Semiconductor junctions are built by chemistry. Doping silicon with phosphorus creates n-type regions; doping with boron creates p-type regions. The junction between them — where the doping profile changes — controls current flow and defines every transistor, diode, and solar cell. The junction is a spatial change in composition. The material on one side is chemically different from the material on the other.
Wu, Zhou, Xing, and Kong (arXiv 2602.23298, February 2026) create a metal-semiconductor-metal junction in bilayer blue phosphorus using only mechanical deformation. No doping, no foreign atoms, no chemical modification. The same material, with the same composition everywhere, switches from metallic to semiconducting based on the distance between its two layers.
Blue phosphorus in the A1B1 stacking configuration is metallic — the two layers hybridize strongly enough that their combined band structure has no gap. When the interlayer spacing increases — through a localized bubble or corrugation in the bilayer — the hybridization weakens. Above a threshold separation, a band gap opens. The region under the bubble is semiconducting. The flat regions on either side remain metallic. The result is a metal-semiconductor-metal homojunction defined entirely by geometry.
The transport properties follow from the spatial variation of the band gap. Electrons traveling through the flat metallic regions encounter a semiconducting barrier at the bubble, where they must tunnel through the gap. The tunneling probability depends exponentially on the barrier height and width, both set by the bubble geometry. Flattening the bubble restores metallicity and allows ballistic transport. The device switches between conducting and insulating states by mechanical deformation — push the layers together and it conducts; pull them apart and it blocks.
The junction exhibits momentum selectivity. Not all electronic states are equally affected by the interlayer separation. States derived from sigma-type bonding between phosphorus atoms within each layer — strong, directional bonds in the plane — continue to conduct through the bubble because their transport channel doesn't depend on interlayer coupling. States derived from pi-type bonding — delocalized orbitals that hybridize between layers — are suppressed when the layers separate. The bubble acts as a momentum filter, transmitting some electronic channels and blocking others.
Two device concepts emerge. A mechanically switchable memory element achieves ON/OFF ratios of 30 by toggling between flat (metallic) and bubbled (semiconducting) configurations. A sliding rheostat provides exponential resistance tuning for angstrom-scale displacements — moving one layer relative to the other by a fraction of a nanometer produces measurable resistance changes, enabling displacement sensing at the atomic scale.
The material is the same everywhere. The junction is pure geometry. Chemistry draws the boundaries in conventional electronics. Here, shape draws them instead.