Strain-Free Layered Semiconductors for 2D Transistors with On-State Current Density Exceeding 1.3 mA μm^-1

High-mobility and air-stable two-dimensional (2D) Bi₂O₂Se semiconductor holds promise as an alternative fast channel material for next-generation transistors. However, one of the key challenges remaining in 2D Bi₂O₂Se is to prepare high-quality crystals to fabricate the high-performance transistors with a high on-state current density. Here, we present the free-standing growth of strain-free 2D Bi₂O₂Se crystals. An ultrahigh Hall mobility of 160 000 cm² V^-1 s^-1 is measured in strain-free Bi₂O₂Se crystals at 2 K, which enables the observation of Shubnikov-de Haas quantum oscillations and shows substantially higher (>4 times) mobility over previous in-plane 2D crystals. The fabricated 2D transistors feature an on-off current ratio of ∼10⁶ and a record-high on-state current density of ∼1.33 mA μm^-1, which is comparable to that of commercial Si and Ge n-type field-effect transistors (FETs) for similar channel length. Strain-free 2D Bi₂O₂Se provides a promising material platform for studying novel quantum phenomena and exploration of high-performance low-power electronics.