Resonant-tunnelling diodes beyond quasi-bound-state lifetime limit

Abstract

Resonant-tunnelling diodes (RTDs) are used for studies of fundamental aspects of tunnelling and also for realization of oscillators at high frequencies, particularly in THz frequency range. Also, the RTDs can be considered as the building blocks of different electronic structures, including optical, e.g., quantum-cascade lasers. It is generally accepted that the inherent limitation of the operating frequency and the charge relaxation (response) time of RTD is determined by the quasi-bound-state lifetime. The simple picture is not generally correct. Here we show, first, that the Coulomb interaction between electrons can lead to large reduction/increase of the relaxation time. Second, we demonstrate that the operating frequencies of RTDs are limited neither by quasi-bound-state lifetime, nor by relaxation-time constants; particularly the differential conductance of RTDs can stay negative at the frequencies far beyond the limits imposed by the time constants. Here we provide the experimental evidences for both effects. We demonstrate negative differential conductance up to the frequency of 12 GHz in our RTDs with the inverse quasi-bound-state lifetime of around 1 GHz. Also the relaxation time in our RTDs was shown to be a factor of 2 shorter/longer (depending on the RTD operating point) than the quasi-bound-state lifetime. According to our theory, the effects are not limited to the low frequencies and the same effects should persist at higher frequencies also. Our results indicate not only that nowadays operating frequencies of RTDs could be increased, but the results also elucidate the fundamental limitations of the whole class of resonant-tunnelling structures: single-electron-transistor-like structures, multi-barrier structures, quantum-cascade lasers, etc.