High-performance Sn-doped Ga₂O₃ FETs by co-sputtering: Depletion mode versus enhancement mode

High-performance Sn-doped gallium oxide (Ga₂O₃) films were deposited on Si/SiO₂ substrates to fabricate field-effect transistors (FETs) via room-temperature co-sputtering. The performances of the SiO₂-supported FETs were optimized by varying the co-sputtering power of Sn to demonstrate a large on-state current (I_ON) of 2.5 mA/mm and a high on/off ratio of over 10⁶. However, it is accompanied by a normally-on behavior with a negative threshold voltage (V_TH), namely depletion mode (d-mode). Importantly, this d-mode can be converted to enhancement mode (e-mode) by adopting high-k Ta₂O₅/pristine Ga₂O₃ dielectrics, and extra nitrogen (N) doping in the Sn-doped Ga₂O₃ channels. Finally, the performances of the e-mode FETs with high-k dielectrics can be promoted via a Sn-doped Ga₂O₃/(Sn, N)-doped Ga₂O₃ dual-layer channel structure, which presents a large I_ON over 2 mA/mm, a high on/off ratio exceeding 10⁷, and a small V_TH below 5 V. Furthermore, the FET with dual-layer channel shows excellent stability, which is reflected by the bias stress measurement and stable performances after 3-month exposure to air. This remarkably-improved performance is attributed to the enhanced field effect by the high-k dielectrics, and the dual-layer channel structure that possesses both high stability of the (Sn, N)-doped layer and high current capability of the Sn-doped layer.