Monolithic fabrication of a planar Gunn diode and a pHEMT side-by-side

This work presents the implementation of planar Gunn diodes and pseudomorphic high electron mobility transistors (pHEMTs) on the same wafer for the first time. The AlGaAs/InGaAs/GaAs heterostructures were designed for the realisation of pHEMTs on a Gallium Arsenide — based wafer. T-gate technology has been used for the maximisation of the transistor performance. Devices with a 70 nm long gate foot showed excellent DC and small-signal characteristics, with 780 mS/mm peak transconductance and 200 GHz fmax. Planar Gunn diodes were fabricated in parallel with the pHEMTs, sharing most of the fabrication steps. The diodes produce oscillations with 87.6 GHz maximum frequency and −40 dBm maximum output power

Integration techniques of pHEMTs and planar Gunn diodes on GaAs substrates

This work presents two different approaches for the implementation of pseudomorphic high electron mobility transistors (pHEMTs) and planar Gunn diodes on the same gallium arsenide substrate. In the first approach, a combined wafer is used where a buffer layer separates the active layers of the two devices. A second approach was also examined using a single wafer where the AlGaAs/InGaAs/GaAs heterostructures were designed for the realisation of pHEMTs. The comparison between the two techniques showed that the devices fabricated on the single pHEMT wafer presented superior performance over the combined wafer technique. The DC and small-signal characteristics of the pHEMTs on the single wafer were enhanced after the use of T-gates with 70 nm length. The maximum transconductance of the transistors was equal to 780 mS/mm with 200 GHz maximum frequency of oscillation (fmax). Planar Gunn diodes fabricated in the pHEMT wafer, with 1.3 μm anode-to-cathode separation (LAC) presented oscillations at 87.6 GHz with maximum power of oscillation equal to -40 dBm

Co-fabrication of planar Gunn diode and HEMT on InP substrate

We present the co-fabrication of planar Gunn diodes and high electron mobility transistors (HEMTs) on an Indium Phosphide (InP) substrate for the first time. Electron beam lithography (EBL) has been used extensively for the complete fabrication procedure and a 70 nm T-gate technology was incorporated for the enhancement of the small-signal characteristics of the HEMT. Diodes with anode-to-cathode separation (Lac) down to 1 μm and 120 μm width where shown to oscillate up to 204 GHz. The transistor presents a cut-off frequency (fT) of 220 GHz, with power gain up to 330 GHz (f<sub>max</sub>). The integration of the two devices creates the potential for the realisation of high-power, high-frequency MMIC Gunn oscillators, circuits and systems

Integration of planar Gunn diodes and HEMTs for high-power MMIC oscillators

This work has as main objective the integration of planar Gunn diodes and high electron mobility transistors (HEMTs) on the same chip for the realisation of high-power oscillators in the millimeter-wave regime. By integrating the two devices, we can reinforce the high frequency oscillations generated by the diode using a transistor-based amplifier. The integration of the planar Gunn diode and the pseudomorphic HEMT was initially attempted on a combined gallium arsenide (GaAs) wafer. In this approach, the active layers of the two devices were separated by a thick buffer layer. A second technique was examined afterwards where both devices were fabricated on the same wafer that included AlGaAs/InGaAs/GaAs heterostructures optimised for the fabrication of pHEMTs. The second approach demonstrated the successful implementation of both devices on the same substrate. Planar Gunn diodes with 1.3 μm anode-to-cathode separation (Lac) presented oscillations up to 87.6 GHz with a maximum power equal to -40 dBm. A new technique was developed for the fabrication of 70 nm long T-gates, improving the gain and the high frequency performance of the transistor. The pHEMT presented cut-off frequency (fT) equal to 90 GHz and 200 GHz maximum frequency of oscillation (fmax). The same side-by-side approach was applied afterwards for the implementation of both devices on an indium phosphide (InP) HEMT wafer for the first time. Planar Gunn diodes with Lac equal to 1 μm generated oscillations up to 204 GHz with -7.1 dBm maximum power. The developed 70 nm T-gate technology was applied for the fabrication of HEMTs with fT equal to 220 GHz and fmax equal to 330 GHz. In the end of this work, the two devices were combined in the same monolithic microwave integrated circuit (MMIC), where the diode was connected to the transistor based amplifier. The amplifier demonstrated a very promising performance with 10 dB of stable gain at 43 GHz. However, imperfections of the material caused large variations at the current density of the devices. As a consequence, no signals were detected at the output of the complete MMIC oscillators

Design, fabrication and testing of a novel W-band monolithic millimetre-wave integrated circuit mixer

Abstract available: p. [1

Design and characterisation of millimetre wave planar Gunn diodes and integrated circuits

