  Two-dimensional (2D) device structures have recently attracted considerable
attention. Here, we show that most 2D device structures, regardless vertical or
lateral, act as a lateral monolayer-bilayer-monolayer junction in their
operation. In particular, a vertical structure cannot function as a vertical
junction as having been widely believed in the literature. Moreover, due to a
larger electrostatic screening, the bilayer region in the junction always has a
smaller band gap than its monolayer counterpart. As a result, a potential well,
aside from the usual potential barrier, will form universally in the bilayer
region to affect the hole or electron quantum transport in the form of
transmission or reflection. Taking black phosphorus as an example, we show that
an oscillation in the transmission coefficient can be clearly resolved in a
two-electrode prototypical device by non-equilibrium Green function combined
with density functional theory calculations and the results can be
qualitatively understood using a simple quantum well model. The presence of the
quantum well is vital to 2D device design, including the effective tuning of
quantum transmission by a vertical electric field.
