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2018.04.05 SiC Power Device

Reliability of SiC-MOSFETs

SiC-MOSFET

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This time, we explain the reliability of SiC-MOSFETs. The information and data presented here is for ROHM SiC-MOSFET products. Development of SiC power devices, including MOSFETs, is constantly progressing, and should readers have any questions or uncertainties, they are urged to ask related questions here.

Reliability of ROHM SiC-MOSFETs

Gate oxide film
At ROHM, formation of gate oxide films for SiC-MOSFETs has been accomplished with reliability comparable to that of Si-MOSFETs through process development and optimization of device structures.

As a result of CCS TDDB(Constant Current Stress Time Dependent Dielectric Breakdown)tests, the QBD (Charge to Breakdown), an index of the reliability of gate oxide film, was found to be 15 to 20 C/cm2, equivalent to that of Si-MOSFETs.

Moreover, in HTGB (High Temperature Gate Bias) tests conducted (at +22 V, 150°C) for the purpose of verifying the reliability of gate oxide films in relation to crystal defects, ROHM has confirmed 1000 operating hours without any failures and characteristic fluctuations.

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Stability of gate threshold voltage against positive gate voltage

Because electron traps are not absent at the interface of a gate oxide film and SiC body, if a positive DC bias is applied to the gate over a long period of time, the threshold value rises due to the capture of electrons by traps.

In HTGB tests, it has been confirmed that the threshold value shift is extremely small, 0.2 to 0.3 V, after 1000 operating hours at Vgs=+22 V and 150°C. Nearly all traps are filled during the several dozen hours of the initial stress application period, and so thereafter the device is stable, with almost no fluctuation.

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Stability of gate threshold voltage against negative gate voltage
When on the other hand a negative DC bias is applied to a gate for a long period of time, holes rather than electrons are trapped, and the threshold value falls. In HTGB test results, the amount of shift in the threshold value under a negative bias is larger than that for a positive bias, and for Vgs of -10 V or greater, the threshold value falls by 0.5 V or more.

In second-generation MOSFETs (the SCT2[xxx] series and SCH2[xxx] series), the guaranty voltage for a negative gate bias is stipulated as -6 V. For a negative bias larger than -6 V, there is the possibility that the threshold value will fall further, and so care must be taken. In the case of an AC (positive-negative) bias, charging and discharging of traps is repeated, and so it is thought that the shift effect is small.

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Body diode conduction degradation
SiC-MOSFETs are thought to have a fault mode called body diode conduction degradation. This is a mode in which, when a forward current continues to flow in the body diode of a MOSFET, faults known as stacking faults are expanded due to the recombination energy of electron-hole pairs, affecting the current path and resulting in increases in the on-resistance and the body diode Vf.

Stacking faults amplify heat generation, and in some cases can cause rated voltage degradation. Hence when used in applications in which commutating through the body diode occurs (inverters, DC/DC converters, and the like), there is the possibility of serious problems. (*Such problems do not occur with SBDs and the first-quadrant operation of MOSFETs)

By developing a proprietary process that prevents expansion of stacking faults, ROHM succeeded in securing reliability with respect to body diode conduction. Below are shown the results of conduction tests for 1000 hours of an 8 A DC current through the body diode of a second-generation 1200 V, 80 Ω SiC-MOSFET product. It was verified that there is no fluctuation in any of the characteristics, including the on-resistance and the leakage current.

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Short-circuit rating
Because of the small chip area and high current density of a SiC-MOSFET compared with a Si-MOSFET, the ability to withstand short-circuits that can cause thermal breakdown tends to be lower than for Si devices. In the case of a 1200 V class MOSFET in a TO247 package, the short-circuit withstand time at Vdd=700 V and Vgs=18 V is roughly 8 to 10 μs. As Vgs falls, the saturation current becomes smaller, so that the withstand time is longer. And, when Vdd is lower, there is less heat generated, so that the withstand time is longer.

Because the time required to turn off a SiC-MOSFET is extremely short, when the Vgs shutoff speed is fast, a steep dI/dt can result in a large surge voltage. A soft turnoff function to gradually lower the gate voltage or some other method should be used to achieve shutoff while avoiding overvoltage conditions.

dV/dt Breakdown
In a Si-MOSFET, there is a mode in which a high dV/dt causes a transient current to flow through the capacitance Cds and turn on the parasitic bipolar transistor, leading to device breakdown. Because ROHM SiC-MOSFETs have a low current amplification factor (hFE) of the parasitic bipolar transistor, current amplification is thought not to occur, and investigations up till now have found no evidence of this breakdown mode even in operation up to about 50 kV/μs. The recovery current of a SiC-MOSFET is extremely small, and dI/dt during recovery is low, so that the dV/dt of the body diode during recovery does not increase too much either, and therefore it is thought that this fault mode does not readily occur.

Cosmic ray neutron-induced single-event effects
In high-altitude applications, there are cases in which single-event effects of semiconductor devices, due to neutrons, heavy ions and other particles that are caused by cosmic rays that only rarely reach the earth, can cause problems. As a result of white neutron irradiation tests (energy: 1 to 400 MeV, conducted at the Research Center for Nuclear Physics, Osaka University (RCNP)) on SiC-MOSFETs (n=15), no failures were found to occur due to single-event effects at Vds=1200 V (100% of the rated breakdown voltage) under neutron irradiation of 1.45×109 neutrons/cm2/s. The failure rate at sea level is 0.92 FIT, and even at an altitude of 4000 m is 23.3 FIT, which is 3 to 4 orders of magnitude lower than that of equivalent Si IGBTs and Si-MOSFETs. With a high effective rated voltage and an ample margin, the risk of failure due to neutrons originating in cosmic rays can be reduced in uses at high altitudes and in numerous quantities.

Electrostatic discharge rating
A feature of SiC-MOSFETs is the ability to reduce chip sizes relative to Si-MOSFETs, but on the other hand, their ability to withstand electrostatic discharge (ESD) failure is limited. Hence adequate electrostatic countermeasures must be taken when handling these devices.

Examples of ESD protection measures

・Eliminate static electricity from human body, devices, and the work environment using ionizers
 (recommended)

・Eliminate static electricity from human body and work environment using wristbands and grounding (This
 measure is ineffective against static charge on devices, so this measure alone is insufficient)

Results of SiC-MOSFET reliability tests
Below are results of reliability tests according to JEITA (formerly EIAJ) ED-4701 standards, which are widely used in Japan as testing methods for semiconductor evaluation. From the results it is clear that ROHM SiC-MOSFETs offer high reliability.

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Key Points:

・The reliability of ROHM SiC-MOSFETs is equivalent to that of Si-MOSFETs currently in use.

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