A large scale automated test set was designed and built to address the varied accelerated life test requirements of the GaAs industry. GaAs low-noise/small-signal MESFETs with 1 x 300 micron gate peripheries and 3 different gate structures were subjected to a 1000 hour high temperature storage test: 1) to compare the reliability performance and manufacturability of a) recessed-gate MESFETs with TiPdAu gates b) realigned self-aligned gate (RSAG) MESFETs with TiWN_x Schottky and TiPdAu overlay c) planarized self-aligned gate (PSAG) MESFETs with TiWN_x Schottky and TiPdAu overlay. 2) to study the changes in I_dss, R_g, R_o, g_m, and V_p over time and their effects upon MAG (Maximum Available Gain). 3) to study failure criteria and their applicability toward accurate life predictions. The recessed-gate devices suffered from Au/GaAs channel interdiffusion resulting in substantial dc parameter degradation above 225°C with an activation energy of 1.7 eV. Although the most widely used device structure in the GaAs industry, its process is not conducive to parameter uniformity. The realigned self-aligned gate (RSAG) devices are an initial attempt at the fabrication of a self-aligned gate analog MESFET. They were found to exhibit excellent electrical characteristics, but their reliability performance was unpredictable due to the critical nature of the .5 micron TiPdAu gate overlay realignment to a 1 micron TiWN_x Schottky. Planarized self-aligned gate (PSAG) devices were found to be readily manufacturable and to exhibit excellent reliability. The use of a decrease in MAG was found to be a more meaningful failure criterion than a 20% change in I_dss, which is employed extensively in the literature. / Master of Science

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Point functionalized carbon nanotubes have recently demonstrated the ability to serve as single-molecule biosensors. Operating as single-molecule Field-Effect Transistors (smFET), the sensors have been used to explore activity ranging in scope from DNA hybridization kinetics to DNA polymerase functionality. High signal levels and an all-electronic label-free transduction mechanism make the smFET an attractive candidate for next-generation medical diagnostics platforms and high-bandwidth basic science research studies. In this work, carbon nanotubes are integrated onto a custom designed CMOS chip. Integration enables arraying many devices for measurement, providing the requisite scale-up for any commercial application of smFETs. Integration also provides substantial benefits towards achieving high bandwidths through the reduction of electrical parasitics. In a first exploitation of these high-bandwidth measurement capabilities, integrated devices are electrically characterized over a 1-MHz bandwidth. Functionalization through electrochemical oxidation of the devices is observed with microsecond temporal resolution, revealing complex reaction pathways with resolvable scattering signatures. High rate random telegraph noise (RTN) is observed in certain oxidized devices, further illustrating the temporal resolution of the integrated sensing platform.

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A unified study on the hot carrier reliability of the Pseudomorphic High Electron Mobility Transistor (PHEMT) is carried out through Sentaurus Device Simulation, measurement, and physical analyses. A trade study of devices with four various geometries are evaluated for DC and RF performance. The trade-off of DC I-V characteristics, transconductance, and RF parameters versus hot carrier induced gate current is assessed for each device. Ambient temperature variation is also evaluated to observe its impact on hot carrier effects. A commercial grade PHEMT is then evaluated and measured to demonstrate the performance degradation that occurs after a period of operation in an accelerated stress regime— one hour of high drain voltage, low drain current stress. This stress regime and normal operation regime are then modeled through Sentaurus. Output characteristics are shown along with stress mechanisms within the device. Lastly, a means of simulating a PHEMT post-stress is introduced. The approach taken accounts for the activation of dopants near the channel. Post-stress simulation results of DC and RF performance are then investigated.

