DEVELOPMENT OF TUNNELING-CURRENT-CONTROLLED THIN FILM TRANSISTORS APPLYING SCHOTTKY CONTACTS

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We made attempts to develop tunneling-current controlled thin film transistor (TFTs) applying Schottky contacts at source and drain, and obtained the results as follws :(1)Improvements of fabrication process for Schottky contacts in poly-Si TFTsPd silicide which was prepared by evaporating Pd and annealing at 250ﾟC was found to be most suitable for Schottky contacts. However, there was still a problem in which the source/drain was short-circuited to the gate electrode due to the deposited Pd on the side wall of gate insulator. This problem was solved by improving the progress in the following way.・The etching period for removing native oxide before Pd evaporation was increased in order to assure sufficient side-etching of the gate oxide.・The thickness of Pd was minimized to form silicide entirely during the annealing at 250ﾟC.(2)Operation as a p-channel conventional FETAlthough the tunneling-current-controlled TFTs were found to have a high transconductance, a large leakage current was observed because of hole injection from the drain. However, when the TFTs were operated in conventional p-channel FET mode, the hole field-effect mobility as high as 15 cm^2/Vs was achieved with a low leakage surrent. This value can be well compared to those for electrons of 20 cm^2/Vs in conventional n-channel poly-Si TFTs. This result may suggest tunneling-current-controlled mode to be suitable for p-channel FETs with a high transconductance.3)Precise measurement of contact resistance at source/drain with Schottky barrier contact by Gate-Four-Probe methodWe measured precisely the contact resistance at source/drain with Schottky barrier contact using a Gate-Four-Probe method. It was found that the contact resistance decreased exponentially with increasing gate voltage ; this confirmed that the tunneling-current through the Schottky barrier was able to be controlled by gate voltage.