  In electric double layer transistors with SrTiO3 single crystals, we found
distinct differences between electrostatic charge accumulation and
electrochemical reaction depending on bias voltages. In contrast to the
reversible electrostatic process below 3.7 V with a maximum sheet charge
carrier density, nS, of 1014 cm-2, the electrochemical process causes
persistent conduction even after removal of the gate bias above 3.75 V. nS
reached 1015 cm-2 at 5 V, and the electron mobility at 2 K was as large as 104
cm2/Vs. This persistent conduction originates from defect formation within a
few micrometers depth of SrTiO3.
