Papers by Author: Brett A. Hull

Abstract: Basal plane dislocations (BPDs) introduced into SiC epitaxial layers, 25 μm thick, by the combination of implantation and activation anneal are directly observed by ultraviolet photoluminescence (UVPL) imaging. BPD loops appear to originate at micron-sized or smaller areas at the surface. These loops expand by gliding along the basal plane in the offcut direction until the loops approach the substrate. The loops can glide perpendicular to the offcut direction by many millimeters.

Abstract: Over the past decade, improvements in silicon carbide growth and materials has led to the development of commercialized unipolar devices such as Schottky diodes and MOSFETs, however, much work remains to realizing the goal of wide-scale commercialization of both unipolar and bipolar devices such as pin diodes or IGBTs, for high applications requiring high powers, operating in elevated temperatures or radiation environments or for many fast switching applications. Despite the great strides that have been made in reducing extended and point defect densities during this period, such defects still remain and with the push to lower off-cut angle substrates are in many cases seeing increases in prevalence. Thus, spectroscopic and imaging techniques for locating and identifying these defects are in high demand. Luminescence imaging and spectroscopy have both been utilized heavily in such work, yet simultaneously obtaining corresponding spectroscopic and spatial information from such defects is problematic. Here we report on hyperspectral imaging of electroluminescence from SiC pin diodes, whereby a stack of luminescence images are collected over a wide spectral range (400-900 nm), thereby providing the ability to both image distinct features and identify their corresponding spectral properties. This process is also equally applicable to collecting either photo- or electroluminescence from other materials or devices emitting in either the UV-Vis or NIR spectral range, as well as to reflectance, transmission or other imaging techniques.

Abstract: DMOSFETs fabricated in 4H-SiC with capabilities for blocking in excess of 1700V and conducting 20A continuous current in the on-state are presented. These SiC DMOSFETs remain functional to temperatures in excess of 225°C, with leakage current at 1700V at 225°C of less than 5 A with VGS = 0V. The DMOSFETs show excellent switching characteristics, with total switching energy of 1.8 to 1.95 mJ over the entire temperature range of testing (25°C to 200°C), when switched from the blocking state at 1200V to conducting at 20A in a clamped inductive load switching circuit. The electrical characteristics are compared to commercially available Si IGBTs rated to 1700V with similar current ratings as the SiC DMOSFET described herein.

Abstract: In this paper, we review the performance, reliability, and robustness of the current 4H-SiC power DMOSFETs. Due to advances in device and materials technology, high power, large area 4H-SiC power DMOSFETs (1200 V, 67 A and 3000 V, 30 A) can be fabricated with reasonable yields. The availability of large area devices has enabled the demonstration of the first MW class, all SiC power modules. Evaluations of 1200 V 4H-SiC DMOSFETs showed that the devices offer avalanche power exceeding those of commercially available silicon power MOSFETs, and have the sufficient short circuit robustness required in most motor drive applications. A recent TDDB study showed that the gate oxides in 4H-SiC MOSFETs have good reliability, with a 100-year lifetime at 375oC if Eox is limited to 3.9 MV/cm. Future work on MOS reliability should be focused on Vth shifts, instead of catastrophic failures of gate oxides.

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.

Abstract: Low-frequency noise in 4H-SiC MOSFETs has been measured for the first time. At drain currents varying from deep subthreshold to strong inversion, the 1/f (flicker) noise dominated at frequencies 1 - 105 Hz. The dependence of relative spectral noise density, , on drain current Id (at a constant drain voltage Vd) differs qualitatively from that in Si MOSFETs. In Si MOSFETs, ~ 1/ in strong inversion, whereas tends to saturate in sub-threshold. In 4H-SiC MOSFETs under study, ~ 1/ over the whole range of currents from deep sub-threshold to strong inversion. Similar noise behavior is often observed in poly- or a-Si TFTs. The effective channel mobility in 4H-SiC MOSFETs, 3 - 7 cm2/Vs, is also as low as that in TFTs. Both noise behavior and transport properties of 4H-SiC MOSFETs are explained, analogously to TFTs, by a high density of localized states (bulk and interface) near the conduction band edge in the ion implanted p-well.

Abstract: Large area (8 mm x 7 mm) 1200 V 4H-SiC DMOSFETs with a specific on-resistance as low as 9 m•cm2 (at VGS = 20 V) able to conduct 60 A at a power dissipation of 200 W/cm2 are presented. On-resistance is fairly stable with temperature, increasing from 11.5 m•cm2 (at VGS = 15 V) at 25°C to 14 m•cm2 at 150°C. The DMOSFETs exhibit avalanche breakdown at 1600 V with the gate shorted to the source, although sub-breakdown leakage currents up to 50 A are observed at 1200 V and 200°C due to the threshold voltage lowering with temperature. When switched with a clamped inductive load circuit from 65 A conducting to 750 V blocking, the turn-on and turn-off energies at 150°C were less than 4.5 mJ.

Abstract: In this paper, large area (0.18cm2) SiC DMOSFETs with 1.2 kV and 20 A rating are evaluated for power electronic switching applications. A drain-to-source voltage drop VDS of 2 V at a forward drain current of 20 A (JD = 110 A/cm2) was obtained and a specific on-resistance of 18 mΩ-cm2 was extracted at room temperature. The device on-resistance was measured up to 150°C and initially decreases with increasing temperature, but remains relatively flat over the entire temperature range, demonstrating stable device behavior. High voltage blocking of 1.2 kV between 25°C and 150°C is also demonstrated with a gate-to-source voltage VGS = 0 V. The drain leakage current under reverse bias and high temperature stress is shown to increase from 10 μA at 25°C to 27 μA at 150°C while maintaining the full blocking rating of the device. Experimental results from double-pulse clamped inductive load tests are presented demonstrating fast high voltage and high current switching capability. High voltage resistive-switching measurements on parallel connected SiC DMOSFETs were performed with VDS having rise and fall times of 49 and 74 ns respectively. Thermal camera images taken of parallel connected DMOSFET die during repetitive switching operation with VDS = 420 V, IDS = 25 A and a 40% duty cycle shows a 2°C difference in die temperature, which suggests even current sharing and temperature stable device operation.

Abstract: Self-heating in high-voltage 4H-SiC PiN diodes has been studied experimentally and theoretically in dc and 8-ms single pulse modes. To simulate the self-heating, an electro-thermal model was used to calculate non-isothermal current-voltage characteristics at dc and current-time dependences at pulsed measurements. The dynamic instability of N-type was observed: the current decreases in spite of increasing of bias applied to the structure. At dc, irreversible diode degradation was found to occur at a current density of about 1700 A/cm2. Under a single current surge 8-ms pulse, the loss of thermal stability has been found at a current density of approximately 9000 A/cm2. Comparison of experimental data and simulations showed that the local temperature in the diode base at the end of the 8-ms, 9000-A/cm2 pulse reaches 2000 – 2300 K.

Abstract: DC characteristics and reverse recovery performance of 4H-SiC Junction Barrier Schottky (JBS) diodes capable of blocking in excess of 10 kV with forward conduction of 20 A at a forward voltage of less than 4 V are described. Performance comparisons are made to a similarly rated 10 kV 4H-SiC PiN diode. The JBS diodes show a significant improvement in reverse recovery stored charge as compared to PiN diodes, showing half of the stored charge at 25°C and a quarter of the stored charge at 125°C when switched to 3 kV blocking. These large area JBS diodes were also employed to demonstrate the tremendous advances that have recently been made in 4H-SiC substrate quality.