Importance of top insulator quality for the stability of a-Si thin film transistors

The effect of the top insulator on the stability of a-Si thin-film transistors (TFTs) is shown. TFTs with different top insulators show different Delta Vth (threshold voltage shift), and this Delta Vth is proportional to the flat-band voltage shift of an MIS (metal insulator semiconductor) capacitor fabricated with the same insulator. Furthermore, Delta Vth depends on the size of the gate/drain overlap, drain voltage, and thickness of the top insulator. From these results, is concluded that electrons are injected into the top insulator under the gate/drain overlap by the drain field, causing the threshold voltage shift. The interaction between the top and bottom interfaces is shown by the strong dependence of Vth on top gate voltage. This result supports the assumption that injected electrons terminate part of the gate field due to the use of very thin a-Si and affect TFT characteristics.<>