| Summary: | Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (Δ<i>E</i><sub>c</sub>) of ~0.4 eV. A depth profile of background charge density indicated that a steep Δ<i>E</i><sub>c</sub> is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (<i>μ</i><sub>FE</sub>) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>, although a channel/gate insulator interface was formed by an IGZO−111 (<i>μ</i><sub>FE</sub> = ~12 cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>). Device simulation analysis revealed that the improvement of <i>μ</i><sub>FE</sub> in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep Δ<i>E</i><sub>c</sub>. Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.
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