Papers by Author: Robin L. Kelley

Abstract: Equivalent sized (4.5 mm2 die area), 1200 V, 4H-SiC, vertical trench Junction Field Effect Transistors (JFETs) were characterized in terms of DC and switching performance. The 100 mΩ Enhancement-Mode (EM) JFET was found to have natural advantages in safe operation being normally-off, whereas the Depletion-Mode (DM) JFET was found to have advantages with ~ twice as high saturation current, less on-resistance (85 mΩ) and no gate current required in the on-state. The JFETs were found to both have radically less (five to ten times) switching energies than corresponding 1200 V Si transistors, with the DM JFET and EM JFET having EON and EOFF of only 115 µJ and 173 µJ, respectively when tested at half-rated voltage (600 V) and 12 A.

Abstract: This work presents the progress in developing an all SiC based power module for use in high frequency and high efficiency applications. Using parallel combinations of 1200V enhancement mode SiC VJFETs (36mm2) and Schottky diodes (23mm2), a total on-resistance of only 10mOhm (2.7m-cm2) was achieved at ID=100A in a commercially available standard module configured as a half-bridge circuit. Careful attention to module layout, gate driver design, and the addition of optimized snubbers resulted in excellent switching waveforms with low total switching losses of 1.25mJ when switching 100A at 150oC.

Abstract: Prototype 800 V, 47 A enhancement-mode SiC VJFETs have been developed for high temperature operation (250 °C). With an active area of 23 mm2 and target threshold voltage of +1.25 V, these devices exhibited a 28 m room temperature on-resistance and excellent blocking characteristics at elevated temperature. With improved device packaging, on-resistance and saturation current values of 15 m and 100 A, respectively, are achievable.

Abstract: The purpose of this paper is to present an all-SiC switched AC-DC converter using active power factor correction. The typical boost-converter approach is employed using continuous conduction mode. A SiC Schottky barrier diode performs the free-wheeling diode function, and a 600 V, 0.12 % SiC vertical junction field effect transistor performs the switching function under the control of a Fairchild ML4821 integrated circuit. The converter is operable off-line over the full universal voltage range (85-260 VAC), but it was optimized for a 400-600 W application operating at 208 VAC. Results are presented that demonstrate extremely high efficiency at a switching frequency of 500 kHz, the highest operating frequency of the ML4821.

Abstract: The power junction field effect transistor (JFET) is the second most mature SiC device, after the SiC Schottky diode, and is commonly associated with normally on functionality; but this feature is often viewed problematically for off-line dc-to-dc converter applications. Two inherently safe, single-switch dc-dc converter designs have been developed that put into practice pure SiC JFET devices (i.e., without cascoded devices) that possess enhancement-mode functionality and bias-enhanced blocking. These ‘Quasi-Off’ devices are designed to block half of the rated blocking voltage at zero gate bias and achieve full rated blocking voltage with a modest negative bias, typically between 0 and -5 V. Inherent safety is provided by utilizing the enhancement mode functionality of these devices as well as appropriate gate driver design. Bias enhanced blocking matches the dynamic stress encountered by modern high-frequency power supply topologies to the ratings of the device while recognizing that the larger dynamic stress is typically encountered only when the power supply (and especially the gate driver) is functioning properly.

Abstract: Wide bandgap semiconductor materials such as SiC or GaN are very attractive for use in high-power, high-temperature, and/or radiation resistant electronics. Monolithic or hybrid integration of a power transistor and control circuitry in a single or multi-chip wide bandgap power semiconductor module is highly desirable for such applications in order to improve the efficiency and reliability. This paper describes a new monolithic SiC JFET IC technology for high-temperature smart power applications that allows for on-chip integration of control circuitry and normally-off power switch. In order to demonstrate the feasibility of this technology, hybrid logic gates with maximum switching frequency > 20 MHz and normally-off 900 V power switch have been fabricated on alumina substrates using discrete enhanced and depletion mode vertical trench JFETs.

Abstract: In this work we have demonstrated the operation of 600-V class 4H-SiC vertical-channel junction field-effect transistors (VJFETs) with 6.6-ns rise time, 7.6-ns fall time, 4.8-ns turn-on and 5.4-ns turn-off delay time at 2.5 A drain current (IDS), which corresponds to a maximum switching frequency of 41 MHz – the fastest ever reported switching of SiC JFETs to our knowledge. At IDS of 12 A, a 19.1 MHz maximum switching frequency has been also achieved. Specific on-resistance (Rsp-on) in the linear region is 2.5 m
·cm2 at VGS of 3 V. The drain current density is greater than 1410 A/cm2 at 9 V drain voltage. High-temperature operation of the 4H-SiC VJFETs has also been investigated at temperatures from 25 °C to 225 °C. Changes in the on-resistance with temperature are in the range of 0.90~1.33%/°C at zero gate bias and IDS of 50 mA. The threshold voltage becomes more negative with a negative shift of 0.096~0.105%/°C with increasing temperature.