High temperature reliability of power module substrates

The thermal cycling reliability of candidate copper and aluminium power substrates has been assessed for use at temperatures exceeding 300°C peak using a combination of thermal cycling, nanoindentation and finite element modelling to understand the relative stresses and evolution of the mechanical properties. The results include the relative cycling lifetimes up to 350°C, demonstrating almost an order of magnitude higher lifetime for active metal brazed Al / AlN substrates over Cu / Si3N4, but four times more severe roughening and cracking of the Ni-P plating's on the Al / AlN (DBA) substrates. The nonlinear finite element modelling illustrated that the yield strength of the metal and the thickness of the ceramic are the main stress controlling factors, but comparisons with the cycling lifetime results demonstrated that the fracture toughness (resistance) of the ceramic is the over-riding controlling factor for the overall passive thermal cycling lifetimes. In order to achieve the highest substrate lifetime for the highly stressed high temperature thermal cycled applications, the optimum solution appears to be annealed copper, brazed on to a thicker than normal or higher fracture toughness Si3N4 ceramic

Novel dispersion representation of rectangular dielectric guides with application to leaky-wave antennas

A novel method for graphically and mathematically representing the dispersion characteristics of electromagnetic dielectric structures is presented. The method has been used to develop closed-form formulas to find the longitudinal propagation constant and useful frequency range for a number of popular rectangular dielectric waveguide types. These formulas are presented in terms of their material dependency and are guide size and operating frequency independent. Their form is so trivial that these characteristics can now be found in just a few seconds using a basic calculator, yet still with second order accuracy. In addition, improvements have been made to a number of well-known formulas normally associated with periodic grating leaky-wave antennas. These improvements allow the width of the metal strip grating to be taken into consideration to derive the correct main beam angle, array factor pattern and grating limits. Previously, the grating width was neglected by the formulas and the resulting error was unacceptably high.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Cryogenic characterisation and modelling of commercial SiC MOSFETs

Two commercial 1.2 kV SiC MOSFETs have been extensively characterised from 30 to 320 K. The temperature dependence of their I/V characteristics, threshold voltage, and breakdown voltage has been examined and are presented in this paper. Overall, the measured characteristics of both devices demonstrate very similar temperature dependencies and it is shown that below ~100 K any further decrease in temperature has little effect on any of the tested characteristics. Increasing temperature beyond 100 K results in a decrease in drain current for a given drain-source and gate-source voltage, a decrease in threshold voltage, and an increase in breakdown voltage. Successful attempts have been made to model the results of these tests by applying theories found in the literature

Functional oxide as an extreme high-k dielectric towards 4H-SiC MOSFET incorporation

MOS Capacitors are demonstrated on 4H-SiC using an octahedral ABO3 ferroic thin-film as a dielectric prepared on several buffer layers. Five samples were prepared: ABO3 on SiC, ABO3 on SiC with a SiO2 buffer (10 nm and 40 nm) and ABO3 on SiC with an Al2O3 buffer (10nm and 40 nm). Depending on the buffer material the oxide forms in either the pyrochlore or perovskite phase. A better lattice match with the Al2O3 buffer yields a perovskite phase with internal switchable dipoles. Hysteresis polarization-voltage loops show an oxide capacitance of ~ 0.2 μF/cm2 in the accumulation region indicating a dielectric constant of ~120

Patient views and correlates of radiotherapy omission in a population‐based sample of older women with favorable‐prognosis breast cancer

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144597/1/cncr31378_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144597/2/cncr31378.pd

Cryogenic characterization of commercial SiC Power MOSFETs

The cryogenic performance of two commercially available SiC power MOSFETs are presented in this work. The devices are characterised in static and dynamic tests at 10 K intervals from 20-320 K. Static current-voltage characterisation indicates that at low temperatures threshold voltage, turn-on voltage, on-state resistance, transconductance, and the body diode turn-on voltage all increase while saturation current decreases. Dynamic, 60 V, 3A switching tests within the cryogenic chamber are also reported and the trends of switching speed, losses, and total power losses, which rise at low temperature, are presented. Overall, both MOSFETs are fully operable down to 30 K with both positive and negative changes in behaviour

On the application of novel high temperature oxidation processes to enhance the performance of high voltage silicon carbide PiN diodes

In this paper, the application of a combined high temperature (1550°C) thermal oxidation / annealing process has been applied to 4H-SiC PiN diodes with 110 μm thick n-type drift regions, for the purpose of increasing the carrier lifetime in the semiconductor. PiN diodes were fabricated on lifetime-enhanced 4H-SiC material, then were electrically characterised and compared against fabricated control sample PiN diodes. Forward current-voltage (I-V) measurements showed that the lifetime-enhanced devices typically had around 15% lower forward voltage drop and 40% lower differential on-resistance (at 100 A/cm2 and 25°C) when compared against control sample PiN diodes. Reverse I-V measurements indicated that the reverse leakage current was strongly dependent on the active area, and hence perimeter-to-area ratio, of the fabricated devices, though large-area PiN diodes were measured to have a reverse leakage current density of around 1 nA/cm2 (at 100 V reverse bias). Analysis of reverse recovery characteristics illustrated the excellent transient characteristics of both types of fabricated device, though, as expected from the increased carrier lifetime, the lifetime-enhanced PiN diodes had around 22% higher reverse recovery charge. The minority carrier lifetime was also extracted from reverse recovery characteristics; PiN diodes fabricated on the lifetime-enhanced 4H-SiC material were found to have a carrier lifetime over 35% higher than the control sample devices. Analysis of the overall power losses of both types of device found that the lifetime-enhanced PiN diodes typically dissipated around 40% less energy over the complete switching cycle than the control sample PiN diodes at 25°C

Heteroepitaxial Beta-Ga2O3 on 4H-SiC for an FET With Reduced Self Heating

A method to improve thermal management of β-Ga 2 O 3 FETs is demonstrated here via simulation of epitaxial growth on a 4H-SiC substrate. Using a recently published device as a model, the reduction achieved in self-heating allows the device to be driven at higher gate voltages and increases the overall performance. For the same operating parameters an 18% increase in peak drain current and 15% reduction in lattice temperature are observed. Device dimensions may be substantially reduced without detriment to performance and normally off operation may be achieved