Realistic RTDs need extremely high doping to provide enough carriers for high current densities. However, Impurity scattering can destroy the RTD performance. The dopants are therefore typically spaced 20-100nm away from the central double barrier structure. Electrons diffuse from high density contacts to low doping region around the central RTD. This diffusion process creates a charge inbalance in the central device region and the electrostatic potential therefore floats up to to repel the electrons from that region. The overall RTD is raised up in its electrostatic potential possibly overall high above the Fermi levels.
An applied bias will primarily drop over the low doping collector region, but some of the potential will also drop over the low doping emitter region, resulting in the formation of a triangular potential well in the emitter. This triangular potential well confines quasi-bound states in the emitter which will couple to the resonance in the central RTD and modulate its current.
The quasi bound states in the emitter can become extremely narrow in the linewidth, due to the strong confinement of the emitter bound states by a very thick emitter potential bulge and a double barrier.
- Realistic RTDs have a non-uniform doping profile that keeps dopants away from the central RTD to avoid ionized impurity scattering
- The non-uniform doping profile results in a non-uniform electrostatic potential profile above the Fermi levels in the high contact regions
- An applied bias causes a potential drop not only in the central RTD region but also in parts of the emitter. That potential drop in the emitter creates a triangular potential well.
- The tri-angular potential well in the emitter binds quantum mechanical states which can interact with the central RTD states.
- The quasi-bound emitter states resonance widths vary exponentially with the applied bias and can become extremely narrow.
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Università di Pisa, Pisa, Italy