Power MOSFETs failure analysis

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Dr. Mizsei János
Department of Electron Devices

Power MOSFET acquire more and more role with spreading of electric drive systems. Next to it they are needed at the application of inverters’ bridge-arms like for example by connecting a solar power plant into the line power. Power MOSFET had also an important role in the replacement of switches with lower current relays or of mechanic switches, in this way they helped in the automation. In the case of power MOSFET, a vertical structure became widespread because with it a lot of transistors can be used in parallel on one chip in the way where the distance between the drain and source does not influence it. The consequence of the performance is one parallel diode can be implemented, too.

In some applications, there would be needed to reduce quickly the current of the inductive load. It can occur by controlling with motors furthermore, such a fast setting to the default stage would be useful in the case of a solenoid magnet valve control, too. In such cases changing of current in the switching off implemented with a normal flyback diode is too slow, there is a claim onto another solution. Because of this we use the body diode of MOS on its breakdown voltage, so the energy stored in the inductor will empty at a higher voltage than a normal operating one. At this higher voltage value, the duration time of switching off is shorter because semiconductor dissipates the same amount of energy at a higher voltage.

TrenchFET, examined in my thesis, were applied in the switching of solenoid magnet valves, where arising from use this quick switching off was needed, but then it resulted slowly the failing of FET with time. However, the planar FET did not fail during operating in the similar conditions. In my thesis, I analysed the inner structures of the two types then I hypothesized a possible mechanism for failure according to current paths appeared because of the avalanche breakdown and according to sources of the literature. I examined it with informative measurements.


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