Transistor structures with a channel semiconductor material that is passivated with two-dimensional (2D) crystalline material. The 2D material may comprise a semiconductor having a bandgap offset from a band of the channel semiconductor. The 2D material may be a thin as a few monolayers and have good temperature stability. The 2D material may be a conversion product of a sacrificial precursor material, or of a portion of the channel semiconductor material. The 2D material may comprise one or more metal and a chalcogen. The channel material may be a metal oxide semiconductor suitable for low temperature processing (e.g., IGZO), and the 2D material may also be compatible with low temperature processing (e.g.,