Safumi Suzuki

Resonant Tunneling Diodes for Sub-Terahertz and Terahertz Oscillators

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 µW 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.

Fundamental oscillation of resonant tunneling diodes above 1 THz at room temperature

Fundamental oscillations up to 1.04 THz were achieved in resonant tunneling diodes at room temperature. A graded emitter and thin barriers were introduced in GaInAs/AlAs double-barrier resonant tunneling diodes for reductions of the transit time in the collector depletion region and the resonant tunneling time, respectively. Output powers were 7 μW at 1.04 THz and around 10 μW in 0.9–1 THz region. A change in oscillation frequency of about 4% with bias voltage was also obtained.

One THz harmonic oscillation of resonant tunneling diodes

One THz harmonic oscillation was observed in a sub-THz oscillating GaInAs∕AlAs resonant tunneling diode integrated with a slot antenna. The fundamental and third-harmonic frequencies were 342GHz and 1.02THz, respectively, for a 50μm long antenna. The maximum output power of the fundamental mode was around 23μW, and that of the third-harmonic component was 2.6% of the fundamental. Theoretical analysis with the van der Pole equation qualitatively explained the measured results.

Experimental and Theoretical Characteristics of Sub-Terahertz and Terahertz Oscillations of Resonant Tunneling Diodes Integrated with Slot Antennas

Experimental and theoretical characteristics of sub-terahertz and terahertz oscillations in resonant tunneling diodes (RTDs) integrated with slot antennas are reported. In the experiments, oscillations up to 0.6 THz were obtained in GaInAs/AlAs double-barrier RTDs. The oscillation characteristics were theoretically analyzed for the total device structure including RTD and slot antenna. The equivalent circuit with all parasitic elements was taken into account for the RTD, and the actual structure of the antenna was analyzed using a three-dimensional electromagnetic simulator. The theoretical analysis was in good agreement with the measurements of oscillation frequency and output power. It was shown from the theoretical results that the RTD itself has the potential to oscillate up to 3.0 THz, and that the RTD oscillator with slot antenna is able to oscillate up to 2.8 THz if the device structure is optimized. The output power analysis showed that 90 µW at 1 THz is possible by optimizing the device structure.

High-Power Operation of Terahertz Oscillators With Resonant Tunneling Diodes Using Impedance-Matched Antennas and Array Configuration

We report the theoretical and experimental results of an examination of the structure needed to achieve high output power in resonant tunneling diode (RTD) oscillators in the terahertz range. An offset-fed slot antenna and antenna width adjustments were employed in a single oscillator to increase the output power by increasing the radiation conductance and impedance matching. A high output power oscillation (~400 μW) at 530-590 GHz was obtained by RTDs with a large negative deferential conductance (NDC) region and offset-fed slot antennas. The maximization of the output power that was obtained by adjusting the antenna width was attributed to the impedance matching between the RTD and antenna. An output power of >;1 mW is theoretically expected in an oscillator that combines an RTD with a large NDC region, offset-fed slot antenna, and antenna width adjustment. In an array configuration, oscillators with an offset structure were employed for array elements and connected together with the metal-insulator-metal stub structure. A single peak was observed in the oscillation spectrum, and combined output powers of 610, 270, and 180 μW at 620, 770, and 810 GHz were obtained in a two-element array.

Fundamental Oscillation of up to 831 GHz in GaInAs/AlAs Resonant Tunneling Diode

A fundamental oscillation of up to 831 GHz was observed at room temperature in GaInAs/AlAs resonant tunneling diodes integrated with planar slot antennas. The thickness of the collector spacer layer was optimized (20 nm) and the mesa area (≪1 µm²) was reduced in order to reduce the resonant tunneling diode capacitance. Reduction in the negative differential conductance in the small mesa area was prevented by increasing the emitter doping concentration (3 × 10¹⁸ cm⁻³) which resulted in an ultra-high peak current density (18 mA/µm²) with a peak-to-valley current ratio of 2. The dependence of oscillation frequency on the mesa area was also studied. The output power was at least 1 µW.

Fundamental Oscillation up to 1.31 THz in Resonant Tunneling Diodes with Thin Well and Barriers

We report the dependence of oscillation frequency on the well and barrier thicknesses in a resonant tunneling diode (RTD) terahertz oscillator integrated with a planar slot antenna. The oscillation frequency increased with decreasing well and barrier thicknesses because of the reduction in dwell time in the resonance region. Room-temperature fundamental oscillation of up to 1.31 THz with an output power of about 10 µW was achieved in the RTD with a 3.9-nm-thick well and 1.0-nm-thick barriers.

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.

High Output Power (∼400 µW) Oscillators at around 550 GHz Using Resonant Tunneling Diodes with Graded Emitter and Thin Barriers

We report resonant tunneling diode (RTD) oscillators with a high output power of around 400 µW at frequencies of 530–590 GHz. RTDs with a graded emitter and thin barriers were employed to obtain large negative differential conductance at high frequencies for high output power. An optimized structure of offset slot antennas was also used to maximize the radiation conductance. The highest output power obtained in this study was 420 µW at 548 GHz for an RTD with a peak current density of 24 mA/µm2; the RTD was placed 58 µm apart from the center of a 130-µm-long slot antenna.

650-GHz Resonant-Tunneling-Diode VCO With Wide Tuning Range Using Varactor Diode

We fabricated a wide-range varactor-tuned terahertz oscillator using a resonant tunneling diode (RTD). An AlAs/InGaAs double-barrier RTD and a varactor-diode mesa were integrated into a 20-μm-long slot antenna. A wide tuning range of ~11% (70 GHz) of the center frequency of 655 GHz was achieved by changing the depletion-layer capacitance of the varactor diode with a dc sweep from -4 to 0.5 V. The dependence of the output power on the varactor-diode bias was also measured. These experimental results agreed well with theory.