Michael N. Feiginov

Resonant-tunnelling-diode oscillators operating at frequencies above 1.1 THz

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.

Operation of resonant-tunneling diodes with strong back injection from the collector at frequencies up to 1.46 THz

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.

Spectral characterization of broadband THz antennas by photoconductive mixing: toward optimal antenna design

The spectral characterization of a broadband antenna using a pump-probe photomixing continuous-wave (CW) terahertz (THz) system is presented. The high dynamic range of the system, comparable to or better than that of similar pump-probe systems reported in the literature, provides an accurate means of antenna characterization. The planar antenna exhibits a log-periodic behavior at low frequencies, a bow-tie behavior at high frequencies, and a resonance characteristic in between, well in agreement with the antenna geometry. It is predicted that an improved geometry that extends the log-periodic behavior to higher frequencies would contribute significantly in enhancing the broadband performance of antenna-coupled photomixers.

Displacement currents and the real part of high-frequency conductance of the resonant-tunneling diode

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.

Does the quasibound-state lifetime restrict the high-frequency operation of resonant-tunnelling diodes?

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.

Effect of the Coulomb interaction on the response time and impedance of the resonant-tunneling diodes

We demonstrated that the response time of the resonant-tunneling structures (τresp) can be much smaller as well as much larger than the quasibound-state lifetime. A simple analytical expression for the impedance of the resonant-tunneling diode has been derived, it takes into account the Coulomb interaction and the quasibound-state lifetime. A simple equation relating τresp to the static differential conductance has also been obtained; it allows one to get τresp in the static measurements of the current–voltage curve.

High-frequency nonlinear characteristics of resonant-tunnelling diodes

The nonlinear response of resonant-tunnelling diodes (RTDs) is analysed theoretically at high frequencies (HFs), which are far above the diode’s tunnel-relaxation-time limit. We show that the HF I-V curve in this regime is substantially different from the static one. The calculated static and oscillation characteristics of a HF RTD oscillator are in good agreement with our measurement results. Our RTD model is applicable to RTDs working at THz frequencies.

Operation of resonant-tunneling diodes beyond resonant-state-lifetime limit

We show, first, that the charge relaxation (response) time of resonant-tunneling diode (RTD) can be significantly shorter or longer than the resonant-state lifetime, depending on RTD operating point and RTD parameters. Coulomb interaction between electrons is responsible for the effect. Second, it is also demonstrated that the operating frequencies of RTDs are limited neither by resonant-state lifetime nor by relaxation time; particularly in the RTDs with heavily doped collector, the differential conductance can stay negative at the frequencies far beyond the limits imposed by the time constants. We provide experimental evidences for both effects.

Operation of resonant-tunnelling oscillators beyond tunnel lifetime limit

The tunnel lifetime of the electrons in the quantum well of a resonant-tunnelling diode (RTD) is usually assumed to be imposing an inherent fundamental limitation on the operating frequencies of RTD oscillators. Here, we experimentally demonstrate that one can overcome the limitation by heavy doping of the RTD collector. We present RTD oscillators with the fundamental oscillation frequency up to a factor of 3 above the tunnel lifetime limitation. Our results indicate that the inherent frequency limitations of RTDs should lie far above the state-of-the-art frequency of the contemporary RTD oscillators.

Analysis of limitations of terahertz p-i-n uni-traveling-carrier photodiodes

The uni-traveling-carrier p-i-n photodiodes have been analyzed both in the ballistic and drift modes of operation. The analytical expressions for the terahertz (THz) power achievable with the photodiodes have been derived in the drift-diffusion approximation, the optimum photodiode parameters have been identified and different THz-power limitation mechanisms (space-charge effects, heating, absorption saturation, etc.) have been considered. It has been shown that the THz powers on the level of 300 μW at ≳1 THz, 10 mW at 0.3 THz and 30 mW at 0.1 THz should be achievable. That would give more than an order of magnitude improvement as compared to the present state-of-the-art results. At the lower end of the THz-frequency range, the main limitation mechanisms are the heating and space-charge effects. At the higher frequencies, at ≳1 THz, the latter mechanism should play the major role.