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I've just read an application note and I was confused about this sentence: "Engineers often think of a MOSFET as a single power transistor, but it is a collection of thousands of tiny power FET cells connected in parallel."

How is this possible ? In every class I learned about the cross section of a MOSFET as a single bulk not as "a collection of thousands power FET cells".

So the question is: Is the application note refering to a special type of MOS or all of my life was a lie ?

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    \$\begingroup\$ A discrete MOSFET that you buy from digi-key or mouser is going to be thousands of parallel FETs--each one of which is represented by that cross section you learned about in class. \$\endgroup\$ – Hearth Jan 27 at 22:02
  • \$\begingroup\$ Most discrete power MOSFETs are actually VDMos devices versus planar devices, which are slightly different \$\endgroup\$ – sstobbe Jan 27 at 22:38
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    \$\begingroup\$ It definitely looks to me like the application note already answered your question "engineers often think of ... as" probably also implies "engineers are often taught of ... as". \$\endgroup\$ – Jasper Jan 28 at 2:21
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If a very large MOSFET (i.e. with a very wide channel) was implemented as a single physical device, like the one you saw in class, then the gate electrode would be very long and thin. This would cause a significant RC delay down the gate and so the MOSFET would turn on and off very slowly. Furthermore, it would be difficult to put such a device in a package because it would be hundreds or thousands of times wider than it was long.

So, it is electrically superior and easier to handle the MOSFET if you break it up into many small MOSFETs. The source, drain, and gate terminals of all of these small devices are connected in parallel. The result is the same as if you had built one huge device.

In CMOS VLSI design these small devices are often called the "fingers" and are actually drawn as parallel structures. Alternate fingers can then share their source/drain regions. Power MOSFETs use other techniques for forming the individual small devices.

Here's an example from the design of digital-to-analog converter: enter image description here Source: pubweb.eng.utah.edu

The yellow layer is polysilicon, and the long vertical stripes are MOSFET gates. The red layer is metal, and the white squares are contacts from the metal down to either the poly gates or source/drain regions. In the top right you see a large PMOS transistor with five parallel gate fingers. In between the gate fingers are the source and drain regions, looks like three parallel sources and three parallel drains. Sharing the source/drain regions like this also reduces the capacitance of those structures to the substrate (N-well) underneath. The linked page has several examples of how this is used in the design of analog CMOS. My experience was mainly in digital devices, but we used the same idea when we needed a high-drive buffer for a global clock or an I/O pin.

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  • \$\begingroup\$ Does the BJT transistor package have the same internal structure ? \$\endgroup\$ – pantarhei Jan 27 at 23:51
  • \$\begingroup\$ Sorry, I don't have experience in BJT design. \$\endgroup\$ – Elliot Alderson Jan 28 at 0:40
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    \$\begingroup\$ Putting in some dimensions, as a reference for those of us who are just lurking around? How huge is a huge device? How small is a small MOSFET? :-) \$\endgroup\$ – motoDrizzt Jan 28 at 11:10
  • \$\begingroup\$ Using 20-year old technology, a 0.25u MOSFET can fit inside a square micron: drain/gate/source/welltie. \$\endgroup\$ – analogsystemsrf Jan 29 at 3:38
  • \$\begingroup\$ @motoDrizzt Big and small are relative and there's no hard and fast rule, but I would guess that if W/L gets above 25 you might think about splitting the device. See the photo example I added. \$\endgroup\$ – Elliot Alderson Jan 29 at 12:50
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I guess this sentence is a reference to the structure of power MOSFETs, like International Rectifier’s HEXFET structure.

See for example http://www.rfwireless-world.com/Terminology/HEXFET-vs-MOSFET.html for more about the HEXFET structure.

EDIT: HEXFET is only one specific design by one specific manufacturer. Other manufacturers certainly have equivalent designs for their power MOSFETs.

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    \$\begingroup\$ @Hearth IMHO this is not spam and a spam flag would be inappropriate here--HEXFET can very well be a representative example of a power MOSFET structure. It points out the example fairly neutrally and points to a third-party source that discusses the structure and properties of this particular technology (as opposed to simply advertising it). That said, this answer can benefit from inclusion of relevant portions of the article (such as the structure diagram or a description thereof) to avoid being an effectively link-only answer. \$\endgroup\$ – Andrey Akhmetov Jan 27 at 22:10
  • \$\begingroup\$ @AndreyAkhmetov I’d rather not copy-paste portions of that article without permission from its author. But i’d be happy to up-vote an answer that’s more comprehensive than mine (and even delete mine). \$\endgroup\$ – user2233709 Jan 27 at 22:25
  • \$\begingroup\$ @Hearth I just don’t know better. I just guessed other manufacturers used similar structures (but I have no idea how much similar). \$\endgroup\$ – user2233709 Jan 27 at 22:28
  • \$\begingroup\$ Alright. Sorry to mistrust you, then! I suppose I was a bit hasty in thinking this could be spam; Andrey is right that it is an example. \$\endgroup\$ – Hearth Jan 29 at 12:51

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