Resonant Tunneling Diode (RTD): Two potential barriers with a quantum well between them. At quasi-bound state energies E_n ≈ n²π²ℏ²/(2m b²), transmission reaches T = 1 (perfect transmission through classically forbidden barriers). This is the quantum analog of Fabry-Pérot resonance. Transfer matrix method: each region contributes a 2×2 matrix; T = |t|² where t is the transmission amplitude. RTDs exhibit negative differential resistance (I decreases as V increases) when a resonance passes below the Fermi energy — enabling THz oscillators and logic devices.