Hidetoshi Kanaya

Oscillation up to 1.92 THz in resonant tunneling diode by reduced conduction loss

A large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss. An n+-InGaAs layer under the air-bridge electrode connected to the RTD was observed to cause a large conduction loss for high-frequency current due to the skin effect. By introducing a new fabrication process removing the InGaAs layer, we obtained 1.92-THz oscillation, which extended the highest frequency of room-temperature electronic single oscillators. Theoretical calculations reasonably agreed with the experiment, and an oscillation above 2 THz is further expected with an improved structure of the slot antenna used as a resonator and radiator.

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.

Structure dependence of oscillation characteristics of resonant-tunneling-diode terahertz oscillators associated with intrinsic and extrinsic delay times

We investigate the effect of intrinsic and extrinsic delay times on the oscillation characteristics of resonant-tunneling-diode (RTD) terahertz oscillators. The intrinsic delay time is composed of the electron dwell time in the resonant tunneling region and the electron transit time in the collector depletion region. We obtain and discuss the structure dependence of these factors in terms of the oscillation frequency and output power measured for RTD oscillators with different quantum-well and collector-spacer thicknesses and different air-bridge widths between the RTD and a slot antenna. The highest oscillation frequency achieved in this experiment is 1.86 THz for the well and spacer thicknesses of 2.5 and 12 nm, respectively, with a 1-µm-wide air bridge. In this structure, the extrinsic delay time (80 fs) estimated from the parasitic elements is more than double the intrinsic delay time (35 fs). It is shown theoretically that an oscillation frequency of over 2 THz is possible upon the reduction in the extrinsic delay time caused by the bulk and spread resistances in RTDs.

Fundamental oscillation up to 1.31 THz in thin-well resonant tunneling diodes

Room-temperature fundamental oscillation of up to 1.31 THz was achieved in thin-well resonant tunneling diodes integrated with planar slot antennas. The output powers were ~10 μW at 1.31 THz and around 30 nW in the 0.8-1.1 THz region. This high frequency oscillation with relatively high output power is attributed to a reduction in the intrinsic delay and an increase in the widths of current density and voltage of the negative differential conductance region due to the thin well structure.

Frequency increase in resonant-tunneling-diode terahertz oscillator by reduction in conduction loss with thick antenna electrode

We obtained frequency increase in resonant-tunneling-diode (RTD) terahertz (THz) oscillators by thick antenna electrode. The THz oscillator was composed of an RTD and slot antenna. Because an oscillation at resonance frequency of the RTD and slot antenna occurs when the negative differential conductance of the RTD compensates for the loss of the antenna, a reduction in the conduction loss and inductance of slot antenna by thick antenna electrode is effective for high frequency oscillation. We obtained ~150 GHz frequency increase after deposition of 1-μm-thick additional gold metal on 0.1-μm-thick antenna electrode. The highest oscillation frequency obtained in this experiment was 1.63 THz for 9-μm-long antenna and 0.22-μm2-area RTD. Theoretical calculation agreed well with the experiment, and an oscillation above 2 THz is expected by this structure and low capacitance RTD with thick spacer thickness.

Terahertz Emission from Resonant Tunneling Diodes without Satisfying Oscillation Condition

Terahertz (THz) emission from resonant tunneling diodes (RTDs) is normally obtained under the oscillation condition in which the negative differential conductance (NDC) exceeds the circuit loss. In this study, we show that a relatively broad band THz emission was observed even for RTDs with an NDC smaller than the circuit loss. The observed output power was on the order of 1–10 nW at 1.2–1.9 THz with spectral widths of 50–100 GHz. The observation was reasonably explained by the theoretical calculation based on the shot noise amplified by the NDC. This emission corresponds to the amplified spontaneous emission in optical devices.

Frequency increase in resonant-tunneling-diode terahertz oscillators using optimum collector spacer

We report an increase in oscillation frequency of room-temperature terahertz oscillators using AIAs/InGaAs resonant tunneling diodes (RTDs) with optimized collector spacer thickness. Because of the trade-off relation between the capacitance and electron transit time for the spacer thickness, an optimum thickness exists in terms of the oscillation frequency. The highest frequency in this experiment was 1.42 THz at the optimum spacer thickness of 12 nm with an output power of ~1 μW. A fundamental oscillation at a frequency >2 THz and output power of ~300 μW at 1 THz are theoretically possible by optimized structures of RTD and antenna.

Terahertz oscillators using resonant tunneling diodes with InAlGaAs/InP composite collector

We proposed a resonant tunneling diode (RTD) with InAlGaAs/InP composite collector for reduction in transit delay caused by the gamma to L valley transition at the collector depletion region. Terahertz oscillators fabricated with this RTD show room-temperature fundamental oscillations of 680-770 GHz with the RTD areas of 1-1.5 square microns. Higher frequency will be possible by reducing the RTD area.