Resonant tunnelling diode (RTD) technology is emerging as one of the promising semiconductor-based solidstate technologies for terahertz (THz) wireless communications. This paper provides a review of the state-of-the-art, with a focus on the THz RTD oscillator, which is the key component of RTDbased THz transmitters and coherent receivers. A brief summary on the device principle of operation, technology, modelling, as well as an overview of oscillator design and implementation approaches for THz emitters, is provided. A new insight to device evaluation and to the reported oscillator performance levels is also given, together with brief remarks on RTD-based THz detectors. Thereafter, an overview of the reported wireless links which utilise an RTD in either transmission or reception, or in both roles, is given. Highlight results include the record single-channel wireless data rate of 56 Gb/s employing an all RTD-based transceiver, which demonstrates the potential of the technology for future short-range communications. The paper concludes with a discussion of the current technical challenges and possible strategies for future progress.

In this paper, we present an In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diode (RTD) epitaxial structure that features high-power capabilities at low-terahertz frequencies (∼ 100−300 GHz). The heterostructure was designed using a TCAD-based quantum transport simulator and experimentally investigated through the fabrication and characterisation of RTD devices. The high-frequency RF power performance of the epitaxial structure was analysed based on the extracted small-signal equivalent circuit parameters. Our analysis shows that a 9 µm 2 , 16 µm 2 , and 25 µm 2 large RTD device can be expected to deliver around 2 mW, 4 mW, and 6 mW of RF power at 300 GHz.

In this paper, we demonstrate a reliable physics-based simulation approach to accurately model high-speed In 0.53 Ga 0.47 As/AlAs double-barrier resonant tunnelling diodes (RTD). It relies on the non-equilibrium Green's function (NEGF) formalism implemented in SIL-VACO Atlas TCAD quantum simulation package to closely mimic the actual device physics, together with the judicious choice of the material parameters, models, and suitable discretisation of the associated epitaxial layer structure. The validity of the approach was proven by comparing simulated data with experimental measurements resulting from fabricated micron-sized RTD devices featuring two different epitaxially grown layer stacks. Our results show that the simulation software can correctly compute the peak current density Jp, peak voltage V p, and the valley-to-peak voltage difference ∆V =V v−V p associated with the negative differential resistance region (NDR) of the RTD heterostructure static current density-voltage (JV ) characteristic at room temperature (RT), all of which are key parameters in the design of these devices for use in oscillator circuits. We believe that this work will now help in optimising the RTD epitaxial structure to maximise its radio-frequency (RF) power performance, accelerating developments in the rapidly evolving RTD technology for emerging applications, including next-generation ultrabroadband short-range wireless communication links and high-resolution imaging systems.

In this paper, we report about an epitaxial structure simulation study of In0.53Ga0.47As/AlAs double-barrier resonant tunneling diodes (RTD) employing Atlas TCAD quantum transport simulation software developed by SILVACO Inc., which is based on the non-equilibrium Green's function formalism. We analyse how epitaxial layers design impacts the heterostructure static current density-voltage characteristic, including barriers, quantum well (QW), and lightly-doped spacer layers, as well as the employment of a high-bandgap emitter region. Our analysis shows that, while barriers and QW thicknesses have a strong impact on the current density operation of the RTD device, accurate asymmetric spacers design can trade-off between the voltage span and relative position of its negative differential resistance region. This work will guide in optimising the RTD epitaxial structure in order to maximise its RF power performance at low-terahertz frequencies (∼ 100−300 GHz).

Low-temperature micro-photoluminescence (μPL) is used to evaluate wafer structural uniformity of current densities >5mA/µm 2 InGaAs/AlAs/InP resonant tunnelling diode (RTD) structures on different length scales. Thin, highly strained quantum wells (QWs) are subject to monolayer fluctuations, leading to a large statistical distribution in their electrical properties. This has an important impact on the RTD device performance and manufacturability. The PL spot size is reduced using a common photolithography mask to reach a typical high Jpeak for a given RTD mesa size (1 ~ 100 µm 2 ). We observe that for lower strain-budget samples, that the PL line-shape is essentially identical for all excitation/collection areas. For higher strain-budget samples, there is a variation in the PL line-shape that is discussed in terms of a variation in long-range disorder brought about by strain relaxation processes. The RTD operating characteristics are discussed in light of these findings, and we conclude that strain model limits overestimate the strain budget that can be incorporated in these devices. We also highlight μPL as a powerful non-destructive characterization method for RTD structures.There is a lack of efficient high-speed technology able to satisfy the ever-growing wireless datademand [1,2]. As a consequence, the THz frequency range (0.1-10THz)[3] has attracted considerable interest as it offers the wide bandwidth required for high data-rate communications. Resonant tunneling diodes (RTDs ) have been demonstrated to be the fastest solid-state device with oscillation near 2 THz [4] with highly attractive characteristics: tunability, compact dimensions, and room temperature operation [5] . As for all quantum-effect devices [6], the RTD performance is critically dependent on crystal purity and heterointerface perfection. We previously demonstrated how ~80% of the parasitic valley current is associated with non-thermal inelastic scattering [7], and as a consequence, the RTD output power is limited by crystal-related imperfections.RTDs are composed of a single double-barrier QW generally growth by molecular beam epitaxy (MBE) or metal-organic vapor-phase epitaxy (MOVPE), despite the outstanding precision offered by these technologies, wafer characterization remains a difficult process, leading to knowledge barriers in