  Separately contacted double layers of a 2d electron - 2d hole gases have been
prepared in GaAs separated by thin AlGaAs barriers with thicknesses down to 15
nm. The molecular-beam-epitaxial growth was interrupted just before the barrier
in order to use in situ focussed-ion-beam implantation to pattern contacts
which extend underneath the barrier. The two charge gases form upon biasing the
p- and n-type contacts underneath and above the barrier in forward direction
and show independent transistor-like behavior.
