RADIATION HARDNESS OF PUNCH-THROUGH AND FET BIASED SILICON MICROSTRIPDETECTORS

Silicon microstrip detectors can be biased with polysilicon resistors or Field Effect Transistor (FET) biasing structures. Polysilicon resis tors are radiation hard, but using the FET biasing principle reduces p rocessing costs and can give better noise performance. A set of micros trip detectors has been manufactured with a standard radiation sensor process in order to assess the radiation hardness of punch-through and FET biasing. Eight different bias geometry designs were used in order to study the effects of bias gap lengths and strip end geometries on the detector characteristics. The test detectors were irradiated at se veral dose levels up to 75 kGy with a Co-60 source. Initially the devi ces had very low oxide charge (3 x 10(10) cm(-2)) and leakage current levels (60 pA per strip). The dynamic resistance was in the 1 G Omega range, which is higher than the values which can be achieved by conven tional polysilicon resistors. Radiation exposure gave significant incr eases in the leakage current of the devices. This causes large reducti ons in the dynamic resistance, and detector performance will degrade, The degradation due to increased leakage current was present for all s trip end geometries, and it could not be compensated by changing the g ate voltage.