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GE researchers develop 250degC+ SiC Transient Voltage Suppressor

Compact chip can replace multiple silicon TVS devices 

Transient voltage suppressors (TVS) are critical components for protecting sensitive electronics from lightning, EMI and other temporary over-voltage events that may occur within the system. However, COTS silicon TVS devices are limited in operation to temperatures of about 150degC. At higher temperatures, leakage currents become excessive and the surge current is highly de-rated.

To address this lack of performance at high temperatures, a team led by Avinash Kashyap at the General Electric Global Research Center at Niskayuna, NY has developed a punch-through physics SiC-based TVS device. This could serve aviation, down-hole applications and high performance power electronics.

Currently, the prevailing design technique for reaching the intended protection specifications using silicon TVS devices (when optimal combinations of breakdown voltage and power ratings may not be available) is to connect several parts in series and/or parallel. This takes up valuable board area in space-limited applications, while drastically reducing reliability due to a combination of: increased number of components; and non-ideal current sharing between the devices under short pulse durations, leading to premature failure.

Multiple devices also increase the cost, not only because of the obvious reasons of using more than one part, but also due to increased component up-screening requirements - as the devices have to be closely matched to one another, leading to lower yields. 

The punch-through physics based SiC TVS developed by Avinash Kashyap and his team, can replace multiple Si TVS devices with a single, smaller SiC device (shown in a hermetic package above) in certain applications.

The SiC device is capable of operating at 250degC and beyond, with minimal de-rating in the surge capability. Surge testing also demonstrated the device conducting current at over 10 kA/cm2, at elevated temperatures (225degC).

The high current density capability allows the SiC devices to be several times smaller than their silicon counterparts, lending themselves towards miniaturisation. Even at these high current densities, the resistance of the device is low, leading to low clamping voltages (about twice the breakdown voltage at 10 kA/cm2). Therefore, it is possible to afford a higher level of protection to downstream electronics because their de-rating can then be decreased, allowing for the maximum utilisation of the transistors.

Pictured above: Comparison of leakage currents of Si (blue) and SiC (red) TVS at 225degC. Note the several orders of magnitude lower leakage for the GE SiC device.

According to Kashyap, these new SiC TVS parts can also pass all the DO-160 surge requirements (up to waveform 5, level 5) for aviation electronics and can hence be used in composite aircrafts requiring enhanced lightning surge protection. Since a single device can be used for passing any DO-160 level, it eliminates the need to parallel several parts, and thereby increases the reliability of the overall system.

The smaller size has another unintended benefit - lower capacitance compared to equivalent silicon devices, thereby not loading communication buses or in some cases, obviating the need to connect pn-diodes in series.

The ruggedness of the SiC TVS devices were demonstrated thorough multiple DO-160 hits, back-to-back in one minute intervals with no apparent degradation in the electrical characteristics. Some of the devices were tested up to 50 consecutive hits with no failures. The die temperature increases to high values during surge events, but the unique material properties of SiC (~1.5 times the thermal conductivity of silicon and 17 times lower intrinsic carrier concentration) allows the device to operate without causing thermal runaway even under extreme ambient conditions (>200degC), which would simply not be possible in silicon.

A validation of these facts are the results from long term HTRB tests that have shown no degradation in the leakage current or breakdown voltage at 225degC after over 8000 hours of stress.

This is the first known wide bandgap transient suppressor device that has successfully undergone surge testing not just in terms of high current densities (which can be achieved through small devices), but also high absolute values of current (~3kA with an 8µs/20µs surge waveform), clearly establishing that the GE design can be scaled to large area die with no apparent current sharing shortcomings.

This work will be presented at the IEEE Workshop on Wide Bandgap Power Devices and Applications to be held at Knoxville, TN in Oct. 2014.


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