Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent terahertz (THz) sources operating at room temperature. In this paper, sub-THz and THz oscillators with RTDs integrated on planar circuits are described. Fundamental oscillation up to 0.65 THz and harmonic oscillation up to 1.02 THz were obtained at room temperature in our recent study. Limiting factors for oscillation frequency and output power are theoretically analyzed including tunneling and transit-time effects and parasitic elements. Oscillation frequency and its dependence on RTD size are in good agreement with the measured results. Based on this result, it is shown that fundamental oscillation up to 2.3 THz and an output power of 60 mW at 1 THz are theoretically expected by improving the structures of the RTD and the antenna. Voltage-controlled oscillation, which is useful for the precise control of frequency, is observed in the RTD oscillators. Coherent power combining in an array configuration to achieve high output power as well as mutual injection locking between the array elements are also described.

We obtained frequency increase of resonant tunneling diode (RTD) oscillators using thick spacer layers at the collector in sub-terahertz range. This is attributed to reduction of parasitic capacitance due to the increase of spacer layer thickness. The oscillation frequency increased from 325 to 425 GHz by the change of spacer layer thickness from 5 to 45 nm in reasonable agreement with theoretical calculation. Frequency switching with bias direction was also obtained for an RTD having an asymmetric structure with the thickness of the collector and emitter spacer layers of 30 and 5 nm, respectively. The oscillation frequency was 394 GHz under forward bias, whereas 336 GHz under reverse bias in which the role of the emitter and collector spacers was exchanged.

We report experimental and theoretical work on the dependence of oscillation frequency and output power on the offset of a feeding point and the aperture width of a slot antenna in sub-terahertz (THz) and THz resonant tunneling diode (RTD) oscillators. Theoretical analysis shows that the oscillation frequency increases with the offset of the RTD from the center of the slot antenna. Output power also increases with the offset up to the point of the best impedance matching. In the experiment, an oscillation frequency of 504 GHz without offset increases to 593 GHz with an offset of 15 mm (60% of half the antenna length) from the antenna center for a 50-mm-long antenna. The experimental frequencies are in good agreement with the calculation. For the dependence on the width of the slot antenna, a slight increase in frequency is expected theoretically, although the output power is almost invariant. The oscillation frequency increases experimentally to about 345 GHz with a width of 2 mm from about 330 GHz with a width of 4 mm, in reasonable agreement with theory. The increase in oscillation frequency without using short antennas, which results in a decrease in output power, is shown by these experimental and theoretical results.

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