Effect of Operating Frequency in the Radiation Induced Buildup of Trapped Holes and Interface States in MOS Devices

摘要

The amount of threshold voltage shift in MOS devices caused by ionizing radiation is strongly dependent on the bias voltage applied to the gate of the MOS device both during and after the radiation. Past experimenters found that n-channel MOSFETs alternately biased off and on did not have a threshold voltage shift that corresponded to the average bias, but the observed shift was more positive than the shift for either the continuously on or off bias conditions. In the work presented here, over 200 test transistors, built in a radiation hardened 4/3 micron process, were irradiated at various operating frequencies, temperatures and bias voltages in an effort to understand the observed threshold voltage shifts were determined through subthreshold current measurements. This research demonstrated the source of this anomalous alternating bias effect on the radiation response of MOS devices is reduced hole trapping and increased interface state buildup in n-channel MOS devices. In p-channel devices, reduced hole trapping is the primary source of this effect. This effect results when holes, trapped at the oxide-silicon interface during the on half of the cycle, are annihilated by radiation-produced electrons pulled into the trapped hole space charged during the off bias portion of the cycle.