Accurate characterisation of Resonant Tunnelling Diodes for high-frequency applications

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 < 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 >6 better compared to commercially available Schottky barrier diode (SBD) detectors

IV Characteristics of a Stabilized Resonant Tunnelling Diodes

The presence of parasitic oscillations found in the negative differential region (NDR), which can distort the current-voltage (I-V) characteristics of the device is one of the main problems when designing resonant tunnelling diode (RTD) circuits. A new method for RTD stabilization is proposed based on work done previously on tunnel diodes and results show that there is a significant difference between the I-V characteristics of a tunnel diode and that of an RTD. This work shows promising potential for further increasing the RTD’s output power, DC-RF conversion efficiency and provides the basis for an accurate model of the NDR regio

Loading Effect of W-band Resonant Tunneling Diode Oscillator by Using Load-Pull Measurement

Resonant tunneling diode (RTD) is the fastest solid-state electronic device with the highest reported frequency at 1.92 THz [1]. RTD-based THz sources have many promising applications such as ultrafast wireless communications, THz imaging, etc. To date, the main limitation of RTD technology is the low output power. Many efforts had been made to increase the power level by such as optimizing the layer structure [2], employing more devices in an array [3], matching impedance by displacing the device in circuit [3], etc. Here we report the loading effect by using E/H impedance tuner. We found that the maximum power is over 20dB higher than the worst impedance matching and the frequency shift is within 14% range of the central frequency. The load-pull measurement provides a convenient way to investigate the power/frequency variation versus the impedance change. Further work will benefit from the measurement results to design corresponding impedance matching network. The power level of RTD oscillator will be increased

15 Gbps Wireless Link Using W-band Resonant Tunnelling Diode Transmitter

A 15 Gbps wireless link over 50 cm distance is reported in this paper. A high power and low phase noise resonant tunneling diode (RTD) oscillator is employed as the transmitter. The fundamental carrier frequency is 84 GHz and the maximum output power is 2 mW without any power amplifier. The measured phase noise value was -79 dBc/Hz at 100 KHz and -96 dBc/Hz at 1 MHz offset. The modulation scheme used was amplitude shift keying (ASK). The 15 Gbps data link showed a correctable bit error rate (BER) of 4.1×10-3, while lower data rates of 10 Gbps and 5 Gbps had BER of 3.6×10-4 and 1.0×10-6, respectively

Resonant Tunneling Diode Oscillator Source for Terahertz Applications

Resonant tunneling diode (RTD) is the fastest solid-state electronic device with the highest reported frequency at 1.92 THz [1]. RTD-based THz sources have many promising applications such as ultrafast wireless communications, THz imaging, etc. To date, the main limitation of RTD technology is the low output power. Here we report the series of nearly/over one half mW output power RTD oscillator. The frequencies range from 125 GHz to 308 GHz and the preliminary wireless communication measurement result demonstrates data rate up to 7Gbps

15 Gb/s 50-cm wireless link using a high power compact III-V 84 GHz transmitter

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

In_0.53Ga_0.47As/AlAs Double-Barrier Resonant Tunnelling Diodes with High-Power Performance in the Low-Terahertz Band

We report about an In0.53Ga0.47As/AlAs doublebarrier 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 µm2, 16 µm2, and 25 µm2 large RTD device can be expected to deliver around 2 mW, 4 mW, and 6 mW of RF power at 300 GHz

High efficiency bias stabilisation for resonant tunneling diode oscillators

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

Accurate small-signal equivalent circuit modelling of resonant tunneling diodes to 110 GHz

This article presents a novel, on-wafer deembedding technique for the accurate small-signal equivalent circuit modeling of resonant tunneling diodes (RTDs). The approach is applicable to stabilized RTDs, and so enables the modeling of the negative differential resistance (NDR) region of the device's current-voltage (I-V) characteristics. Furthermore, a novel quasi-analytical procedure to determine all the equivalent circuit elements from the deembedded S-parameter data is developed. Extraction results of a 10 μm × 10 μm stabilized, low-current density RTD at different bias points show excellent fits between modeled and measured S-parameters up to 110 GHz

Long-range millimetre wave wireless links enabled by travelling wave tubes and resonant tunnelling diodes

High data rate wireless links are an affordable and easily deployable solution to replace or complement fibre. The wide frequency band available at millimetre waves above 100 GHz can support multi-gigabit per second data rate. However, the high attenuation due to rain and humidity poses a substantial obstacle to long-range links. This study describes a wireless system being developed for point-to-point links at D-band (DLINK), above 150 GHz, to enable a full fibre-on-air link with more than 1 km range and unprecedented data rate up to 45 Gb/s. The upper end of the D-band spectrum is used (151.5–174.8 GHz) in full frequency division duplex transmission. The DLINK system consists of a transmitter using a directly modulated resonant tunnelling diode oscillator powered by novel travelling wave tubes. The performance and the small footprint of the front end will make the DLINK system highly competitive to the point-to-point links presently available in the market at frequencies below 100 GHz. The innovative approach and the design are oriented to large-scale productions to satisfy the high data traffic demand of the new 5G infrastructure