  Two-dimensionally confined electrons showing unusually large thermopower (S)
have attracted attention as a potential approach for developing high
performance thermoelectric materials. However, enhanced S has never been
observed in electric field induced two-dimensional electron gas (2DEG). Here we
demonstrate electric field modulation of S for a field effect transistor (FET)
fabricated on a SrTiO3 crystal using a water-infiltrated nanoporous glass as
the gate insulator. An electric field application confined carrier electrons up
to ~2E15 /cm^2 in an extremely thin (~2 nm) 2DEG. Unusually large enhancement
of |S| was observed when the sheet carrier concentration exceeded 2.5E14 /cm^2,
and it modulated from ~600 (~2E15 /cm^2) to ~950 {\mu}V/K (~8E14 /cm^2), which
were approximately five times larger than those of the bulk, clearly
demonstrating that an electric field induced 2DEG provides unusually large
enhancement of |S|.
