Radiation-Hard ZnO Thin Film Transistors

We report effects for up to 100 Mrad ( ${{\rm SiO}_2}$ ) gamma-ray exposure on polycrystalline ZnO thin film transistors (TFTs) deposited by two different techniques. The radiation related TFT changes, either with or without electrical bias during irradiation, are primarily a negative ${{\rm V}_{\rm ON}}$ shift and a smaller ${{\rm V}_{\rm T}}$ shift ( $\Delta {{\rm V}_{\rm ON}}\sim - 2.5~\hbox{V}$ and $\Delta {{\rm V}_{\rm T}}\sim - 1.5~\hbox{V}$ for 100 Mrad ( ${{\rm SiO}_2}$ ) exposure). Field-effect mobility remains nearly unchanged. Both, ${{\rm V}_{\rm ON}}$ and ${{\rm V}_{\rm T}}$ shifts are nearly completely removed by annealing at $200^\circ {\rm C}$ for 1 minute and some recovery is seen even at room temperature. We find that our ZnO TFTs are insensitive to electrical bias during irradiation; that is, unbiased measurements are useful worst case test results. To the best of our knowledge, these are the most radiation-hard thin film transistors reported to date.