  InAs/GaSb double quantum wells (QWs) separated by a 100 \AA\ AlSb middle
barrier are grown by molecular beam epitaxy. We report a nanofabrication
technique that utilizes the surface Fermi level pinning position in InAs
$[E_f^s(\rm InAs)]$ for realizing independent electric contacts to each well.
In particular, separate ohmic contacts to the upper InAs quantum well are
achieved by selectively etching down to the InAs, while contacts to the lower
GaSb quantum well are obtained by the depletion method. For the latter, the
upper InAs quantum well is locally pinched off by top etched trenches capped
with a remaining 2-3 nm InAs layer. As a result of a relatively low $E_f^s(\rm
InAs)$, applying a negative bias gate potential will create a conducting hole
channel in GaSb, and hence a separate ohmic contact to the lower quantum well.
This method is demonstrated with experiment and the support of a
self-consistent band bending calculation. A number of experiments on separately
probing Coulomb and tunnel-coupled InAs/GaSb systems now become accessible.
