  We report the operation of LaAlO_3 / SrTiO_3 depletion mode top-gated
junction field-effect transistors using a range of LaAlO_3 thicknesses as the
top gate insulator. Gated Hall bars show near ideal transistor characteristics
at room temperature with on-off ratios greater than 1000. Lower temperature
measurements demonstrate a systematic increase in the Hall mobility as the
sheet carrier density in the channel is depleted via the top gate, providing a
route to higher mobility, lower density electron gases in this system.
