Planar Heterostructure Barrier Varactor Diodes for Millimetre Wave Applications

This thesis deals with fabrication, characterisation and modelling of the Heterostructure Barrier Varactor (HBV) diode and its use in frequency multiplier applications. Different aspects of material structures and frequency multipliers are described. The aim of the work presented is to develop design methods and processes to fabricate state-of-the-art planar HBVs and multipliers in the millimetre and submillimetre wave length region.<p /> Results from AlGaAs HBV frequency tripler measurements are presented. Simulations and cooled measurements show that excessive conduction current due to self-heating degrades the multiplier efficiency. A new design of planar GaAs-based HBVs with reduced thermal resistance and series resistance have been fabricated. A state-of-the-art performance of 4,8% efficiency and an output power of 4 mW at 246 GHz was achieved.<p /> A novel fabrication process where HBV diodes are fabricated on a copper substrate is proposed. This reduces thermal resistance and parasitic resistance without degrading the electrical characteristics.<p /> A 141 GHz quasi-optical HBV tripler is presented. A peak flange-to-flange efficiency of 8% and an output power of 11,5 mW was achieved.<p /> Different III-V material systems for HBVs have been tested. The results of lattice matched and pseudomorphic GaAs/AlGaAs, InGaAs/InAlAs, InAs/AlSb and phosphide containing materials for HBVs are presented. The state-of-the-art material for millimetre and submillimetre wave HBVs is the In_0,53GaAs/In_0,52AlAs system with a thin AlAs layer (30 Å) in the middle of the barrier.<p /> Both simple analytical models and a self-consistent Poisson/Schroedinger approach are used to predict and optimise HBV diodes. Finally, a simple quick-design method for calculation of optimum embedding impedances, optimum conversion efficiency and pump power for HBV triplers are presented

A 185-215-GHz Subharmonic Resistive Graphene FET Integrated Mixer on Silicon

A 200-GHz integrated resistive subharmonic mixer based on a single chemical vapor deposition graphene field-effect transistor (G-FET) is demonstrated experimentally. This device has a gate length of 0.5 μm and a gate width of 2x40 μm. The G-FET channel is patterned into an array of bow-tie-shaped nanoconstrictions, resulting in the device impedance levels of ~50 Ω and the ON-OFF ratios of ≥4. The integrated mixer circuit is implemented in coplanar waveguide technology and realized on a 100-μm-thick highly resistive silicon substrate. The mixer conversion loss is measured to be 29 ± 2 dB across the 185-210-GHz band with 12.5-11.5 dBm of local oscillator (LO) pump power and >15-dB LO-RF isolation. The estimated 3-dB IF bandwidth is 15 GHz

High/low-impedance transmission-line and coupled-line filter networks for differential phase shifters

Two compact and simple to design differential phase shifter topologies, based on high/low-impedance transmission-line sections and open-ended coupled-line sections, are presented for the first time. The basic circuit theory for single section topologies is reviewed, leading to design equations and graphs for direct circuit synthesis. Balanced topologies and multiple section designs are also proposed improving the performance and feasibility of the phase shifters. Two planar microstrip single section differential phase shifter hybrids, at a centre frequency of 12 GHz and a 45 and 135° phase difference have been designed and manufactured. The designs have a simulated 0.5 dB amplitude and 1° phase imbalance over more than 25 and 40% bandwidth, respectively. Experimental results verify the circuit performance and feasibility of the proposed differential phase shifters

A 30-GHz Integrated Subharmonic Mixer based on a Multichannel Graphene FET

A 30 GHz integrated subharmonic mixer based on a single graphene field effect transistor (G-FET) has been designed, fabricated and characterized. The mixer is realized in microstrip technology on a 250 um high resistivity silicon substrate. In order to enhance the current on-off ratio, the G-FET utilizes a channel consisting of an array of bow-tie structured graphene, yielding a current on-off ratio of 7. A conversion loss (CL) of 19 ± 1 dB over the frequency range of 24 to 31 GHz is obtained with an LO to RF isolation better than 20 dB at an LO power of 10 dBm. The overall minimum CL is 18 dB at 27 GHz. The mixer has a 3 GHz ±1-dB IF bandwidth, which is achieved with a fixed LO signal of 15 GHz. The mixer linearity is characterized and the highest third order intercept point is measured to be 12.8 dBm

