D.C. Sheridan

University of South Carolina, Columbia, South Carolina, United States

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Publications (34)15.13 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: 1200V SiC vertical trench JFETs have been evaluated for their reverse conduction properties. Absent of a traditional body diode, the SiC trench JFET is shown to be able to operate effectively in reverse mode when used with or without an antiparallel diode in applications requiring reverse commutation. Device characteristics and experimental results are given for both traditional half-bridge and cascode topologies.
    Power Semiconductor Devices and ICs (ISPSD), 2012 24th International Symposium on; 01/2012
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    ABSTRACT: A novel SiC junction field-effect transistor (JFET) model that uses a unified description of linear and saturated conduction modes is proposed. Advantages of the proposed model are improved robustness and convergence, inclusion of field-dependent mobility effects, and more physical description of the current saturation phenomenon. The model is validated against a normally off JFET sample over a wide temperature range. Finite-element simulations are used to demonstrate the physics-based nature of the proposed model.
    IEEE Transactions on Industry Applications 09/2011; · 1.67 Impact Factor
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    ABSTRACT: A practical parameter extraction procedure for a power silicon carbide (SiC) junction field-effect transistor (JFET) is presented. The carrier mobility and carrier concentration are very important parameters, strongly affecting the device current capability and dynamic characteristics for a given design. When modeling JFETs, the values of these parameters are usually based on assumptions and given by a vendor in a range. As a result, model accuracy is compromised. In this paper, a step-by-step parameter extraction procedure is described that includes the extraction of mobility and carrier concentration in the channel and drift regions based on knowledge of device geometrical parameters. For the first time, carrier mobilities in the channel and drift regions of a power JFET are extracted individually. It is found that channel and drift region mobilities can be very different for a given device since they are strongly dependent on the fabrication process. The separate extraction of these two mobilities can also improve model accuracy in the case of imperfect knowledge of the device geometry. The developed procedure includes the extraction of empirical parameters describing the temperature dependence of mobilities in the channel and drift regions. A simple static I - V characterization and C - V measurements are the only measurements required for the parameter extraction. In this paper, the procedure is experimentally validated for both normally off (enhancement mode) and normally on (depletion mode) JFETs.
    IEEE Transactions on Industry Applications 09/2011; · 1.67 Impact Factor
  • MRS Online Proceeding Library 01/2011; 423.
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    ABSTRACT: In this paper, a physical model for a SiC Junction Field Effect Transistor (JFET) is presented. The novel feature of the model is that the mobility dependence on both tempera-ture and electric field is taken into account. This is particularly important for high-current power devices where the maximum conduction current is limited by drift velocity saturation in the channel. The model equations are described in detail, emphasizing the differences introduced by the field-dependent mobility model. The model is then implemented in Pspice. Both static and dynamic simulation results are given. The results are validated with ex-perimental results under static conditions and under resistive and inductive switching conditions. Index Terms—Field-dependent mobility, junction field effect transistor (JFET), physics-based model, silicon carbide (SiC).
    IEEE Transactions on Industry Applications 01/2011; 47. · 2.05 Impact Factor
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    ABSTRACT: The unique wide bandgap properties of SiC allow the creation of high performance normally-off vertical JFET power device. Due to the vertical nature of the device, it can easily be designed with blocking capability exceeding 2kV with R<sub>DS(ON), sp</sub> > 3mΩ-cm<sup>2</sup>, resulting in the lowest specific on-resistance for enhancement mode SiC devices with V<sub>BR</sub> <; 1200V. The low R<sub>DS(ON), sp</sub> yields a small die size that translates into switching losses that are 5-10X smaller than similarly rated Si IGBTs. When used as an IGBT replacement, a significant reduction in losses can be achieved, greatly increasing the overall system efficiency of solar inverters and other renewable power systems.
    Power Electronics Conference (IPEC), 2010 International; 07/2010
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    ABSTRACT: Excellent performance and record inverter efficiency have been reported for 1200 V normally-off silicon carbide (SiC) vertical channel junction field effect transistors (VJFETs) with uniform channel doping. Optimally designed normally-off SiC VJFETs typically have a threshold voltage of approximately +1 V and pentode-like output characteristics with clear saturation. Some applications require significant surge or pulsed current capability thus it is desirable to increase saturation current density while maintaining normally-off operation. This paper reports the use of non-uniform channel doping in a normally-off SiC power VJFET to achieve a 28-48% increase in saturation current and 13% decrease in on-resistance compared to the uniform channel case. This results in a specific on-resistance of 2.5 mΩ·cm<sup>2</sup> and saturation current density of 1275 A·cm<sup>-2</sup> at 25°C. Forward drain leakage at V<sub>gs</sub>= 0 V and V<sub>ds</sub>= 1100 V (measurement setup limit) is very similar for both channel doping profiles and remains less than 55 uA (1.75 mA·cm<sup>-2</sup>) at 150°C for the non-uniform channel SiC VJFET. The total switching energy for non-uniform channel devices was 194 μJ compared to 190 μJ for uniform channel devices.
