Papers by Author: Karl D. Hobart

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Authors: Marko J. Tadjer, Travis J. Anderson, Karl D. Hobart, Tatyana I. Feygelson, James E. Butler, Fritz J. Kub
Abstract: Nanocrystalline diamond (NCD) films were deposited using plasma-enhanced chemical vapor deposition. The NCD films were Boron-doped for p-type conductivity, yielding sheet resistances from 6.17x1011 to 522.5 /. Four different metals were deposited as Ohmic contacts and investigated for contact resistance and thermal stability. Contact and film annealing was performed under different atmospheric conditions with variable N2 content.
Authors: Rachael L. Myers-Ward, Stephen E. Saddow, Shailaja P. Rao, Karl D. Hobart, M. Fatemi, Francis J. Kub
Authors: Marko J. Tadjer, Karl D. Hobart, Michael A. Mastro, Travis J. Anderson, Eugene A. Imhoff, Fritz J. Kub, Jennifer K. Hite, Charles R. Eddy
Abstract: Field-effect transistors were fabricated on GaN and Al0.2Ga0.8N epitaxial layers grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates. The threshold voltage VTH was higher when AlGaN was used as an active layer. VTH also increased with temperature due to the increased positive polarization charge at the GaN/AlN buffer/sapphire interfaces. Drain current increased at high temperatures even with more positive threshold voltage, which makes GaN-based FET devices attractive for high temperature operation.
Authors: Travis J. Anderson, Karl D. Hobart, Luke O. Nyakiti, Virginia D. Wheeler, Rachael L. Myers-Ward, Joshua D. Caldwell, Francisco J. Bezares, D. Kurt Gaskill, Charles R. Eddy, Francis J. Kub, Glenn G. Jernigan, M.J. Tadjer, Eugene A. Imhoff
Abstract: Graphene, a 2D material, has motivated significant research in the study of its in-plane charge carrier transport in order to understand and exploit its unique physical and electrical properties. The vertical graphene-semiconductor system, however, also presents opportunities for unique devices, yet there have been few attempts to understand the properties of carrier transport through the graphene sheet into an underlying substrate. In this work, we investigate the epitaxial graphene/4H-SiC system, studying both p and n-type SiC substrates with varying doping levels in order to better understand this vertical heterojunction.
Authors: Kendrick X. Liu, Robert E. Stahlbush, Karl D. Hobart, Joseph J. Sumakeris
Abstract: With a simple processing sequence using only patterned aluminum (Al), dislocations and stacking faults were examined in thick n- epitaxial layers including a grown p+ layer on top. The thicknesses of the n- layers were 100 and 150 μm, values that are typical for fabricating 10 kV PiN diodes. High temperature sintering of the metal film was avoided making this a potentially nondestructive scheme for evaluating SiC epitaxy. Faulting of basal plane dislocations (BPDs) and the resulting forward voltage, Vf, increase were examined at current densities up to 20 A/cm2. A simple guard ring structure defined in the Al pattern was successfully used to confine the current through the epitaxial layers to the area inside of the ring. Kelvin contacts compensated for voltage drops at the Al/SiC interfaces. As a result, current-voltage characteristics and electroluminescent imaging were obtained across a known area of the PiN layer at currents densities ranging from 20 A/cm2 to 7 × 10-3 A/cm2.
Authors: Eugene A. Imhoff, Fritz J. Kub, Karl D. Hobart
Abstract: In silicon carbide devices used above around 2.4 kV, effective anode edge termination usually requires a high-resolution floating guard ring implant or multiple lithography/implant cycles to effect a multi-zone junction termination extension. In general the goal is to produce a smoothly tapered field profile to prevent high-voltage field-crowding that causes premature breakdown at the edge of the high voltage electrode. Using a much simpler grayscale photolithographic technique and a single termination implant, we directly produce the desired tapered doping profile. The effectiveness of this termination is shown by the near-ideal (6.1 kV) breakdown measured in PiN diodes made with a 38 µm intrinsic layer. The simple method is applicable to the fabrication of many high-voltage devices.
Authors: Rachael L. Myers-Ward, Luke O. Nyakiti, Jennifer K. Hite, Orest J. Glembocki, Francisco J. Bezares, Joshua D. Caldwell, Eugene A. Imhoff, Karl D. Hobart, James C. Culbertson, Yoosuf N. Picard, Virginia D. Wheeler, Charles R. Eddy, D. Kurt Gaskill
Abstract: Homo- and heteroepitaxial 3C-SiC layers were grown on 4H-SiC step-free mesas. The yields of smooth, defect-free mesas were ~ 17% for both intentionally and unintentionally doped films, while those with screw dislocations and multiple stepped surfaces were ~ 22%. The electronic and structural properties of the mesas were found on a micrometer-sized length scale using µ-PL and µ-Raman, respectively. 3C-SiC mesas were found to have complete 3C-SiC coverage with some of the mesas having electronic defects, while other mesas were found to be defect-free.
Authors: Eugene A. Imhoff, Karl D. Hobart
Abstract: Forward and reverse bias performance of 10kV, 10A and 20A junction barrier-controlled Schottky 4H silicon carbide rectifiers are presented. Over a temperature range of 30 to 200°C, the forward current-voltage curves show a normal Schottky rectifier relationship and the reverse current-voltage curves show typical PiN blocking. When operated in reverse-blocking at 125°C and 8kV, the 10A JBS rectifiers are notably stable at less than 5μA of leakage current, despite the large active area of the devices.
Authors: Joshua D. Caldwell, A.J. Giles, Robert E. Stahlbush, M.G. Ancona, Orest J. Glembocki, Karl D. Hobart, Brett A. Hull, Kendrick X. Liu
Abstract: Since it was determined that the formation and expansion of intrinsic stacking faults (SFs) induced a drift in the forward voltage (Vf) in 4H-SiC bipolar devices, significant effort has been made to understand the driving force causing SF motion as well as the various associated luminescence processes. The observation that annealing of faulted SiC devices and epilayers induced SF contraction and a recovery of the Vf drift enabled the studying of the impact of various parameters such as temperature, injection level and operation time upon SF motion, the Vf drift and luminescence within the same device. However, these observations in many cases contradicted the previously reported driving force models. Here we report on a basic driving force model explaining SF expansion in hexagonal SiC as well as discuss the observation of green luminescence from C-core partial dislocations bounding the SFs that may indicate an enhanced mobility of point defects within forward biased SiC pin diodes.
Authors: Karl D. Hobart, Eugene A. Imhoff, Fritz J. Kub, A.R. Hefner, T.H. Duong, J.M. Ortiz-Rodriguez, Sei Hyung Ryu, David Grider, Scott Leslie, J. Sherbondy, B. Ray
Abstract: The performance of Junction Barrier Schottky (JBS) diodes developed for medium voltage hard-switched Naval power conversion is reported. Nominally 60 A, 4.5kV rated JBS freewheeling diodes were paired with similarly rated Si IGBTs and evaluated for temperature dependent static and dynamic characteristics as well as HTRB and surge capability. The SiC JBS/Si IGBT pair was also directly compared to Si PiN diode/Si IGBT with similar ratings. Compared to Si, the SiC freewheeling diode produced over twenty times lower reverse recovery charge leading to approximately a factor-of-four-reduction in turn-on loss. Alternatively, for equivalent total switching loss, the SiC JBS/Si IGBT hybrid configuration allows for at least a 50% increase in specific switched power density. Reliability testing showed the devices to be robust with zero failures.
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