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This is a newbie question on MOSFET structure. If I need a heat sink to be attached to MOSFET, it should be closer to the source of heat. Here channel is the source of heat. Hence heat sink should be either to drain or source.

The bulk area of the body (substrate) has nothing to do with heat dissipation. Is it correct?

My confusion arises because we say collector is made larger for better heat dissipation in BJT and in Power MOSFET drain is made larger for better thermal management.

Pic Ref

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    \$\begingroup\$ I would connect it to the big tab on the mosfet that's designed for you to connect a heatsink to. The datasheet will tell you which terminal this is electrically connected to. \$\endgroup\$ – immibis Jun 20 at 4:02
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    \$\begingroup\$ @immibis I think the OP is asking from the perspective if a silicon designer who designs transistors, not an end user. \$\endgroup\$ – DKNguyen Jun 20 at 4:24
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It really depends on what all the terminals are connected to.

You'd probably want to connect it to the body because it has the most surface area (or in this case drain). The body is usually the best thermal pathway as there are only small leads attached to the source and gate.

Here is a pic of a mosfet that I dencapsulated (all it takes is a bit of warm nitric acid be careful), they connected it to the drain with thermally conductive epoxy. Most of the heat will be dissipated in the small square and out through the copper base/drain.

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