    Power Semiconductor Devices & IC's (ISPSD), 2010 22nd International Symposium on; 07/2010
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    ABSTRACT: Normally-OFF SiC VJFETs have been proved to be advantageous as a ¿drop-in¿ replacement of MOSFETs and IGBTs in a variety of applications. As this device's acceptance continues to grow, developers are investigating optimized driver methods that will yield the best possible switching performance leading to higher system efficiencies. This paper presents new results for an alternative and more optimized gate driver to the capacitive coupled driver used in past literature. Additionally switching energy measurements are documented for the 50 mOhm enhancement-mode SiC VJFET in the newly optimized two-stage, DC-coupled gate driver and compared against past results obtained using the initial driver design. Specific design guidelines are included for achieving the best possible results using the two stage gate driver design presented here.
    Applied Power Electronics Conference and Exposition (APEC), 2010 Twenty-Fifth Annual IEEE; 03/2010
  • Materials Science Forum - MATER SCI FORUM. 01/2010;
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    ABSTRACT: A novel SiC junction field effect transistor (JFET) model is proposed that uses a unified description of linear and saturated conduction modes. Advantages of the proposed model are improved robustness and convergence, inclusion of field-dependent mobility effects, and more physical description of the current saturation phenomenon. The model is validated against a normally-off JFET sample over a wide temperature range. Finite element simulation are used to demonstrate the physics-based nature of the proposed model.
    Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE; 10/2009
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    ABSTRACT: A practical parameter extraction procedure for power junction field effect transistor (JFET) is presented. The carrier mobility and carrier concentrations are very important parameters, strongly affecting the current capability and dynamic characteristics of the device for a given design. When modeling JFETs, values of these parameters usually are based on assumptions and given by a vendor in a range. As a result, model accuracy is compromised. In this paper, a step-by-step parameter extraction procedure is described that includes extraction of the mobility and the carrier concentration in the channel and drift regions based on knowledge of the device geometrical parameters. For the first time, carrier mobility in channel and drift regions of power JFET are extracted individually. It is found that channel and drift region mobilities can be very different for a given device, since they are strongly fabrication-process dependent. The developed procedure includes extraction of parameters for proposed empirical temperature dependencies of mobilities in the channel and drift regions. A simple static I-V characterization and C-V measurements are the only measurements required for the parameter extraction.
    Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE; 10/2009
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    ABSTRACT: Twenty amp normally-off enhancement mode 4H-SiC VJFETs are demonstrated with 1.9 kV avalanche breakdown voltage and a specific on-resistance of 2.8 mOmega-cm<sup>2</sup>. The VJFETs shown near ideal subthreshold characteristics and maintain enhancement mode functionality to temperatures exceeding 175degC due to the optimized channel design with low DIBL characteristics. The low specific on-resistance enables the VJFET to have low intrinsic capacitances that result in low total switching times of less than 150 ns at 15 A at a Tj = 175degC, and low total switching energies of 97 muJ when switching 12 A at Tj = 25degC. Short circuit performance was also investigated with the VJFET exhibiting a rugged short circuit withstand capability in excess of 700 mus at a V<sub>DS</sub> = 600 V.
    Power Semiconductor Devices & IC's, 2009. ISPSD 2009. 21st International Symposium on; 07/2009
  • Materials Science Forum - MATER SCI FORUM. 01/2009;
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    Materials Science Forum - MATER SCI FORUM. 01/2009;
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    ABSTRACT: In this work a physical model for a SiC Junction Field Effect Transistor (JFET) is presented. The novel feature of the model is that the mobility dependence on both temperature and electric field is taken into account. This is particularly important for high-current power devices, where the maximum conduction current is limited by drift velocity saturation in the channel. The model equations are described in detail, emphasizing the differences introduced by the field-dependent mobility model. The model is then implemented in Pspice. Both static and dynamic simulation results are given. The results are validated with experimental results under static conditions and under resistive switching conditions.
