THz RTD oscillators

We have set records for the operating frequencies of RTD oscillators at 1.1 THz in 2011 (Ref. 1) and 1.46 THz in 2014, see Ref. 2. That were the record frequencies for all existing active semiconductor devices. Our RTD oscillators are among the smallest (and perhaps the smallest) THz sources, their size is only a fraction of mm2 (see the picture). RTD oscillators are probably the most promising THz sources for practical applications: RTDs consume current in the mA range and operate at the bias below 1 V, the oscillators work at room temperature and provide rather high output power. Our analysis shows that there is much room for further improvement of the characteristics and parameters of RTD oscillators.

Picture of an RTD oscillator (smaller than 1 square mm) fabricated on a membrane. The oscillator contains an RTD, resonator and an additional emitting (Vivaldi) antenna.

© Michael Feiginov

Membrane THz RTD oscillator

Picture of an on-chip RTD oscillator. Slot antenna: thin dielectric between 2 metallisation layers, RTD connected to one of them by a narrow bridge. Slot emits radiation into chip substrate.

© Michael Feiginov

On-chip slot-antenna THz RTD oscillator

Graph: Spectral density over frequency. Highest frequency: 1.46THz at 0.36µW. Lowest frequency: 0.5THz at 50µW. Slot antennas with 12 and 16µm lengths have comparable performance.

© Michael Feiginov

Emission spectra of slot-antenna RTD oscillators

Traveling-wave microstrip RTD oscillators

We have shown that realization of a hybrid THz source, which combines advantages of both THz quantum-cascade lasers and RTD oscillators, is possible. That could be done in microstrip RTD oscillators, Ref. 3. Such oscillators are similar to THz QCLs with a metal-metal waveguide and with the active part of only a single QCL period (an RTD) as their active core. Assuming realistic parameters of RTD layers, we have shown that such oscillators should be working at sub-THz and THz frequencies, Ref. 3. The oscillators are room-temperature devices and potentially they could provide high output power due to large active volume of the RTD layers.

Diagram: Cross-section. A travelling-wave RTD oscillator consists of two (e.g., gold) metal stripes containing RTD active layers and doped (n++) layers between them.

© Michael Feiginov

Travelling-wave microstrip RTD oscillator

RTD response time

The quasi-bound-state lifetime (τ) is usually supposed to be imposing a fundamental inherent limitation on the operating frequency and the charge relaxation time (τrel) of RTDs. However, we have shown theoretically that the simple picture is not generally correct, Ref. 6-8. First, we have shown that the Coulomb interaction effects can lead to large reduction/increase of τrel, Ref. 6,8. Second, we have shown that the operating frequencies of special RTDs with heavily-doped collector should be limited neither by τ, nor by τrel specifically, the differential conductance of such RTDs should stay negative at the frequencies far beyond the limits imposed by both time constants, i.e., when ωτ»1 and ωτrel»1, Ref. 7,8. We have proved both effects experimentally, Ref. 4.

Further on, we have demonstrated that RTD oscillators can also operate far beyond τ and τrel limits. The parameters ωτ and ωτrel in our oscillators were ωτ≈3 and ωτrel≈10 at 109 GHz, Ref. 5. Then we have demonstrated similar behavior of RTD oscillators at much higher sub-THz frequencies: ωτ≈1.2 and ωτrel≈564 GHz, Ref. 9. That shows that RTD oscillators with proper design of RTDs are limited neither by τ, nor by τrel.

3 Graphs: comparison of measured and simulated RTD conductance as a function of frequency. The theory is well in agreement with the experiment.

© Michael Feiginov

Frequency dependence of the RTD differential conductance

Graph: oscillation frequency of an RTD oscillator (ca. 110GHz). Oscillation frequency is 10x higher than inverse relaxation time and 3x higher than inverse tunnel lifetime of the RTD.

© Michael Feiginov

Oscillation frequency and the inverse tunnel-lifetime and relaxation time of an RTD

RTD theory

Accurate static and dynamic theoretical models for description of RTDs and RTD oscillators are developed, Ref. 1,2,4-8 including a non-linear analysis of large-signal oscillations in RTDs, Ref. 10. Realization of the record RTD oscillators and analysis of the Coulomb-interaction and relaxation-time effects in RTDs mentioned above are based on the developed theory. The theory has also allowed us to identify a new operation regime of RTDs: RTDs with strong back injection from collector, when the quantum-well subband stays emersed under the collector Fermi level, Ref. 2. Such RTDs have been working at the record frequency of 1.46 THz, Ref. 2.

Graph: measured and simulated RTD I-V curves. Both are in good agreementh with each other, in the oscillation region of the RTD as well as regarding output power and oscillation amplitude.

© Michael Feiginov

Simulated and measured I-V curves of an RTD in an oscillator with account of non-linear high-frequency RTD characteristics