Amorphous InGaZnO Thin-Film Transistors-Part I: Complete Extraction of Density of States Over the Full Subband-Gap Energy Range

A combination of the multifrequency C-V and the generation-recombination current spectroscopy is proposed for a complete extraction of density of states (DOS) in amorphous InGaZnO thin-film transistors (a-IGZO TFTs) over the full subband-gap energy range (E-V <= E <= E-C) including the interface trap density between the gate oxide and the a-IGZO active layer. In particular, our result on the separate extraction of acceptor- and donor-like DOS is noticeable for a systematic design of amorphous oxide semiconductor TFTs because the former determines their dc characteristics and the latter does their threshold voltage (V-T) instability under practical operation conditions. The proposed approach can be used to optimize the fabrication process of thin-film materials with high mobility and stability for mass-production-level amorphous oxide semiconductor TFTs.