Resonant tunneling diode (RTD) technology is emerging as one of the promising semiconductor-based solid-state technologies for terahertz (THz) wireless communications. This article provides a review of the state of the art, with a focus on the THz RTD oscillator, which is the key component of RTD-based THz transmitters and coherent receivers. A brief summary on the device principle of operation, technology, modeling, 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 utilize 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 article concludes with a discussion of the current technical challenges and possible strategies for future progress.

Terahertz (THz) oscillators based on resonant tunneling diodes (RTDs) have relatively low output power, tens to hundreds of microwatts. The conventional designs employ sub-micron sized RTDs to reduce the device self-capacitance and, as a result, realise higher oscillation frequencies. However, reducing the RTD device size leads to lower output power. In this paper we present RTD oscillators which can employ one or two RTD devices of relatively large size, 9 - 25 μm2, for high power and, at the same time, can oscillate at THz frequencies. This is achieved through low resonating inductances realized by microstrip or coplanar waveguide (CPW) transmission line short stubs with low characteristic impedances (Z0), which have lower inductance values per unit length and so compensate the increase of the self-capacitance of large area RTD devices. Thus, fabrication using only photolithography is possible. It is also shown that device sizing, which is limited only by bias stability considerations, does not limit device bandwidth. Further, we report a new way to estimate the RTD oscillator output power with frequency. A series of oscillators with oscillation frequencies in the 245 - 309 GHz range and output powers from 0.1 - 1 mW have been demonstrated showing the feasibility of the proposed approach.

A new high-performance normally-off gallium nitride (GaN)-based metal-oxide-semiconductor high electron mobility transistor that employs an ultrathin subcritical 3 nm thick aluminium gallium nitride (Al _0.25 Ga _0.75 N) barrier layer and relies on an induced two-dimensional electron gas for operation is presented. Single finger devices were fabricated using 10 and 20 nm plasma-enhanced chemical vapor-deposited silicon dioxide (SiO ₂ ) as the gate dielectric. They demonstrated threshold voltages (V _th ) of 3 and 2 V, and very high maximum drain currents (IDSmax) of over 450 and 650 mA/mm, at a gate voltage (V _GS ) of 6 V, respectively. The proposed device is seen as a building block for future power electronic devices, specifically as the driven device in the cascode configuration that employs GaN-based enhancement-mode and depletion-mode devices.

This paper reports on a 15-Gb/s wireless link that employs a high-power resonant tunneling diode (RTD) oscillator as a transmitter (Tx). The fundamental carrier frequency is 84 GHz and the maximum output power is 2 mW without any power amplifier. The reported performance is over a 50-cm link, with simple amplitude shift keying modulation utilized. The 15-Gb/s data link shows correctable bit error rate (BER) of 4.1 x 10⁻³, while the lower data rates of 10 and 5 Gb/s show a BER of 3.6 x 10⁻⁴ and 1.0 x 10⁻⁶, respectively. These results demonstrate that the RTD Tx is a promising candidate for the next-generation low-cost, compact, ultrahigh data rates wireless communication systems.

This paper presents a series of monolithic microwave integrated circuit (MMIC) resonant tunneling diode (RTD) oscillators. The oscillator circuit topology employs two InGaAs/AlAs RTDs in parallel and each device is biased individually. The oscillators operate at 125/156/206/308 GHz with -1.7/-3.3/-14.6/-4.8 dBm output power. With improved epitaxial layer structures and oscillator designs, it is expected that the output power will reach several milliwatts. This work shows the promising potential of RTD oscillators as terahertz (THz) sources for high speed wireless communications, etc.

We report on high-efficiency, high-power, and low-phase-noise resonant tunneling diode (RTD) oscillators operating at around 30 GHz. By employing a bias stabilization network, which does not draw any direct current (dc), the oscillators exhibit over a tenfold improvement in the dc-to-RF conversion efficiency (of up to 14.7%) compared to conventional designs (~ 0.9%). The oscillators provide a high maximum output power of around 2 dBm, and low phase noise of -100 and -113 dBc/Hz at 100 kHz and 1 MHz offset frequencies, respectively. The proposed approach will be invaluable for realizing very high efficiency, low phase noise, and high-power millimeter-wave (mm-wave) and terahertz (THz) RTD-based sources.