Components and circuits for tunneling diode based high frequency sources

Terahertz (THz) technology has been generating a lot of interest because of the potential applications for systems working in this frequency range. However, to fully achieve this potential, effective and efficient ways of generating controlled signals in the terahertz range are required. Devices that exhibit negative differential resistance (NDR) in a region of their current-voltage (I-V ) characteristics have been used in circuits for the generation of radio frequency signals. Of all of these NDR devices, resonant tunneling diode (RTD) oscillators, with their ability to oscillate in the THz range are considered as one of the most promising solid-state sources for terahertz signal generation at room temperature. There are however limitations and challenges with these devices, from inherent low output power usually in the range of micro-watts (uW) for RTD oscillators when milli-watts (mW) are desired. At device level, parasitic oscillations caused by the biasing line inductance when the device is biased in the NDR region prevent accurate device characterisation, which in turn prevents device modelling for computer simulations. This thesis describes work on I-V characterisation of tunnel diode (TD) and RTD (fabricated by Dr. Jue Wang) devices, and the radio frequency (RF) characterisation and small signal modelling of RTDs. The thesis also describes the design and measurement of hybrid TD oscillators for higher output power and the design and measurement of a planar Yagi antenna (fabricated by Khalid Alharbi) for THz applications. To enable oscillation free current-voltage characterisation of tunnel diodes, a commonly employed method is the use of a suitable resistor connected across the device to make the total differential resistance in the NDR region positive. However, this approach is not without problems as the value of the resistor has to satisfy certain conditions or else bias oscillations would still be present in the NDR region of the measured I-V characteristics. This method is difficult to use for RTDs which are fabricated on wafer due to the discrepancies in designed and actual resistance values of fabricated resistors using thin film technology. In this work, using pulsed DC rather than static DC measurements during device characterisation were shown to give accurate characteristics in the NDR region without the need for a stabilisation resistor. This approach allows for direct oscillation free characterisation for devices. Experimental results show that the I-V characterisation of tunnel diodes and RTD devices free of bias oscillations in the NDR region can be made. In this work, a new power-combining topology to address the limitations of low output power of TD and RTD oscillators is presented. The design employs the use of two oscillators biased separately, but with the combined output power from both collected at a single load. Compared to previous approaches, this method keeps the frequency of oscillation of the combined oscillators the same as for one of the oscillators. Experimental results with a hybrid circuit using two tunnel diode oscillators compared with a single oscillator design with similar values shows that the coupled oscillators produce double the output RF power of the single oscillator. This topology can be scaled for higher (up to terahertz) frequencies in the future by using RTD oscillators. Finally, a broadband Yagi antenna suitable for wireless communication at terahertz frequencies is presented in this thesis. The return loss of the antenna showed that the bandwidth is larger than the measured range (140-220 GHz). A new method was used to characterise the radiation pattern of the antenna in the E-plane. This was carried out on-wafer and the measured radiation pattern showed good agreement with the simulated pattern. In summary, this work makes important contributions to the accurate characterisation and modelling of TDs and RTDs, circuit-based techniques for power combining of high frequency TD or RTD oscillators, and to antennas suitable for on chip integration with high frequency oscillators

Diced and grounded broadband bow-tie antenna with tuning stub for resonant tunnelling diode terahertz oscillators

Radiation from antennas integrated with indium phosphide (InP)-based resonant tunnelling diode (RTD) oscillators is mainly through the substrate because of the effects of the large dielectric constant. Therefore, hemispherical lenses are used to extract the signal from the backside of the substrate. In this study the authors present a broadband bow-tie slot antenna with a tuning stub which is diced and mounted on a ground plane to alleviate the substrate effects. Here, the large dielectric constant substrate around the antenna conductor is removed. In addition, the ground plane underneath the diced substrate acts as a reflector and, ultimately, the antenna radiates to the air-side direction. Antenna integration with RTD oscillators is described in this study as well. Two-port bow-tie slot antennas were designed and characterised and showed the suitability of integration with power combining RTD oscillator circuits which are based on mutual coupling. The antennas were fabricated using electron beam lithography on a semi-insulating InP substrate. Simulated and measured bandwidth almost extends the entire frequency range 230–325  GHz. Simulations shows air-side radiation pattern and antenna gain of around 11  dB at 280  GHz. Simulations also show that the antenna may be fed with a 50-Ω or 30-Ω feed line, i.e. suitable feed lines, without compromising its performance which may prove beneficial for optimum loading of RTD oscillators

