Sol–Gel-Derived Zn_(1-x)Mg_xO Thin Films Used as Active Channel Layer of Thin-Film Transistors

Sol–gel derived n-type Zn_(1-x)Mg_xO (x=0–0.45) thin films and thin-film transistors (TFTs) with active channel layers made of the films were investigated. The films were prepared at 500°C. The effects of Mg doping on the crystallinity, optical transparency, grain size, and charge-carrier concentration (n) of the films were examined. The Fermi level of the films, as derived from the temperature dependence of n, was ∼0.12 eV below the conduction band. The donor concentration and donor level (E_d) were derived by a curve fitting method based on the electrical neutrality condition. E_d was found to be ∼0.3 eV below the conduction band. The composition dependence of the TFT output characteristics was interpreted and correlated to the width of the depletion region adjacent to the grain boundaries. When the grains were almost depleted at x=0.2, the TFT showed an enhancement mode and an on/off ratio of 10⁶.