We present resonant-tunnelling-diode (RTD) oscillators operating at the fundamental frequency of 1111 GHz. We show that our RTDs and RTD oscillators have much room for further improvement of their parameters and for further increase of their operating frequencies. The operating frequencies of several THz should be achievable with RTD oscillators. Our study also shows that operation of RTDs beyond the relaxation-time limit at THz frequencies should be possible. RTD oscillators under study 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 applications.

We have shown, firstly, that the response time (τresp) of the double-barrier resonant-tunnelling diode (RTD) can be much smaller as well as much larger than the quasibound-state lifetime in the quantum well (τdwell). Secondly, the real part of the RTD conductance can be negative and large at the frequencies higher than the reciprocal τdwell in the RTDs with a heavily doped collector without spacer layers. The Coulomb interaction of the electrons in the quantum well with emitter and collector is responsible for the effects. A simple analytical expression for the impedance of the RTD has been derived and an equivalent circuit has been proposed.

In search for possibilities to increase the operating frequencies of resonant-tunneling diodes (RTDs), we are studying RTDs working in an unusual regime. The collector side of our diodes is so heavily doped that the collector depletion region is fully eliminated in our RTDs and the ground quantum-well subband stays immersed under (or stays close to) the collector quasi-Fermi level. The electron injection from the collector into the RTD quantum well is very strong in our diodes and stays comparable to that from the emitter in the whole range of RTD operating biases. Our RTDs exhibit well pronounced negative-differential-conductance region and peak-to-valley current ratio around 1.8. We demonstrate operation of our diodes in RTD oscillators up to 1.46 THz. We also observe a fine structure in the emission spectra of our RTD oscillators, when they are working in the regime close to the onset of oscillations.

I have shown that weak variation of the tunnel transparency of the collector barrier with bias has substantial (and frequently crucial) effect on the high-frequency properties of the resonant-tunneling diodes (RTDs). Also it has been shown that the real part of the RTD conductance can be negative and large at the frequencies much higher than the reciprocal quasibound-state lifetime in the quantum well between the barriers of RTD, if (as opposed to common practice) the RTD collector is heavily doped and does not have thick spacer layers. The displacement currents are responsible for the effects. A simple equivalent circuit of RTD is proposed, and it fairly well describes the published experimental data.

The review outlines the basic principles of operation of resonant-tunnelling diodes (RTDs) and RTD oscillators followed by an overview of their development in the last decades. Further, we discuss different types of RTDs and RTD oscillators, the limitations of RTDs due to parasitics, inherent limitations of RTDs and operation of RTDs as detectors. We also give an overview of the present status of sub-THz and THz RTD oscillators and give several examples of their applications.