A quantum particle scatters off a double-barrier potential. At special energies, the transmission probability reaches unity (T=1) — resonant tunneling, the basis of tunnel diodes.
Two rectangular barriers separated by a quantum well. For E < V₀, the particle tunnels through classically forbidden regions. When the well supports a quasi-bound state, the transmission T(E) → 1 exactly at resonance — constructive interference of reflections cancels backscattering. The transfer matrix method gives the exact analytic solution. Resonant tunneling diodes (RTDs) exploit this for THz oscillators.