Heterojunction planar Gunn devices were first demonstrated by Khalid et al in 2007. This new design of Gunn device, or transferred electron device, was based on the well-established material system of GaAs as the oscillation media. The design did not only breakthrough the frequency record of GaAs for conventional Gunn devices, but also has several advantages over conventional Gunn devices, such as the possibility of making multiple oscillators on a single chip and compatibility with monolithic integrated circuits. However, these devices faced the challenge of producing high enough RF power for practical applications and circuit technology for integration. This thesis describes systematic work on the design and characterisations of planar Gunn diodes and the associated millimetre-wave circuits for RF signal power enhancement. Focus has been put on improving the design of planar Gunn diodes and developing high performance integrated millimetre-wave circuits for combining multiple Gunn diodes. Improvement of device design has been proved to be one of the key methods to increase the signal power. By introducing additional δ-doping layers, electron concentration in the channel increases and better Gunn domain formation is achieved, therefore higher RF power and frequency are produced. Combining multiple channels in the vertical direction within devices is another effective way to increase the output signal power as well as DC-to-RF conversion efficiency. In addition, an alternative material system, i.e. In0.23Ga0.77As, has also been studied for this purpose. Planar passive components, such as resonators, couplers, low pass filters (LPFs), and power combiners with high performance over 100 GHz have been developed. These components can be smoothly integrated with planar Gunn diodes for compact planar Gunn oscillators, and therefore contribute to RF power enhancement. In addition, several new measurement techniques for characterising oscillators and passive devices have also been developed during this work and will be included in this thesis

Design of Circuits to enhances the performace of high frequency planar Gunn diodes

The project contains adventurous research, with an aim to understand and design a planar Gunn diode with a novel integrated circuit configuration to extract the 2nd harmonic. This will potentially enhance the Gunn diode as a high frequency source towards frequencies in excess of 600 GHz. The RF performance from the above integrated circuit was achieved by design and simulation of radial and diamond stub resonators, which were used to short the fundamental oscillation frequency while allowing the second harmonic frequency to pass through to the load. The diamond stub resonator is a new configuration offering a number of advantages which include a higher loaded quality factor and occupies 55% less chip area than a comparable radial stub resonator. The designed novel circuits with integrated planar Gunn diode were fabricated using microwave monolithic integrated circuits (MMIC) technology at the James Watt Nanofabrication centre in Glasgow University. Full DC and microwave characterisation of the diodes and integrated circuits with diodes was carried out using a semiconductor analyser, network analyser (10 MHz to 110GHz) and spectrum analyser (10 MHz to 125GHz). The microwave measurements were carried out at the high frequency RF laboratories in Glasgow University. Both GaAs and InP based Gunn diodes were characterised and RF characterisation work showed that higher fundamental frequencies could be obtained from Gunn diodes fabricated on InGaAs on a lattice matched InP substrate. Planar Gunn diodes with an anode to cathode spacing of 4 microns giving a fundamental frequency of oscillation of 60 GHz were fabricated as an integrated circuit with coplanar waveguide (CPW) circuit elements to extract the second harmonic. A second harmonic frequency of 120 GHz with an RF output power of -14.11 dBm was extracted with very good fundamental frequency suppression. To the authors knowledge this was the first time second harmonic frequencies have been extracted from a planar Gunn diode technology. Aluminium gallium arsenide (AlGaAs) planar Gunn diodes were also designed with an integrated series inductor to match the diode at the fundamental frequency to obtain higher RF output powers. Devices with a 1 micron anode to cathode separation gave the highest fundamental oscillation frequency of 121 GHz the highest reported for a GaAs based Gunn diode and with an RF output power of -9 dBm. These circuits will have potential applications in secure communications, terahertz imaging etc. The research programme was in collaboration with the University of Glasgowwould like to thank the staff of the James Watt Nanofabrication Centre at the University of Glasgow for help in fabricating the devices which is reported in this thesis. ‘Part of this work was supported by ESPRC through EP/H011862/1, and EP/H012966/1

Millimeter-Wave MMICs and Applications

As device technology improves, interest in the millimeter-wave band grows. Wireless communication systems migrate to higher frequencies, millimeter-wave radars and passive sensors find new solid-state implementations that promise improved performance, and entirely new applications in the millimeter-wave band become feasible. The circuit or system designer is faced with a new and unique set of challenges and constraints to deal with in order to use this portion of the spectrum successfully. In particular, the advantages of monolithic integration become increasingly important. This thesis presents many new developments in Monolithic Millimeter-Wave Integrated Circuits (MMICs), both the chips themselves and systems that use them. It begins with an overview of the various applications of millimeter waves, including a discussion of specific projects that the author is involved in and why many of them demand a MMIC implementation. In the subsequent chapters, new MMIC chips are described in detail, as is the role they play in real-world projects. Multi-chip modules are also presented with specific attention given to the practical details of MMIC packaging and multi-chip integration. The thesis concludes with a summary of the works presented thus far and their overall impact on the field of millimeter-wave engineering.</p

