How to recognize an original or counterfeit MOSFET transistor

Recently I bought two IRF3205 MOSFET transistors in one store. I also bought the same IRF3205 transistor in another store, but cheaper. And I had suspicions about the cheap transistor. Is it really a fake? But how can I tell if there is a good inscription on the case and it is impossible to determine a bad transistor by its appearance.

You can distinguish an original MOSFET from a fake by measuring its input capacitance. The IRF3205 datasheet indicates its input capacitance of 3700 pF on average. I touch the gate-source of the transistor with measuring tweezers and the capacitance is about 4300 pF. This is normal.

Now I measure the input capacitance of a fake transistor, which was cheaper. It is 1570 pF. About two and a half times less than the capacitance. What does this mean? And this means that the crystal area of ​​such a transistor will be less than the crystal of the original transistor. Such original transistors are formed using HEXFET technology, this is when thousands of field-effect transistors are connected in parallel in one transistor and therefore such a transistor has a large current and open channel resistance. The capacitance of the transistors will be as indicated in the datasheet. The counterfeit has a smaller capacitance, which means the crystal will be smaller, and this means that the transistor can withstand less current than the original. Put the counterfeit in the equipment and after a short period of works, replace the counterfeit transistor again, it will fail.

Broke the original and the fake transistors to see the crystal and measure it. The original has a semiconductor crystal area of ​​about 4.5 mm * 4 mm

You can see the fake crystal in the photo. Its dimensions are approximately 2 mm. Although this is also a MOSFET, it is clearly not for 110 Amperes but much smaller.

Conclusion. Cheap transistors can be fake and you need to check the input capacity, so we will know that the crystal area is normal or less. But there is another nuance, this concerns the purchase of high-voltage field-effect transistors. The crystal of the original and the fake can be approximately the same size and normal capacity, but the fake cannot withstand high voltage