1,721,071 research outputs found
Two-Dimensional Heterojunction Interlayer Tunneling Field Effect Transistors (Thin-TFETs)
Full text linkLayered 2-D crystals embrace unique features of atomically thin bodies, dangling bond free
interfaces, and step-like 2-D density of states. To exploit these features for the design of a steep slope
transistor, we propose a Two-dimensional heterojunction interlayer tunneling field effect transistor (Thin-
TFET), where a steep subthreshold swing (SS) of ∼14 mV/dec and a high on-current of ∼300 μA/μm are
estimated theoretically. The SS is ultimately limited by the density of states broadening at the band edges
and the on-current density is estimated based on the interlayer charge transfer time measured in recent
experimental studies. To minimize supply voltage VDD while simultaneously maximizing on currents,
Thin-TFETs are best realized in heterostructures with near broken gap energy band alignment. Using
the WSe2/SnSe2 stacked-monolayer heterostructure, a model material system with desired properties for Thin-TFETs, the performance of both n-type and p-type Thin-TFETs is theoretically evaluated. Nonideal effects such as a nonuniform van der Waals gap thickness between the two 2-D semiconductors and finite total access resistance are also studied. Finally, we present a benchmark study for digital applications, showing the Thin-TFETs may outperform CMOS and III–V TFETs in term of both switching speed and energy consumption at low-supply voltages
Quantitative Scanning Microwave Microscopy for Transfer Characteristics of GaN High-Electron-Mobility Transistors
No full textThis article demonstrates the feasibility in using a scanning microwave microscope (SMM) to probe the transfer characteristics of an ungated GaN high-electron-mobility transistor (HEMT). To guide the experiment and to interpret the result, an equivalent circuit is proposed to model the probe-sample near-field interaction, and the model is validated by simulation and experimentation. In the experiment, the SMM probe with a dc bias voltage acts as a surrogate to locally modulate the 2-D electron gas (2DEG) at the GaN heterojunction. Because the present SMM is most sensitive to a 2DEG sheet resistance RSH between 10 4 Ω / □ and 10 6 Ω / □ , the unbiased RSH is determined to be (3 ± 3) × 10 3 Ω / □ , in contrast to ∼ 450 Ω / □ determined by Hall measurements. However, with the bias decreasing from 0 to − 8 V, the 2DEG is depleted and its resistance is increased to (5 ± 2) × 10 5 Ω / □ with an on/off ratio of 160, a peak transconductance around − 5 V, and a threshold voltage of − 6 V. These results agree with the dc-measured current–voltage characteristics on a gated HEMT after its fabrication is completed. This shows that the SMM could be a powerful tool for in-process monitoring and material/device correlation
TWO-DIMENSIONAL HOLE GAS IN N-POLAR ALGAN/GAN HETEROSTRUCTURES: GROWTH AND TRANSPORT ANALYSIS
No full text117 pagesN-polar Ⅲ-nitride heterostructures and devices have been a subject of intense investigation recently, as the inverted polarization fields provide unique advantages over the metal-polar devices. From the molecular beam epitaxy (MBE) growth point of view, however, obtaining p-type conductivity in Mg-doped N-polar heterostructures is challenging. This is because unintentional oxygen donor incorporation is higher but Mg incorporation is lower for N-polar growth compared to the metal-polar counterpart, both by roughly an order of magnitude. Polarization-induced doping provides an alternate route to obtain p-type conductivity. In this work, we report the observation of polarization-induced two-dimensional hole gases (2DHGs) in N-polar AlGaN/GaN heterostructures grown by plasma-assisted MBE. AlGaN/GaN heterostructures were grown on N-polar semi-insulating GaN bulk substrates. The existence of 2DHGs in these heterostructures is confirmed by the stable p-type conductivity and high hole mobilities observed in temperature dependent Hall effect measurements. Through transport analysis, we have shown that acoustic piezoelectric scattering has significant influence on the mobility of 2DHGs with relatively low density (< 2×1013 cm-2)
N-polar n-GaN MOSCAP and Tunnel Junctions Toward p-GaN MOSCAP
No full text92 pagesWith remarkable attributes of unique polarization characteristics, high electron mobility, and saturation velocity, gallium nitride (GaN) material has emerged as a key contender in the high-performance semiconductor market, paving the way for the future optoelectronics and high-frequency electronics. The oxide/semiconductor interface quality could significantly impact device performance and reliability if not properly managed. This thesis intends to categorize different types of charge trapping effects at the oxide/semiconductor interface and highlight their influence on the device performance. The approach to identify and separate the influence of different oxide traps is suggested based on electrical characterization methods. The quality of the oxide/GaN interface is discussed in the context of experimentally fabricated MOS capacitors on n-type GaN (000), utilizing dielectric materials grown by plasma-enhanced atomic layer deposition. Additionally, this thesis presents findings from the 1st and 2nd generation GaN homojunction tunnel diodes, contributing to the fabrication of MOS capacitors on p-type GaN (0001) substrate for interface quality studies. Lastly, recommendations for optimized device design are provided based on literature survey
Electron transport in 2D crystal semiconductors and their device applications
No full textIn this work, we investigate the transport properties of 2D crystal semiconductors from two angles. For drift transport in traditional FETs, scattering mechanisms that limit the electron mobility are identified, and means to improve them vastly over currently reported values are presented. For low-power electronics, tunneling transport currents within monolayer p-n junctions, and interlayer tunneling currents between adjacent 2D semiconductor layers is discussed for TFETs
DEMONSTRATION OF FERROELECTRIC HZO UNDER THE GATE OF ALN/GAN HIGH-ELECTRON-MOBILITY TRANSISTORS