The thesis presents experimental studies of device characteristics and vibrational properties of atomic layer thin graphene and molybdenum disulphide (MoS2). We carried out Raman spectroscopic studies on field effect transistors (FET) fabricated from these materials to investigate the phonons renormalized by carrier doping thus giving quantitative information on electron-phonon coupling. Below, we furnish a synoptic presentation of our work on these systems. Chapter1: Introduction Chapter1, presents a detailed introduction of the systems studied in this the¬sis, namely single layer graphene (SLG), bilayer graphene (BLG) and single layer molybdenum disulphide (MoS2). We have mainly discussed their electronic and vibrational properties in the light of Raman spectroscopy. A review of the Raman studies on graphene layers is presented. Chapter2: Methodology and Experimental Techniques Chapter 2 starts with a description of Raman instrumentation. The steps for isolating graphene and MoS 2 flakes and the subsequent device fabrication procedures involving lithography are discussed in detail. A brief account of the top gated field effect transistor (FET) using solid polymer electrolyte is presented. Chapter3: Band gap opening in bilayer graphene and formation of p-n junction in top gated graphene transistors: Transport and Raman studies In Chapter3 the bilayer graphene (BLG) field effect transistor is fabricated in a dual gate configuration which enables us to control the energy band gap and the Fermi level independently. The gap in bilayer energy spectrum is observed through different values of the resistance maximum in the back gate sweep curves, each taken at a fixed top gate voltage. The gate capacitance of the polymer electrolyte is estimated from the experimental data to be 1.5μF/cm2 . The energy gap opened between the valence and conduction bands using this dual-gated geometry is es¬timated invoking a simple model which takes into account the screening of gate induced charges between the two layers. The presence of the controlled gap in the energy band structure along with the p-n junction creates a new possibility for the bilayer to be used as possible source of terahertz source. The formation of p-n junction along a bilayer graphene (BLG) channel is achieved in a electrolytically top gated BLG FET, where the drain-source voltage VDS across the channel is continuously varied at a fixed top gate voltage VT(VT>0). Three cases may arise as VDS is varied keeping VT fixed: (i) for VT-VDS0, the entire channel is doped with electron, (ii) for VT-VDS= 0, the drain end becomes depleted of carriers and kink in the IDS vs VDS curve appears, (iii) for VT-VDS « 0, carrier reversal takes place at the drain end, accumulation of holes starts taking place at the drain end while the source side is still doped with electrton. The verification of the spatial variation of carrier concentration in a similar top gated single layer graphene (SLG) FET device is done using spatially resolved Ra¬man spectroscopy. The signature 2D Raman band in a single layer graphene shows opposite trend when doped: 2D peak position decreases for electron doping while it increases for hole doping. On the other hand, the G mode response being symmetric in doping can act as a read-out for the carrier concentration. We monitor the peak position of the G and the 2D bands at different locations along the SLG FET channel. For a fixed top gate voltage V T , both G and the 2D band frequencies vary along the channel. For a positive VTsuch that VT-VDS= 0, the peak frequencies ωGand ω2DωG/2D occur at the undoped frequency (ωG/2D)n=0 near the drain end while the source end corresponds to non-zero concentration. When VT-VDS<0, Raman spectra from hole doped regions (drain end) in the channels show an blue-shift in ω2Dwhile from the electron doped regions (near source) ω2Dis softened. Chapter4: Mixing Of Mode Symmetries In Top Gated Bilayer And Multilayer Graphene Field Effect Devices In Chapter4, the effect of gating on a bilayer graphene is captured by using Raman spectroscopy which shows a mixing of different optical modes belonging to differ¬ent symmetries. The zone-center G phonon mode splits into a low frequency (Glow) and a high frequency (Ghigh) mode and the two modes show different dependence on doping. The two G bands show different trends with doping, implying different electron-phonon coupling. The frequency separation between the two sub-bands in¬creases with increased doping. The mode with higher frequency, termed as Ghigh, shows stiffening as we increase the doping whereas the other mode, Glow, shows softening for low electron doping and then hardening at higher doping. The mode splitting is explained in terms of mixing of zone-center in-plane optical phonons rep¬resenting in-phase and out-of-phase inter-layer atomic motions. The experimental results are combined with the theoretical predictions made using density functional theory by Gava et al.