AN2344

MOSFET fundamentals

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MOSFET fundamentals

Figure 1. shows a basic, simplified MOSFET structure. The actual MOSFET is an infinite

parallel of these 'microscopic ' structures that work together, sharing the same Drain with all

of the Gates which are connected together by a deposited polysilicon mesh, and all of the

Sources are linked by the top metal layer.

In this case, the ‘mesh’ is the ST-patented, high voltage Mesh Overlay™ technology, which

optimizes the body-drain junction shape as well as improves other aspects of the MOSFET

structure. However, the overall concept of this vertical structure can be considered valid for

various older technologies (e.g. cellular) as well.

During the ON state, while the gate source voltage is above the threshold, the conduction

current is localized in the drain and the region below the gate (channel). During the OFF

state, the voltage drop across the drain and source is sustained by the PN junction at

reverse bias, and a very small current (leakage) flows through the junction.

If the voltage increases too much and the electrical field reaches the critical value, the

junction goes into breakdown, and the current starts to flow through the body region. If an

overvoltage is applied to the junction, a current flows through it while the MOSFET limits the

actual drain-source breakdown voltage.

The breakdown mechanism itself is not destructive for a PN junction. However, overheating

caused by the high breakdown current and voltage damages the PN junction unless

sufficient heat sinking is provided.

Looking at the MOSFET structure, one can see that the PN junction is not a simple or

perfect diode. The MOSFET diode is the collector-base junction of a Bipolar Junction

front metal.

The capability of a MOSFET to withstand the avalanche condition takes into account these

concerns. In fact, two kinds of failure arise: one is related to current, and the other to power

dissipation. In the former, failure is caused by the latching of the parasitic bipolar due to the

current that flows through its base resistance, multiplied by the gain. The second is reached

when the temperature of the junction rises to a critical value that provokes the formation of

hot spots caused by regenerative thermal runaway, with average temperatures of about

650°C, that peak at approximately 1000°C, which then triggers extremely rapid device

destruction.

Figure 1.

MOSFET vertical structure and parasitic elements

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