title: Accurate characterisation of Resonant Tunnelling Diodes for high-frequency applications
Creators: name: Morariu, Razvan
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Resonant Tunnelling Diode, RTD, terahertz, THz, device modelling.

description: Recent scientific advancements regarding the generation and detection of terahertz (THz) radiation have led to a rapid increase in research interest in this frequency band in the context of its numerous potential applications including high-speed wireless communications, biomedical diagnostics, security screening and material science. Various proposed solutions have been investigated in the effort to bridge this relatively unexplored region of the electromagnetic spectrum, and thus exploit its untapped potential. Among them, the resonant tunnelling diode (RTD) has been demonstrated as the fastest electronic device with its room temperature operation extending into the THz range. The RTD exhibits a negative differential resistance (NDR) region in its I-V characteristics, with this feature being key to its capabilities. Even though the unique capabilities of RTD devices have been experimentally proven in the realisation of compact NDR oscillators and detectors, with fundamental frequencies of about 2 THz, and high-sensitivity detectors up to 0.83 THz, the reliable design procedures and methodologies of RTD-based circuits are yet to be fully developed. In this regard, significant effort has been devoted primarily to the accurate theoretical description of the high-frequency behaviour of RTDs, using various small-signal equivalent circuit models. However, many of these models have had either limited or no experimental validation, and so a robust and reliable RTD device model is desirable. The aim of this thesis is to describe a systematic approach regarding the design, fabrication and characterisation of RTD devices, providing a universal methodology to accurately determine their radio-frequency (RF) behaviour, and so this way enable a consistent integrated circuit design procedure for high-frequency circuits. A significant challenge in the modelling of RTD devices is represented by the presence of parasitic bias oscillations within the NDR region. This has been identified as one of the main restricting factors with regards to the accurate high-frequency characterisation of this operating region. The common approach to overcoming this limitation is through a stabilising technique comprising of an external shunt-resistor network. This approach has been successfully demonstrated to suppress bias oscillations in RTD-based circuits which require operation within the NDR region. However, the introduction of the additional circuit component associated with this method increases the complexity of the de-embedding procedure of the extrinsic parasitic elements, rendering the overall device characterisation generally difficult at high-frequencies. In this work, a novel on-wafer bond-pad and shunt resistor network de-embedding technique was developed in order to facilitate the characterisation of RTDs throughout the complete bias range, without limitation to device sizing or frequency, under a stable operating regime. The procedure was demonstrated to accurately determine the circuit high-frequency behaviour of the RTD device from S-parameter measurements up to 110 GHz. The universal nature of this procedure allows it to be easily adapted to accommodate higher complexity stabilising networks configuration or different bond-pad geometries. Furthermore, the de-embedding method has also enabled the development of a novel quasi-analytical procedure for high accuracy extraction of the device equivalent circuit parameters, which is expected to provide a strong experimental foundation for the further establishment of a universal RTD RF model. The applicability of the developed high-frequency model, which can be easily scaled for various device sizes, together with the measured RTD I-V characteristics was further demonstrated in the development of a non-linear model, which was integrated in a commercial simulator, the Advanced Design Systems (ADS) software from Keysight Technologies. From an application perspective, the model was used in the design of an RTD as a square-law detector for high-frequency data transmission systems. The simulated detector performance was validated experimentally using an RTD-based transmitter in the W-band (75 – 110 GHz) up to 4 Gbps (error free transmission: BER &lt; 10-10 in a waveguide connection), and in the Ka-band (26.5 – 50 GHz) up to 2.4 Gbps (error free transmission in a wireless data link), which demonstrated the accuracy of the developed RTD modelling approach. Lastly, a sensitivity analysis of the RTD-based detector within the Ka-band showed a superior RTD performance over commercially available solutions, with a peak (corrected) detector responsivity of 13.48 kV/W, which is a factor of &gt;6 better compared to commercially available Schottky barrier diode (SBD) detectors.