[PRB 80, 155422 (2009)]. Similar G band splitting is observed in the Raman spectra from multilayer graphene showing influence of stacking on the symmetry properties. Chapter5: Anomalous dispersion of D and 2D modes in graphene and doping dependence of 2D ′and 2D+G bands Chapter 5 consists of two parts: Part A titled “Doping dependent anomalous dispersion of D and 2D modes in graphene” describes the tunability of electron-phonon coupling (EPC) associated with the highest optical phonon branch (K-A) around the zone corner K using a top gated single layer graphene field effect transistor. Raman D and 2D modes originate from this branch and are dispersive with laser excitation energy. Since the EPC is proportional to the slope of the phonon branch, doping dependence of the D and 2D modes is carried out for different laser energies. The dispersion of the D mode decreases for both the electron and the hole doping, in agreement with the recent theory of Attaccalite et. al [Nano Letters, 10, 1172 (2010)]. In order to observe D-band in the SLG samples, low energy argon ion bombardment was carried out. The D peak positions for variable carrier concentration using top-gated FET geometry are determined for three laser energies, 1.96 eV, 2.41 eV and 2.54 eV. However, the dispersion of the 2D band as a function of doping shows an opposite trend. This most curious result is quantitatively explained us¬ing a fifth order process rather than the usual fourth order double resonant process usually considered for both the D and 2D modes. Part B titled “Raman spectral features of second order 2D’ and 2D+G modes in doped graphene transistor” deals with doping dependence of 2D’ and 2D+G bands in single layer graphene transistor. The phonon frequency blue shifts for the hole doping and whereas it red shifts for electron doping, similar to the behaviour of the 2D band. The linewidth of the 2D+G combination mode too follows the 2D trend increasing with doping while that of 2D’ mode remains invariant. Chapter6: New Raman modes in graphene layers using 2eV light Unique resonant Raman modes are identified at 1530 cm−1 and 1445 cm−1 in single, bi, tri and few layers graphene samples using 1.96 eV (633 nm) laser excitation energy (EL). These modes are absent in Raman spectra using 2.41 eV excitation energy. In addition, the defect-induced D band which is observed only from the edges of a pristine graphene sample, is observed from the entire sample region using E L = 1.96 eV. Raman images with peak frequencies centered at 1530 cm−1, 1445 cm−1 and D band are recorded to show their correlations. With 1.96 eV, we also observe a very clear splitting of the D mode with a separation of ∼32 cm−1, recently predicted in the context of armchair graphene nanoribbons due to trigonal warping effect for phonon dispersion. All these findings suggest a resonance condition at ∼2eVdue to homo-lumo gap of a defect in graphene energy band structure. Chapter7: Single and few layer MoS2: Resonant Raman and Phonon Renormalization Chapter 7 is divided into two parts. In Part A “Layer dependent Resonant Raman scattering of a few layer MoS2”, we discuss resonant Raman scattering from single, bi, four and seven layers MoS2. As bulk crystal of MoS2is thinned down to a few atomic layers, the indirect gap widens turning into a direct gap semiconductor with a band gap of 1.96 eV in its monolayer form. We perform Raman study from MoS 2 layers employing 1.96 eV laser excitation in order to achieve resonance condition. The prominent Raman modes for MoS 2 include first order E12g mode at ∼383 cm−1 and the A1gmode at ∼408 cm−1 which are observed under both non resonant and resonant conditions. A1gphonon involves the sulphur atomic vibration in opposite direction along the c axis (perpendicular to the basal plane) whereas for E12g mode, displacement of Mo and sulphur atoms are in the basal plane. With decreasing layer thickness, these two modes shifts in opposite direction, the E12g mode shows a blue shift of ∼2cm−1 while the A1gis red shifted by ∼4cm−1 . Under resonant condi¬tion, apart from E12g and A1gmodes, several new Raman spectral features, which are completely absent in bulk, are observed in single, bi and few layer spectra pointing out the importance of Raman characterization. New Raman mode attributed to the longitudinal acoustic mode belonging to the phonon branch at M along the Γ-M direction of the Brillouin zone is seen at ∼230 cm−1 for bi, four and seven layers. The most intense region of the spectrum around 460 cm−1 is characterized by layer dependent frequencies and spectral intensities with the band near 460 cm−1 becoming asymmetric as the sample thickness is increased. In the high frequency region between 510-630 cm−1, new bands are seen for bi, four and seven layers. In Part B titled “Symmetry-dependent phonon renormalization in monolayer MoS2transistor”, we show that in monolayer MoS2the two Raman-active phonons, A1g and E21 g, behave very differently as a function of doping induced by the top gate voltage in FET geometry. The FET achieves an on-off ratio of ∼ 105 for electron doping. We show that while E12g phonon is essentially unaffected, the A1gphonon is strongly influenced by the level of doping. We quantitatively understand our experimental results through the use of first-principles calculations to determine frequencies and electron-phonon coupling for both the phonons as a function of carrier concentration. We present symmetry arguments to explain why only A1g mode is renormalized significantly by doping. Our results bring out a quantitative under¬standing of electron-phonon interaction in single layer MoS2.

