Radiation damage of amorphous silicon, thin-film, field-effect transistors

The effect of 60Co radiation on the noise and drain-source current characteristics of hydrogenated amorphous silicon (alpha-Si:H) field-effect transistors (FETs) was examined as a function of dose to cumulative doses as high as approximately 2 x 10(4) Gy. Following these measurements, room-temperature and elevated-temperature annealing of induced radiation damage was examined. The FETs examined are representative of those incorporated in alpha-Si:H arrays under development for various x-ray medical imaging applications. No significant effect upon the noise characteristics of the FETs was observed as a result of the radiation. The predominant drain-source current effect with increasing dose was a shift of the transfer characteristic toward negative gate voltage and/or a decrease of the transfer characteristic subthreshold slope. This resulted in large increases in leakage current for gate voltages where the FETs were initially highly nonconducting. This leakage current increase was less pronounced for more negative gate voltages and was further diminished by maintaining the FETs at a more negative gate voltage during the irradiation. Following the radiation measurements, room-temperature annealing resulted in a 10% to 50% reduction in the leakage current in the first day followed by a logarithmic decrease thereafter. Elevated-temperature annealing for 2 h at 200 degrees C restored FET leakage current and threshold voltage properties to their preirradiation values. The irradiation effects are small for cumulative doses less than approximately 100 Gy, which is larger than the clinical lifetime dose for an imaging detector for chest radiography or for fluoroscopy (with infrequent exposure to the direct beam). For significantly higher dose applications such as mammography, fluoroscopy (with frequent direct beam exposure), and radiotherapy imaging, the results suggest that periodic elevated-temperature annealing or operation of the arrays at more negative gate voltages may be necessary.