Radiation tolerance of the FOXFET biasing scheme for AC-coupled Si microstrip detectors

The radiation response of FOXFETs (field oxide FETs) has been studied for proton, gamma, and neutron exposures. It is shown that the punch-through (PT) conduction mechanism is much less sensitive to radiation damage than the usual surface conduction in FOXFETs. The threshold voltage is strongly affected by the huge buildup of positive charge in the thick gate oxide. This effect can be reduced by decreasing the oxide thickness, but this modification seems to be unnecessary for detector applications. In fact, variations of a few volts of the PT voltage after irradiations in the Mrad range will result in a corresponding similar increase of the strip self-bias in detectors. Such modifications are still acceptable, and can be properly compensated by a suitable increase of the detector backside bias in order to ensure full depletion operation. It is also shown that the radiation-induced variations of the PT voltage can be tailored by the proper choice of the gate bias. Computer simulations have shown that the main radiation effects affecting the PT mechanism are the charge accumulation in the oxide and substrate-type inversion