Like cell to the human body, transistors are the basic building blocks of any electronics circuits. Silicon has been the industries obvious choice for making transistors. Transistors with large size occupy large chip area, consume lots of power and the number of functionalities will be limited due to area constraints. Thus to make the devices smaller, smarter and faster, the transistors are aggressively scaled down in each generation. Moore's law states that the transistors count in any electronic circuits doubles every 18 months. Following this Moore's law, the transistor has already been scaled down to 14 nm. However there are limitations to how much further these transistors can be scaled down. Particularly below 10 nm, these silicon based transistors hit the fundamental limits like loss of gate control, high leakage and various other short channel effects. Thus it is not possible to favor the silicon transistors for future electronics applications. As a result, the research has shifted to new device concepts and device materials alternative to silicon. Carbon is the next abundant element found in the Earth and one of such carbon based nanomaterial is graphene. Graphene when extracted from Graphite, the same material used as the lid in pencil, have a tremendous potential to take future electronics devices to new heights in terms of size, cost and efficiency. Thus after its first experimental discovery of graphene in 2004, graphene has been the leading research area for both academics as well as industries. This dissertation is focused on the analysis and optimization of graphene based circuits for future electronics. The first part of this dissertation considers graphene based transistors for analog/radio frequency (RF) circuits. In this section, a dual gate Graphene Field Effect Transistor (GFET) is considered to build the case study circuits like voltage controlled oscillator (VCO) and low noise amplifier (LNA). The behavioral model of the transistor is modeled in different tools: well accepted EDA (electronic design automation) and a non-EDA based tool i.e. \simscape. This section of the dissertation addresses the application of non-EDA based concepts for the analysis of new device concepts, taking LC-VCO and LNA as a case study circuits. The non-EDA based approach is very handy for a new device material when the concept is not matured and the model files are not readily available from the fab. The results matches very well with that of the EDA tools. The second part of the section considers application of multiswarm optimization (MSO) in an EDA tool to explore the design space for the design of LC-VCO. The VCO provides an oscillation frequency at 2.85 GHz, with phase noise of less than -80 dBc/Hz and power dissipation less than 16 mW. The second part of this dissertation considers graphene nanotube field effect transistors (GNRFET) for the application of digital domain. As a case study, static random access memory (SRAM) hs been design and the results shows a very promising future for GNRFET based SRAM as compared to silicon based transistor SRAM. The power comparison between the two shows that GNRFET based SRAM are 93% more power efficient than the silicon transistor based SRAM at 45 nm. In summary, the dissertation is to expected to aid the state of the art in following ways: 1) A non-EDA based tool has been used to characterize the device and measure the circuit performance. The results well matches to that obtained from the EDA tools. This tool becomes very handy for new device concepts when the simulation needs to be fast and accuracy can be tradeoff with. 2)Since an analog domain lacks well-design design paradigm, as compared to digital domain, this dissertation considers case study circuits to design the circuits and apply optimization. 3) Performance comparison of GNRFET based SRAM to the conventional silicon based SRAM shows that with maturation of the fabrication technology, graphene can be very useful for digital circuits as well.

