mV. For reduction of gamma irradiation influence, we proposed the MOSFETs structure which has field plate (FP) electrodes connected to isolation layer electrically. We indicated that the proposal device had the potential of gamma irradiation capability of 100 kGy experimentally.

We modified the active layout of an operational amplifier (op-amp) to exhibit high gamma irradiation resistance of over 100-kGy using our 4H-SiC complementary MOS technology, which can be applied for measuring instruments installed in nuclear power plants. The op-amp with the modified active layout features both a thin gate oxide and newly developed gate-electrode structure for suppressing the leakage current. From an experiment we conducted, the leakage current of the p-channel MOSFET with modified active layout remained unchanged from the initial value after irradiation, although that of it with the conventional layout we previously evaluated increased by about two orders of magnitude. The offset voltage of the improved op-amp was maintained below 2.8 mV up to 100-kGy irradiation. The improved op-amp also showed a healthy amplification characteristic without distortion.