Mobility degradation and series resistance in graphene field-effect transistors

Accurate device models and parameter extraction methods are of utmost importance for characterizing graphene field-effect transistors (GFETs) and for predicting their performance in circuit applications. For DC characterization, accurate extraction of mobility and series resistance is of particular concern. In this paper, we show how a first-order mobility degradation model can be used to separate information about mobility degradation and series resistance for a set of GFETs of different channel lengths. Data from a large set of top-gated GFETs based on chemical vapor deposited (CVD) graphene was analyzed to validate the proposed model and extraction procedures. For removing any uncertainties caused by observed device-to-device data variations due to the uneven quality of CVD graphene, the same methods were applied to a set of closely located bottom-gated GFETs found in literature. Those GFETs were designed for transfer length methods and fabricated on exfoliated graphene of homogenous quality. Similar mobility degradation behavior was observed for both sets of devices with the mobility being reduced to half for a voltage-induced charge carrier density of 1013 cm-2

Effect of idler terminations on the conversion loss for THz Schottky diode harmonic mixers

Efficient and reliable frequency converters, preferably operating at room temperature, are critical components for frequency-stabilizing terahertz sources. In this work, we present the analysis of optimum configurations for Schottky diode-based x4, x6, and x8 harmonic mixers operating at 2.3 THz, 3.5 THz, and 4.7 THz respectively. Detailed large-signal analysis of the two basic single-ended Z- and Y-mixers was carried out using a standard Schottky-diode model. For each case, the conversion loss was minimized by finding optimal embedding impedances at RF, LO, and IF frequencies. The analysis shows that the Y-mixer has less conversion loss at a low LO pump power. However, the Z-mixer provides reduced loss with increasing harmonic index and pump power due to the associated power dissipation in idler circuits. The results provide preliminary design guidelines for room- temperature frequency converters and their use in phase-locked loop applications

340 GHz FMCW pulse-Doppler radar to characterize the dynamics of particle clouds

This is a document that will describe a 340 GHz pulse Doppler radar. It will describe the system and its performanc

Microwave noise characterization of graphene field effect transistors

The microwave noise parameters of graphene field effect transistors (GFETs) fabricated using chemical vapor deposition graphene with 1  μm gate length in the 2 to 8 GHz range are reported. The obtained minimum noise temperature (Tmin) is 210 to 610 K for the extrinsic device and 100 to 500 K for the intrinsic GFET after de-embedding the parasitic noise contribution. The GFET noise properties are discussed in relation to FET noise models and the channel carrier transport. Comparison shows that GFETs can reach similar noise levels as contemporary Si CMOS technology provided a successful gate length scaling is performed

A System for THz Imaging of Low-Contrast Targets Using the Born Approximation

A THz imaging system, operating at 346 GHz and tailored for implementation of an imaging algorithm based on the Born approximation, is presented. The imaging algorithm provides focusing by compensating for the antenna footprint. This allows for using a more simple antenna system without optical focusing. Several aspects of implementing an imaging algorithm based on the Born approximation in THz imaging are discussed and key system properties are highlighted. The performance of the imaging algorithm is verified by imaging two simple dielectric targets. The results indicate that this approach provides a qualitative indication of the distribution of contrast in the samples complex permittivity and is a potential complement to existing imaging techniques

A compact 128-element Schottky diode grid frequency doubler generating 0.25 W of output power at 183 GHz

This paper presents a compact varactor grid frequency doubler encapsulated in a waveguide environment, thus providing single mode (H₁₀) waveguide connection at both input and output. Schottky diodes are used as varactors in this 128-element grid frequency doubler. By packaging the grid and its embedding network together with a stepped waveguide taper on the output, a module measuring 9 mm x 19 mm by 19 mm is created. A peak output power of 0.25 W is produced at 183 GHz with 1.32 W of input power and a corresponding conversion efficiency of 19%. The peak conversion efficiency is 23% at 183 GHz with 666 mW of input power