Toshikazu Mukai

Terahertz imaging system with resonant tunneling diodes

We report a feasibility study of a terahertz imaging system with resonant tunneling diodes (RTDs) that oscillate at 0.30 THz. A pair of RTDs acted as an emitter and a detector in the system. Terahertz reflection images of opaque samples were acquired with our RTD imaging system. A spatial resolution of 1 mm, which is equal to the wavelength of the RTD emitter, was achieved. The signal-to-noise ratio (SNR) of the reflection image was improved by 6 dB by using polarization optics that reduced interference effects. Additionally, the coherence of the RTD enabled a depth resolution of less than 3 µm to be achieved by an interferometric technique. Thus, RTDs are an attractive candidate for use in small THz imaging systems.

4H-SiC Trench Metal Oxide Semiconductor Field Effect Transistors with Low On-Resistance

The fabrication and characteristics of high-performance 4H-SiC trench metal oxide semiconductor field effect transistors (MOSFETs) are presented. Vertical trench etching of SiC without sub trenches was performed by inductive coupled plasma-reactive ion etching (ICP-RIE) with SF6, O2, and HBr. It was found that the drain–source current (Ids) of a single channel plane was strongly dependent on the crystallographic planes, but that of unit cells was almost independent of the crystallographic planes. Specific on-resistance (Ron,sp) at gate–source voltage (Vgs)=20 V, drain–source voltage (Vds)=1 V is estimated to be 2.9 mΩ cm2, and the blocking voltage is 900 V. Moreover, Ids at Vds=5 V is over 100 A. The chip is 3.0×3.0 mm2. The lowest on-resistance in the fabricated trench MOSFETs is 1.7 mΩ cm2 and the blocking voltage is 790 V. The chip is 0.5×0.5 mm2.

Modeling and Simulation of Terahertz Resonant Tunneling Diode-Based Circuits

Circuit models of transmission line elements and of a terahertz resonant tunneling diode (RTD) have been developed. The models allow for a reliable design of RTD-based oscillator and detector circuits. The transmission line elements have been modeled based on electromagnetic field simulations and dc measurements. Their accuracy has been verified through S-parameter measurements. The RTD has been modeled on the basis of dc and S-parameter measurements. The models have been used for the circuit design. A new circuit has been developed that can provide a load impedance that allows for high-output-power oscillators and high-sensitivity detectors. The circuit has been manufactured and measured as an oscillator and as a detector at frequencies around 300 GHz. An excellent agreement between measurement and simulation has been obtained, proving the accuracy of the developed models.

Integration of resonant tunneling diode with Terahertz photonic-crystal waveguide and its application to gigabit terahertz-wave communications

We integrate a resonant tunneling diode chip with a terahertz-wave photonic-crystal waveguide for the development of terahertz-wave integrated circuits. The propagation frequency band of the photonic-crystal waveguide is successfully observed from the integrated device through the resonant tunneling diode as a terahertz detector. Finally, we achieve 3-Gbit/s error-free terahertz-wave communication using the device in the 300-GHz band.

Current Injection Laser Oscillation in TlInGaAs/GaAs Double Quantum Well Diodes with InGaP Cladding Layers

TlInGaAsN/AlGaAs heterostructures were proposed for use in the fabrication of temperature-stable lasing wavelength and threshold current laser diodes. As a first step, we grew TlInGaAs/GaAs quantum well (QW) structures on GaAs(100) substrates and demonstrated an electroluminescence (EL) emission of up to 300 K. Compared with InGaAs/GaAs QWs, we confirmed that the temperature variation of the EL peak energy was decreased by the addition of Tl into InGaAs. We also demonstrated the pulsed current injection laser oscillation in the TlInGaAs/GaAs double QW laser diodes with InGaP cladding layers up to 176 K.

Optimization of the Epitaxial Design of High Current Density Resonant Tunneling Diodes for Terahertz Emitters

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows, with constraints on strained layer epitaxy introduced. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well that satisfies strained layer epitaxy constraints.

Terahertz resonant tunneling diode systems for next generation wireless communication

For low cost and short distance wireless data communication systems in the THz frequency range, resonant tunneling diodes (RTDs) are very promising. To obtain a large signal-to-noise ratio, thus high data rate, the transmitter output power and receiver sensitivity have to be high. In this paper, we discuss how to overcome the limitations of current RTD-based systems for wireless communication with respect to circuit design, showing our latest experimental results.

Fabrication, Characterisation, and Epitaxial Optimisation of MOVPE-Grown Resonant Tunnelling Diode THz Emitters

Resonant tunnelling diodes (RTDs) are a strong candidate for future wireless communications in the THz region, offering compact, room-temperature operation with Gb/s transfer rates. We employ the InGaAs/AlAs/InP material system, offering advantages due to high electron mobility, suitable band-offsets, and low resistance contacts. We describe an RTD emitter operating at 353GHz, radiating in this atmospheric transmittance window through a slot antenna. The fabrication scheme uses a dual-pass technique to achieve reproducible, very low resistivity, ohmic contacts, followed by accurate control of the etched device area. The top contact connects the device via the means of an air bridge. We then proceed to model ways to increase the resonator efficiency, in turn improving the radiative efficiency, by changing the epitaxial design. The optimization takes into account the accumulated stress limitations and realities of reactor growth. Due to the absence of useful in-situ monitoring in commercially-scalable metal-organic vapour phase epitaxy (MOVPE), we have developed a robust non-destructive epitaxial characterisation scheme to verify the quality of these mechanically shallow and atomically thin devices. A dummy copy of the active region element is grown to assist with low temperature photoluminescence spectroscopy (LTPL) characterisation. The resulting linewidths limits the number of possible solutions of quantum well (QW) width and depth pairs. In addition, the doping levels can be estimated with a sufficient degree of accuracy by measuring the Moss-Burstein shift of the bulk material. This analysis can then be combined with high resolution X-ray diffractometry (HRXRD) to increase its accuracy.

Valley current characterization of high current density resonant tunnelling diodes for terahertz-wave applications

We report valley current characterisation of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for THz emission, with a view to investigate the origin of the valley current and optimize device performance. By applying a dual-pass fabrication technique, we are able to measure the RTD I-V characteristic for different perimeter/area ratios, which uniquely allows us to investigate the contribution of leakage current to the valley current and its effect on the PVCR from a single device. Temperature dependent (20 – 300 K) characteristics for a device are critically analysed and the effect of temperature on the maximum extractable power (PMAX) and the negative differential conductance (NDC) of the device is investigated. By performing theoretical modelling, we are able to explore the effect of typical variations in structural composition during the growth process on the tunnelling properties of the device, and hence the device performance.

Optimization of high current density resonant tunneling diodes for terahertz emitters

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well.