The huge demand for switching components exceeding silicon's (Si) current density limitation of 200 A/cm2 has pushed the enhancement of alternative semiconductor materials such as silicon carbide (SiC), gallium nitride, and diamond. The enhanced material properties of SiC, such as high thermal conductivity, large critical field, wide bandgap, large elastic modulus, and high saturation velocity, make it a viable candidate for pulsed power systems. Using SiC would increase both current and power densities, improve dI/dt and dV/dt capabilities, reduce recovery time, and minimize switching losses in various power electronic systems. Furthermore, a significant reduction in the volume and weight of pulsed power systems can be realized by implementing SiC SGTOs, reducing the thermal management requirements of the pulsed power system.

A cross-sectional view of the 1.0 cm2 SiC SGTO.

To achieve the highest performance capability with SiC pulsed power devices, new packaging schemes must be explored. Wide-bandgap devices offer high switching speed and high-temperature operation capabilities. However, the advanced capabilities of these switches are a challenge to demonstrate due to the limitations of the present power packages being used to accommodate these devices.

A silicon carbide super gate turn off (SGTO) thyristor device is a superior switching component for pulsed power applications. This device was fabricated using five epitaxial layers grown on 4° off-axis 4H-SiC in a single run to achieve a high turn-on gain. The bottom n+-type and p+-type buffer layers are a few microns thick in order to form a punch-through structure to reduce the emitter efficiency of the npn transistor portion of the GTO for easier turn-off of the device. A multi-zone junction termination edge (JTE) and a drift region thickness of 90 μm permits the device to hold off voltages greater than 9 kV.

This device has an asymmetric pnpn structure, and was fabricated on a thick (350 μm), highly doped n-type SiC substrate, which results in a much higher conductivity compared to highly doped p-type substrates in SiC (Figure 1). The anode layer was etched using reactive ion etch, and then the mesa of the device was etched for total isolation.

The SGTO cell-based design has identical lengths and widths for uniform gate and anode current distribution. Each cell-based design uses interdigitated gate and anode-based fingers. The SGTO offers several advantages over standard GTOs including a much larger cell density, which improves dI/dt capability and turn-off capability while maintaining a comparable forward voltage; a smaller net base charge, which improves turn-on time and turn-off delay; and a much better thermal capability due to a finer cell structure, which minimizes current crowding.

Computer-aided design layout of the terminal modifications of the improved package.

The redesign of the high-voltage (HV) package focused on improvements to the terminals; however, one additional improvement was the application of room-temperature vulcanization (RTV) silicone sealant in between the plastic case and the base plate where they join together to prevent the uncured encapsulant from leaking out of the cavity before it fully cured. Without this seal, air leaks into the cavity from the bottom of the package, and air bubbles form inside the cavity.

The superior switching and peak current handling capability of the SiC SGTO makes it a viable candidate for pulsed power applications. This research demonstrates the successful implementation of an optimized, fluxless, solder-based power package using a SiC GTO for extreme pulsed power switching conditions. The SiC GTO was pulsed up to 2.14 kA with a 1-ms pulse width. The alternative solder bond packaging approach process was implemented on one device. Future work will include the pulsed power performance and reliability of solder bond packaging on additional HV pulsed power devices greater than 10 kV.

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Tech Briefs Magazine

This article first appeared in the June, 2017 issue of Tech Briefs Magazine.

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