Graphene integrated circuits for micro- and millimetre-wave applications
Ghareeb, Ahmed Hamed Abdel Razek; Negra, Renato (Thesis advisor); Vescan, Andrei (Thesis advisor)
Aachen (2019) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xxii, 172 Seiten) : Illustrationen, Diagramme
Graphene is one of the first two-dimensional (2D) materials to be isolated in nature. It has the thinnest crystal-structure known, with a thickness of one carbon atom. Since more than seventy years, properties of graphene have been theoretically studied as a thermodynamically-unstable material, which can not exist in a free state. Therefore, it was considered for a long time as academic material. In 2004, thermodynamically-stable single-layer graphene (SLG) was successfully isolated from a graphite surface. This breakthrough stimulated intense theoretical and experimental studies, exploring the outstanding mechanical strength, chemical stability, optical transparency, and the excellent electronic properties of graphene. This includes the extraordinary high mobility and saturation-velocity of the charge carriers in graphene, promising a great potential in high-frequency electronics. However, the research in graphene-based circuits and systems shows a significantly slower pace for two main reasons. First, the fabrication technology of the graphene field-effect transistors (GFETs) results in poor current-saturation and relatively low cut-off frequency, fT, and maximum oscillation frequency, fmax. The second reason is that most of the graphene-based circuits have been presented as the extension to the study of the transistor itself. Accordingly, they are limited to single-transistor designs and not adapted to large-scale integration. These two reasons raise some doubts about the true feasibility of using graphene in microelectronics. The goal of this thesis is to establish the mandatory foundation from the design and technology perspectives to push forward the activities in graphene electronics towards the applied research. In this way, useful applications can be demonstrated exploiting the excellent properties of graphene and competing with conventional semiconductor technologies. As a first step, a graphene-compatible Monolithic Microwave Integrated Circuit (MMIC) process has been developed and optimised to enable the fabrication of high-quality passive components. In addition, graphene-based diodes are introduced as Metal-Insulator-Graphene (MIG) junctions, integrated alongside with the developed MMIC process. MIG diodes exhibit superior performance, in terms of high current-density, low series-resistance, large current-asymmetry, adequate nonlinearity, and high voltage-responsivity. In addition, the proposed fabrication technology, using graphene grown by chemical vapour deposition (CVD), ensures the reproducibility of the MIG diode and the repeatability of its electrical properties. These performance metrics boost the realisation of high-frequency graphene-based circuits, overcoming the limited speed of state-of-the-art GFETs. Moreover, small-signal and nonlinear models for the MIG diodes have been extracted and verified in order to facilitate the circuit design process. The study of the MIG diode as a high-frequency mixing and frequency multiplying device reveals a promising performance in terms of conversion-loss (CL). Accordingly, several microwave circuits have been proposed employing MIG diodes and the developed MMIC process. This includes a single-diode mixer at 2.4 GHz and fully-integrated balanced fundamental and subharmonic microwave mixers for X-band applications, i.e., 8-12 GHz. The measured performance of the fabricated mixers sets a new record compared to state-of-the-art GFET-based counterparts and competes with the commercial Schottky-diode mixers. Moreover, fully-integrated and wideband resistive frequency multiplier circuits have been demonstrated in frequency doubler and tripler configurations. The theoretical analysis, together with the measurement results, proves a low-loss performance which, in turn, opens the opportunity to realise higher-order graphene-based frequency multipliers circuits. Moreover, using GFETs as power detectors enables operation beyond their fT and fmax. This concept has been utilised to realise a fully-integrated millimetre-wave receiver frontend in a six-port configuration. Successful demodulation of an On-Off Keying digitally modulated signal has been demonstrated at 90 GHz. The proposed graphene technology and design concepts in this thesis represent the basis for future demonstrations of graphene-based micro- and millimetre-wave integrated circuits. This endorses the potential of graphene as the material for future microelectronics, such as wearable and flexible devices for biomedical and sensing applications.