Organic electronics based on π-conjugated semi-conductor raises new technology, such as organic film transistors, e-paper, and organic photovoltaic cells that can be implemented cost-effectively on large-area applications. Currently, the device performance is limited by low charge carrier mobility. Poly(3-hexylthiophene) (P3HT) and organic field effect transistors (OFET) is used as a model to investigate morphology of the organic film and corresponding electronic properties. In this thesis, processing parameters such as boiling points and solubility are controlled to impact the micro- and macro-morphology of the film to enhance the charge transport of the device. Alternative approach to improve ordering of polymer chains and increase in charge transport without post-treatment of P3HT solution is studied. The addition of high boiling good solvent to the relatively low boiling main solvent forms ordered packing of π-conjugated polymers during the deposition process. We show that addition of 1% of dichlorobenzene (DCB) to the chloroform based P3HT solution was sufficient to improve wetting and molecular structures of the film to increase carrier mobility. Systematic study of solvent-assisted re-annealing technique, which has potential application in OFET encapsulation and fabrication of top-contact OFET, is conducted to improve mobility of OFET, and, to suggest a cost-effective processing condition suitable for industrial application. Three process parameters: boiling point, polarity, and solubility are investigated to further understand the trend of film response to the solvent-assisted technique. We report the high boiling non-polar solvents with relatively high RED values promote highest improvement in molecular packing and formulate crystalline structure of the thin film, which increases the device performance.

Les nanomatériaux et nanotechnologies sont devenus un élément incontournable dans l'électronique de faible puissance, la production énergétique / gestion et les réseaux sans fil, offrant la possibilité de construire une vision pour les capteurs autonomes. Cette thèse s’intéresse au concept de systèmes basse température utilisant des structures de matériaux hybrides organique/inorganique pour la réalisation de dispositifs électroniques faible coût, dont les transistors à effet de champ (FET) et les nanogénérateurs piézoélectriques (nommés PENGs) et ce, sur divers substrats en particulier plastiques. Pour atteindre ces objectifs, ce travail décrit d'abord la croissance contrôlée de nanostructures monocristallines de ZnO en utilisant des approches vapeur-liquide-solide (VLS) et hydrothermales à haute et basse température respectivement. Pour les dispositifs FET, les nanostructures ZnO obtenues par VLS sont utilisées en raison de leur haute qualité structurale et optique. Les sections suivantes présentent des différentes études menées pour optimiser les prototypes FET, comprenant (i) les contacts métal-semiconducteur, (ii) la qualité de l'interface semi-conducteur/isolant et (iii) l'épaisseur de diélectrique organique. La dernière section examine la possibilité de fabriquer des systèmes hybrides organiques/inorganiques pour PENGs utilisant l'approche hydrothermale. Certaines des questions clés, ce qui limitent les performances PENG sont abordés : (i) l'effet de porteurs libres et (ii) l'encapsulation polymère. Ce travail démontre le fort potentiel des ZnO nanostructures pour l'avenir de l'électronique. / Nanomaterials and nanotechnology has become a crucial feature in low-power electronics, energy generation/management and wireless networks, providing the opportunity to build a vision for autonomous sensors. The present thesis delivers the concept of low-temperature processable organic / inorganic hybrid systems for the realization of inexpensive electronic devices including field-effect transistors (FETs) and piezoelectric nanogenerators (PENGs) on various substrates including plastics. To achieve these objectives, this work first describes the controlled growth of single-crystalline ZnO nanostructures using high-temperature vapor-liquid-solid (VLS) and low-temperature hydrothermal approaches. For the FET devices, VLS grown ZnO nanostructures are used, owing to their high structural and optical quality. Later sections present different studies conducted to optimize the FET prototypes, includes: (i) metal-semiconductor contacts, (ii) semiconductor/insulator interface quality and (iii) organic dielectric thickness. The last section investigates the possibility to fabricate organic / inorganic hybrid systems for PENGs using hydrothermal approach. Some of the key issues, restricting the PENG performances are addressed: (i) screening effect from free charge carriers and (ii) polymer encapsulation. This work demonstrates the high potential of ZnO nanostructure for the future of electronics.