Stability of hydrogenated amorphous silicon thin film transistors on polyimide substrates

The inverted staggered hydrogenated amorphous silicon thin film transistors (a-Si:H TFT) were fabricated on 6 μm thick polyimide substrate at temperatures below 300 °C. The a-Si:H TFT off current is below 10−12 A, the on/off current ratio is ∼107, the threshold voltage is ∼2-3 V, field effect mobility is ∼0.5 cm2/Vs, and the subthreshold slope is ∼0.4 V/decade. The stabilities of a-Si:H TFTs were studied and the device parameters determined before and after a bias stress of VGS = 20 V for t = 10, 102, 103 and 104s. The threshold voltages shifted to higher values and on/off ratio decreased with the duration of bias stress. The device characteristics were measured in the dark and under the light illumination. Threshold voltages and on/off current ratio both decreased. Temperature dependant measurements of transfer and output characteristic of a-Si:H TFT in the range from 22 °C to 125°C were also investigated. When temperature is increased threshold voltages decreased and the field effect mobility increased.