  Planar semiconductor heterostructures offer versatile device designs and are
promising candidates for scalable quantum computing. Notably, heterostructures
based on strained germanium have been extensively studied in recent years, with
emphasis on their strong and tunable spin-orbit interaction, low effective
mass, and high hole mobility. However, planar systems are still limited by the
fact that the shape of the confinement potential is directly related to the
density. In this work, we present the successful implementation of a backgate
for a planar germanium heterostructure. The backgate, in combination with a
topgate, enables independent control over the density and the electric field,
which determines important state properties such as the effective mass, the
$g$-factor and the quantum lifetime. This unparalleled degree of control paves
the way towards engineering qubit properties and facilitates the targetted
tuning of bilayer quantum wells, which promise denser qubit packing.
