Frequency limit of double-barrier resonant-tunneling oscillators

The speed of operation of negative differential resistance (NDR) devices based on resonant tunneling in a double-barrier quantum-well structure is considered. It is shown that the intrinsic RC delay of a single barrier limits the frequency of active oscillations to fmax =1/(2πτ), where τ=εα−1(λ/c)exp(4πd/λ) with λ being the de Broglie wavelength of the tunneling electron, d the barrier thickness, ε the dielectric permittivity, c the speed of light, and α≊1/137 the fine-structure constant. The relevance of this estimate to recent experimental results is discussed. An alternative mechanism for the NDR is proposed—not involving resonant tunneling. It should be observable in various single-barrier structures in which tunneling occurs into a two-dimensional system of states. In a double-barrier structure, specially designed experiments are required to distinguish this effect from resonant tunneling.