Papers by Author: Michael J. Uren

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Authors: D.J. Morrison, A. Keir, I.H. Preston, Keith P. Hilton, Michael J. Uren, C. Mark Johnson
Authors: Michael J. Uren, Martin Kuball
Abstract: Recent work on the thermal and electrical challenges in realizing AlGaN/GaN microwave heterojunction field effect transistors grown on SiC substrates is discussed. Raman thermography has been used to directly measure the self-heating induced lattice temperature rise with dramatically improved resolution and accuracy compared to traditional infrared techniques. It is demonstrated that defects in the SiC substrate can influence the temperature distribution within the active device with potential consequences for reliability. Microwave devices require an insulating GaN substrate material for device isolation. It is shown that the net deep level acceptor concentration has to be accurately controlled to suppress short-channel effects and to achieve radio frequency power efficient operation.
Authors: D.J. Morrison, A.J. Pidduck, V. Moore, P.J. Wilding, Keith P. Hilton, Michael J. Uren, C. Mark Johnson
Authors: Praneet Bhatnagar, Nicolas G. Wright, Alton B. Horsfall, C. Mark Johnson, Michael J. Uren, Keith P. Hilton, A.G. Munday, A.J. Hydes
Abstract: Silicon Carbide (SiC) power devices are increasingly in demand for operations which require ambient temperature over 300°C. This paper presents circuit applications of normally-on SiC VFETs at temperatures exceeding 300°C. A DC-DC boost converter using a 4H-SiC VJFET and a SiC Schottky Diode was fabricated and operated up to 327°C. A power amplifier achieved a voltage gain of 3.88 at 27°C dropping to 3.16 at 327°C. This 20 % reduction is consistent with the fall in transconductance of the device.
Authors: Praneet Bhatnagar, Nicolas G. Wright, Alton B. Horsfall, Konstantin Vassilevski, C. Mark Johnson, Michael J. Uren, Keith P. Hilton, A.G. Munday, A.J. Hydes
Abstract: 4H-SiC depletion mode (normally-on) VJFETs were fabricated and characterised at temperatures up to 377 °C. The device current density at drain voltage of 50 V drops down from 54 A/cm2 at room temperature to around 42 A/cm2 at 377 °C which is a 20 % reduction in drain current density. This drop in drain currents is much lower than previously reported values of a 30 % drop in JFETs at high temperatures. The average temperature coefficient of the threshold voltage was found to be -1.36 mV/°C which is smaller than for most Si FETs. We have found that these devices have shown good I-V characteristics upto 377 °C along with being able to retain its characteristics on being retested at room temperature.
Authors: Konstantin Vassilevski, Irina P. Nikitina, Praneet Bhatnagar, Alton B. Horsfall, Nicolas G. Wright, Anthony G. O'Neill, Michael J. Uren, Keith P. Hilton, A.G. Munday, A.J. Hydes, C. Mark Johnson
Abstract: 4H-SiC diodes with nickel silicide (Ni2Si) and molybdenum (Mo) Schottky contacts have been fabricated and characterised at temperature up to 400°C. Room temperature boron implantation has been used to form a single zone junction termination extension. Both Ni2Si and Mo diodes revealed unchanging ideality factors and barrier heights (1.45 and 1.3 eV, respectively) at temperatures up to 400°C. Soft recoverable breakdowns were observed both in Ni2Si and Mo Schottky diodes at voltages above 1450 V and 3400 V depending on the epitaxial structure used. These values are about 76% and 94% of the ideal avalanche breakdown voltages. The Ni2Si diodes revealed positive temperature coefficients of breakdown voltage at temperature up to 240°C.
Authors: Konstantin Vassilevski, Irina P. Nikitina, Alton B. Horsfall, Nicolas G. Wright, Anthony G. O'Neill, Keith P. Hilton, A.G. Munday, A.J. Hydes, Michael J. Uren, C. Mark Johnson
Abstract: High voltage 4H-SiC Schottky diodes with single-zone junction termination extension (JTE) have been fabricated and characterised. Commercial 4H-SiC epitaxial wafers with 10, 20 and 45 +m thick n layers (with donor concentrations of 3×1015, 8×1014 and 8×1014 cm-3, respectively) were used. Boron implants annealed under argon flow at 1500°C for 30 minutes, without any additional protection of the SiC surface, were used to form JTE’s. After annealing, the total charge in the JTE was tuned by reactive ion etching. Diodes with molybdenum Schottky contacts exhibited maximum reverse voltages of 1.45, 3.3 and 6.7 kV, representing more than 80% of the ideal avalanche breakdown voltages and corresponding to a maximum parallel-plane electric field of 1.8 MV/cm. Diodes with a contact size of 1×1 mm were formed on 10 +m thick layers (production grade) using the same device processing. Characterisation of the diodes across a quarter of a 2-inch wafer gave an average value of 1.21 eV for barrier heights and 1.18 for ideality factors. The diodes exhibited blocking voltages (defined as the maximum voltage at which reverse current does not exceed 0.1 mA) higher than 1 kV with a yield of 21 %.
Authors: D. Sands, P.H. Key, M. Schlaf, C.D. Walton, C.J. Anthony, Michael J. Uren
Authors: Konstantin Vassilevski, Keith P. Hilton, Nicolas G. Wright, Michael J. Uren, A.G. Munday, Irina P. Nikitina, A.J. Hydes, Alton B. Horsfall, C. Mark Johnson
Abstract: Trenched and implanted vertical JFETs (TI-VJFETs) with blocking voltages of 700 V were fabricated on commercial 4H-SiC epitaxial wafers. Vertical p+-n junctions were formed by aluminium implantation in sidewalls of strip-like mesa structures. Normally-on 4H-SiC TI-VJFETs had specific on-state resistance (RO-S ) of 8 mW×cm2 measured at room temperature. These devices operated reversibly at a current density of 100 A/cm2 whilst placed on a hot stage at temperature of 500 °C and without any protective atmosphere. The change of RO-S with temperature rising from 20 to 500 °C followed a power law (~ T 2.4) which is close to the temperature dependence of electron mobility in 4H-SiC.
Authors: Keith P. Hilton, Michael J. Uren, D.G. Hayes, H.K. Johnson, P.J. Wilding
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