= Solution
A common implementation uses a <gallium arsenide>/<aluminium gallium arsenide> heterostructure with a <two-dimensional electron gas> near the interface. <electrons> supplied by remote donors occupy quantized normal-direction states, while an undoped spacer reduces ionized-impurity scattering. Surface metallic gates, insulated from the conducting layer, deplete selected regions when negatively biased.
Leave a small conducting island bounded by depleted regions and connect it weakly to source and drain reservoirs through two tunable constrictions. A plunger gate controls the island potential/population; barrier gates control tunnel coupling. Ohmic contacts connect the reservoirs to the measurement circuit. \b[Vertical heterostructure confinement plus lateral <depletion force> defines the dot.] A 2DEG without lateral gates would remain extended in its plane.
The original device diagram labels the gates, island, tunnel barriers and reservoirs. Charge stability and spectroscopy versus gate/bias should verify that an isolated weakly coupled island has actually formed; a lithographic outline alone is insufficient.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-2-dot-charging.png]
{title=Gate-defined dot, charging parabolas, charge steps and Coulomb diamonds}
{description=Original schematic and constant-capacitance model plots. The conductance map is an idealized symmetric-bias cartoon, not experimental data or a microscopic tunnelling calculation.}
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