Thin film transistor structures may include a regrown source or drain material between a channel material and source or drain contact metallization. The source or drain material may be selectively deposited at low temperatures to backfill recesses formed in the channel material. Electrically active dopant impurities may be introduced in-situ during deposition of the source or drain material. The source or drain material may overlap a portion of a gate electrode undercut by the recesses. With channel material of a first composition and source or drain material of a second composition, thin film transistor structures may display low external resistance and high channel mobility.