    Industry Applications Society Annual Meeting, 2008. IAS '08. IEEE; 11/2008
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    ABSTRACT: The conventional wisdom that the SiC JFET is a normally on device has recently been superseded by the first practical normally off SiC JFET. The new true enhancement mode, three-terminal, pure-SiC design provides designers with a normally off solution that retains all the benefits of the normally on SiC JFET. With a simple change in the series gate impedance, the EM SiC JFET can be used with common IC drivers and is a drop-in replacement for current power devices in most applications. Device characteristics are superior to MOSFETs and IGBTs and offer the possibility of efficiency improvements from reduced conduction and switching losses. The work presented in this paper demonstrates the drop-in replacement of an IGBT with a normally off SiC JFET in a PFC demo circuit. System efficiency using each device was observed and compared. An improvement was noted with the JFET as expected.
    Power Electronics Specialists Conference, 2008. PESC 2008. IEEE; 07/2008
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    ABSTRACT: This paper reports on the development of a normally- off 4H-SiC VJFET power switch technology suitable for drop in replacement in switching-mode power supplies (SMPS). The fabricated devices exhibited a low specific on-resistance (Ron-sp) measured at V<sub>DS</sub>=1 V and V<sub>Gs</sub>=2-5 V. The transistors designed for 800 V applications had R<sub>ON-SP</sub> < 2.9 mOmegaldrcm<sup>2</sup> and R<sub>ON-SP</sub> < 6.6 mOmegaldrcm<sup>2</sup> at 25degC and 200degC, respectively. The devices designed for 1200 V application had R<sub>ON-SP</sub> < 4.3 mOmegaldrcm<sup>2</sup> at 25degC and R<sub>ON-SP</sub> < 12.8 mOmegaldrcm<sup>2</sup> at 200degC. The total delay time of 73 ns was measured on a 1200 V device when switching from 600 V to 4.9 A with the gate bias ranging from 0 V to 2.75 V. The highest measured off-state drain voltage blocked by a 1200 V device at V<sub>GS</sub>=0 V exceeded 1800 V with the total drain leakage of 1 mA.
    Power Semiconductor Devices and IC's, 2008. ISPSD '08. 20th International Symposium on; 06/2008
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    ABSTRACT: Proton irradiation is used to probe the physics of 4H-silicon carbide (SiC) Schottky barrier diodes (SBDs) and negative channel metal oxide semiconductor (nMOS) capacitors for the first time. Both 4H-SiC SBD diodes and SiC MOS structures show excellent radiation tolerance under high-energy, high-dose proton exposure. Unlike for SiC JBS diodes, which show a strong increase in series resistance after proton irradiation, these SiC SBDs show very little forward bias I--V degradation after exposure to 63.3 MeV protons up to a fluence of 5×10<sup>13</sup> p/cm<sup>2</sup>. An improvement in reverse leakage current after irradiation is also observed, which could be due to a proton annealing effect. The small but observable increase in blocking voltage for these SiC SBDs is attributed to a negative surface charge increase, consistent with earlier gamma results. The resultant Q<sub>eff</sub> change of 4H-SiC nMOS capacitors under proton irradiation was used to quantify the radiation induced changes to the blocking voltage in the SBD diodes in MEDICI simulations, and showed a good agreement with the experimental data. Characterization of these capacitors also suggests that 4H-SiC MOS structures are radiation hard.
    IEEE Transactions on Nuclear Science 01/2005; · 1.22 Impact Factor
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    ABSTRACT: The effects of proton irradiation on the static ( dc) and dynamic (switching) performance of high-voltage 4H-SiC Junction Barrier Schottky (JBS) diodes are investigated for the first time. In contrast to that observed on a high-voltage Si p - i - n diode control device, these SiC JBS devices show an increase (degradation) in series resistance (R<sub>S</sub>), a decrease (improvement) of reverse leakage current, and increase (improvement) in blocking voltage after high-fluence proton exposure. Measured breakdown voltages of post-irradiated SiC diodes increase on average by about 200 V after irradiation. Dynamic reverse recovery transient measurements show good agreement between the various dc observations regarding differences between high-power SiC and Si diodes, and show that SiC JBS diodes are very effective in minimizing switching losses for high-power applications, even under high levels of radiation exposure.
    IEEE Transactions on Nuclear Science 01/2004; · 1.22 Impact Factor
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    ABSTRACT: The effects of radiation on high-voltage 4H-SiC Schottky diodes for use in extreme environment power supplies are investigated for the first time. Diodes were fabricated with a wide range of barrier heights (1.06-1.60 eV) and were evaluated after being exposed to a <sup>60</sup>Co gamma radiation source. DC current-voltage characteristics were measured after several radiation doses, with no observable degradation in the diode forward or reverse characteristics up to a total dose of 4 Mrad(Si). Measured breakdown voltages of post-irradiated diodes increase approximately 200 V compared to the virgin devices and are attributed to increased negative interface charge, as determined by MOS capacitor measurements and correlated with numerical breakdown simulations
    IEEE Transactions on Nuclear Science 01/2002; · 1.22 Impact Factor