Series coupled resonant tunneling diode oscillators for terahertz applications

A series of resonant tunneling diode oscillators with frequencies up to W-band and output power around one milliwatt are presented. To our knowledge, the 75.2 GHz RTD oscillator with -0.2 dBm output power is the highest power reported. The technique demonstrated here shows the great potential to scale up the design to terahertz frequencies. Jue Wang, Khalid Alharbi, Afesomeh Ofiare, Ata Khalid, Liquan Wang, David Cumming and Edward Wasig

Broadband Bow-Tie Slot Antenna with Tuning Stub for Resonant Tunnelling Diode Oscillators with Novel Configuration for Substrate Effects Suppression

Radiation from antennas integrated with InP-based resonant tunnelling diode (RTD) oscillators is usually degraded because of the effects of the large dielectric constant substrate. The common solution has been to use hemispherical lenses to extract the signal from the backside of the substrate. In this paper we present a broadband bow-tie slot antenna with tuning stub which is diced and mounted on a ground plane to alleviate the substrate effects. Here, the large dielectric constant substrate around the antenna conductor is removed. In addition, the ground plane underneath the diced substrate acts as a reflector and, ultimately, the antenna radiates to air-side direction. The antenna was designed and fabricated using photolithography techniques to offer wide bandwidth (return loss S11 <-10dB) between 200-350 GHz on semi-insulating InP substrate with dielectric constant of ϵr = 12.56. Simulated and measured bandwidth almost extends the frequency range 230-325 GHz. Simulations shows air-side radiation pattern, an antenna gain of around 11 dB at 290 GHz and 98% radiation efficiency

Novel Tunnel Diode Oscillator Power Combining Circuit Topology Based on Synchronisation

Devices with negative differential resistance (NDR) regions in their current-voltage (I-V) characteristics such as tunnel diodes (TD) and resonant tunneling diodes (RTDs) have been used for realizing high frequency oscillators. In this paper, a new power combining technique is presented which combines output power through synchronisation of two coupled tunnel diode oscillators. The measured output power of the two synchronised tunnel diode oscillators realized in microstrip hybrid technology was -6.72 dBm at 716.2 MHz, while that of single tunnel diode oscillator was -9.09 dBm at 575.7 MHz. The circuit topology proposed in this paper can be utilized to realize high power and high frequency RTD terahertz sources

Wideband planar Yagi antennas for millimetre wave frequency applications.

Abstract: In this paper, a very wide-band Yagi antenna suitable for high-data rate communication in the millimetre-wave frequency range is presented. The coplanar waveguide (CPW) fed antenna is realised on InP substrate using the CPW ground planes as the reflector elements. The measured <i>S</i><sub>11</sub> return loss under -10 dB shows a 100% bandwidth for the 140-220 GHz frequency band antenna and 50% for the 220-325 GHz frequency band antenna. The maximum measured gain of the antenna is 7.35 dB at 202 GHz and 4.75 dB at 260 GHz

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

Resonant tunneling and planar Gunn diodes: a comparison of two solid state sources for terahertz technology

The demand for higher frequency applications is growing and a solid-state source for THz frequencies is needed. We compare experimentally demonstarted results of resonant tunneling diode and planar Gunn diodes for terahertz technology. The highest power demonstrated for W-band RTD oscillators at 75.2 GHz with -0.2 dBm (0.96 mW) and at 300GHz for submicron planar Gunn with -16dBm (28μW) are compared as the potential solid-state source for Terahertz applications

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

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