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Terahertz (THz) oscillators based on resonant tunneling diodes (RTDs) have relatively low output power, tens to hundreds of microwatts. The conventional designs employ submicron 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 µm 2 , 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 selfcapacitance 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. Index Terms-Resonant tunnelling diode (RTD), terahertz (THz) sources, photolithography.

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.

Presented is a monolithic microwave integrated circuit (MMIC) resonant tunnelling diode (RTD) oscillator that employs two In 0.53 Ga 0.47 As/AlAs RTDs (5 × 5 µm 2 ) in parallel. The oscillator works at 28.7 GHz with −0.7 dBm output power. The phase noise was −95 dBc/Hz at 100 kHz and −114 dBc/Hz at 1 MHz offset. This reported work demonstrates the potential of RTD oscillators in achieving high output power at high frequencies by utilising suitable power maximising and combining techniques.

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