We investigate experimentally resonant-tunnelling-diode (RTD) oscillators, which are based on RTDs with heavily
doped collector. We demonstrate that such RTD oscillators can work at frequencies, which are far beyond the
limitations imposed by resonant-state lifetime and relaxation time. Exploiting further such RTDs, we have
achieved the record operating frequency of 1.1 THz and show that substantially higher frequencies should be
also achievable with RTD oscillators. RTD oscillators are extremely compact (less than a square millimeter)
room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz
In the paper, we present the experimental data that demonstrate the conductance decrease with bias in the tunnel
Al/GaAs Schottky structure with delta-n-doped 2D channel. The conductance is decreasing due to the increase
in the tunnel-barrier height and the corresponding drop in the barrier tunnel transparency with bias. Theoretical
calculations are in very good agreement with the experimental data, they also show that the mechanism should
lead to the negative value of the differential conductance, if the separation between the subbands in the 2D
channel is sufficiently large. The Al/InAlGaAs/InAlAs and Ti/GaN/AlGaN heterostructures with tunnel
Schottky-barriers are suggested, where the negative differential conductance should be achievable.
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.
A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.