No full text93 pagesGallium Nitride-based high electron mobility transistors (GaN HEMTs) are at the cutting edge of technological innovation. Renowned for their high speed and power capabilities, GaN HEMTs are utilized across a diverse array of sectors, including telecommunications, power electronics, aerospace, defense, industrial, medical, and consumer electronics. Integrating GaN with an aluminum nitride (AlN) barrier enhances speed, power output, and thermal management. However, traditional HEMTs have reached the physical limits of their operational speed. This thesis investigates the addition of a ferroelectric hafnium zirconium oxide (HZO) layer beneath the gate of AlN/GaN HEMTs to overcome these limitations. This is achieved through the use of the remnant polarization of ferroelectrics to modulate the threshold voltage and demonstrate the ability for memory. Through the development of multiple HEMT devices with varying HZO thicknesses, hysteresis in device output currents is verified and alterable, exhibiting a threshold voltage tuning range of 0.5-1.2 V. The thesis also addresses the challenges and uncertainties associated with processing devices and working with ferroelectrics, emphasizing the necessity for meticulous care throughout the process. Overall, this work lays the foundation for the benefits of incorporating ferroelectric layers under the gate of AlN/GaN HEMTs, paving the way for faster devices in the future
Surface/interface states in GaN-based transistors: gate dielectrics and surface passivation
No full textGaN has raised wide attention owing to its high breakdown voltage and capability of performing efficiently even at high temperature. However, there are several challenges to overcome for achieving the superior performance of GaN HEMTs, including management and minimization of gate leakage and interface states located at the insulator-semiconductor interface. In this work, I studied several gate insulators readily available at CNF and their insulator-semiconductor interface in the metal-insulator-semiconductor capacitor structure. I found that the Al2O3/HfO2 bilayer is helpful in reducing gate leakage by one order of magnitude and improving the breakdown field from 4.5 MV/cm to 6.5 MV/cm compared to the single HfO2 layer with a similar equivalent oxide thickness. The observed improvement can be attributed to the large bandgap and excellent conduction band offset of Al2O3. Moreover, I found that a UV/ozone oxidation surface treatment prior to dielectric deposition helps to decrease the interface density of states at 0.39 eV below the conduction band to 1.2 × 10^12 eV^−1 cm^−2 characterized by the frequency-dependent conductance method. I also studied PECVD SiNx passivation of AlN/GaN/AlN quantum well HEMTs with several surface treatment methods. With NH3 plasma pretreatment, I observed an improvement in the maximum drain current by 30 % and peak transconductance by 60 % in these HEMTs. These are positive indicators for the effectiveness of NH3 plasma pretreatment in managing dispersion in HEMTs
Physical Properties Enhancement of Epitaxial Transitional Metal Nitrides Grown on Sapphire by Substrate Miscut Angles: Mechanism, Property Characterization, and Applications
No full text111 pagesThis study explores the plasma-assisted molecular beam epitaxy (PAMBE) growth and characterization of high-quality niobium nitride (NbN) thin films on miscut c-plane sapphire substrates, with the ultimate goal of improving their superconducting properties for potential applications in low microwave noise Josephson junctions and superconducting quantum circuits. Specifically, the study investigates the effects of substrate miscut angles on the critical temperature, coherence length, and critical magnetic field of NbN thin films, providing valuable insights for developing advanced superconducting devices.The research begins with the PAMBE growth of NbN films using the Veeco GENxplor MBE system. The study focuses on achieving cubic -phase NbN, known for its superior superconducting properties, with controlled growth parameters such as substrate temperature. Characterization techniques, including X-ray diffraction (XRD) and atomic force microscopy (AFM), are employed to assess the structural properties and surface morphology of the NbN films. Then, the superconducting properties of NbN were measured and examined via the Quantum Design Physical Property Measurement System. For the annealed substrate sample series, the results reveal a monotonic increase in critical temperature and a decrease in coherence length with increasing substrate miscut angle, which is favorable for applications requiring precise control over tunneling properties and critical current. The critical magnetic field also increases with increasing substrate miscut angle, indicating enhanced superconducting stability in high magnetic field environments. At last, the challenges associated with critical temperature and coherence length measurements are discussed
Fabrication of Novel Devices using Selective In-Situ Thermal Sublimation Etching and Selective Area Regrowth of GaN in MBE
No full text95 pagesGallium Nitride (GaN), a wide band gap semiconductor, has gained widespread attention owing to its unique properties like spontaneous and piezoelectric polarization, high electron mobility, high saturation velocity, combined with its high thermal stability and versatility. The significance of GaN-based lateral PN junctions cannot be overstated, given their role as fundamental components of CMOS technology. However, conventional techniques like ion implantation for p-GaN doping pose challenges due to lattice defects and disorder. Additionally, doping redistribution occurs at high annealing temperatures, reaching up to 1450°C. This thesis proposes an alternative approach utilizing in-situ selective sublimation etching and regrowth techniques in an MBE environment for novel device fabrication. Through systematic experimentation and optimization of process parameters, novel insights into the sublimation etching process are obtained. The thesis presents a comprehensive process flow for lateral PN diode realization using in-situ selective sublimation etching and regrowth, showcasing precise control over device morphology. Challenges like sample property variations and substrate coatings are addressed, underscoring the importance of uniformity in the fabrication process. Overall, this work lays the foundation for employing this novel techniques in the fabrication of high-performance semiconductor devices, including GaN-based HEMT devices with regrown contacts, thereby extending the applicability of the proposed approach
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