High efficiency and high frequency resonant tunneling diode sources

Terahertz (THz) technology has been generating a lot of interest due to the numerous potential applications for systems working in this previously unexplored frequency range. THz radiation has unique properties suited for high capacity communication systems and non-invasive, non-ionizing properties that when coupled with a fairly good spatial resolution are unparalleled in its sensing capabilities for use in biomedical, industrial and security fields. However, in order to achieve this potential, effective and efficient ways of generating THz radiation are required. Devices which exhibit negative differential resistance (NDR) in their current-voltage (I – V) characteristics can be used for the generation of these radio frequency (RF) signals. Among them, the resonant tunnelling diode (RTD) is considered to be one of the most promising solid-state sources for millimeter and submillimeter wave radiation, which can operate at room temperature. However, the main limitations of RTD oscillators are producing high output power and increasing the DC-to-RF conversion efficiency. Although oscillation frequencies of up to 1.98 THz have been already reported, the output power is in the range of micro-Watts and conversion efficiencies are under 1 %. This thesis describes the systematic work done on the design, fabrication, and characterization of RTD-based oscillators in monolithic microwave/millimeter-wave integrated circuits (MMIC) that can produce high output power and have a high conversion efficiency at the same time. At the device level, parasitic oscillations caused by the biasing line inductance when the diode is biased in the NDR region prevents accurate characterization and compromises the maximum RF power output. In order to stabilise the NDR devices, a common method is the use of a suitable resistor connected across the device, to make the differential resistance in the NDR region positive. However, this approach severely hinders the diode’s performance in terms of DC-to-RF conversion efficiency. In this work, a new DC bias decoupling circuit topology has been developed to enable accurate, direct measurements of the device’s NDR characteristic and when implemented in an oscillator design provides over a 10-fold improvement in DC-to-RF conversion efficiency. The proposed method can be adapted for higher frequency and higher power devices and could have a major impact with regards to the adoption of RTD technology, especially for portable devices where power consumption must be taken into consideration. RF and DC characterization of the device were used in the realization on an accurate large-signal model of the RTD. S-parameter measurements were used to determine an accurate small-signal model for the device’s capacitance and inductance, while the extracted DC characteristics where used to replicate the I-V characteristics. The model is able to replicate the non-stable behavior of RTD devices when biased in the NDR region and the RF characteristics seen in oscillator circuits. It is expected that the developed model will serve in future optimization processes of RTD devices in millimeter and submillimeter wave applications. Finally, a wireless data transmission link operating in the Ka-band (26.5 GHz – – 40 GHz) using two RTDs operating as a transmitter and receiver is presented in this thesis. Wireless error-free data transfer of up to 2 gigabits per second (Gbit/s) was achieved at a transmission distance of 15 cm. In summary, this work makes important contributions to the accurate characterization, and modeling of RTDs and demonstrates the feasibility of this technology for use in future portable wireless communication systems and imaging setups

RECONFIGURABLE POWER AMPLIFIER WITH TUNABLE INTERSTAGE MATCHING NETWORK USING GaAs MMIC AND SURFACE-MOUNT TECHNOLOGY

As the demand of reconfigurable devices increases, the possibility of exploiting the interstage matching network in a two-stage amplifier to provide center frequency tuning capability is explored. While placement of tuning elements at the input and/or output matching network has some disadvantages, placement of tuning elements in the interstage absorbs the lossy components characteristics into useful attributes. The circuit design methodology includes graphical method to determine the bandpass topology that achieves high Q-contour on the Smith chart thus result in narrow bandwidth. T-section and π-section topologies are used to match reactive terminations provided by the first and second amplifier stages. The design methodology also includes utilization of interstage mismatch loss that decreases as increasing frequency to compensate for amplifier gain roll-off and equalize the gain at different tuning states. In prototype realization, three design configurations are discussed in this thesis: 1) a discrete design for operation between 0.1 – 0.9 GHz with the total layout area of 7.5 mm x 12.5 mm, 2) a partial monolithic design (Quasi-MMIC) for operation between 0.9 – 2.4 GHz that is 25 times smaller layout area compared to the discrete design, and 3) a conceptual design of integrated monolithic reconfigurable PA for operation between 0.9 – 2.4 GHz that is 130 times smaller layout area compared to the discrete design. One variant of the fabricated reconfigurable PA offers advantage of 4-states center frequency tuning from 1.37 GHz to 1.95 GHz with gain of 21.5 dB (+ 0.7 dB). The feasibility of interstage matching network as tuning elements in reconfigurable power amplifier has been explored. The input and output matching networks are fixed while the interstage impedances are varied using electronic switching (discrete SP4T and GaAs FET switches). The discrete design is suited for the operation at low frequency (fo < 1GHz), while monolithic implementation of the tunable interstage matching network is required for higher frequency operation due to size limitation and parasitic effects. The reconfigurable PA using MMIC tuner was designed at higher frequency to possibly cover GSM, CDMA, Bluetooth, and WiMAX frequency (0.9 – 2.4 GHz)