  Inversion- and depletion-regions generally form at the interfaces between
doped leads (cladding layers) and the active region of polar heterostructures
like AlN/GaN and other nitride compounds. The band bending in the depletion
region sets up a barrier which may seriously impede perpendicular electronic
transport. This may ruin the performance of devices such as quantum-cascade
lasers and resonant-tunneling diodes. Here we introduce the concepts of
polarization balance and polarization-balanced designs: A structure is
polarization balanced when the applied bias match the voltage drop arising from
spontaneous and piezeolectric fields. Devices designed to operate at this bias
have polarization-balanced designs. These concepts offer a systematic approach
to avoid the formation of depletion regions. As a test case, we consider the
design of AlN/GaN double barrier structures with
Al$_{\tilde{x}}$Ga$_{1-\tilde{x}}$N leads. To guide our efforts, we derive a
simple relation between the intrinsic voltage drop arising from polar effects,
average alloy composition of the active region, and the alloy concentration of
the leads. Polarization-balanced designs secure good filling of the ground
state for unbiased structures, while for biased structures with efficient
emptying of the active structure it removes the depletion barriers.
