Diode in MOSFET

[sonnytiger]

Taking this datasheet as an example, http://www.vishay.com/docs/91291/91291.pdf
what is the purpose of the diode in the symbol? do all MOSFETS have this diode?

[c4757p]

Yes, all MOSFETs have it. The substrate (body) forms a diode into the channel, and on discrete MOSFETs, it is connected to the source. You can't avoid having the body diode, though some applications for MOSFETs inside integrated circuits (e.g. the ones in analog switches) do tie it to something other than the source.

[mark-r]

It's part of the structure of the device, it doesn't do anything useful but its shown on the symbol as a reminder that it's there so your design must take account of it.

[c4757p]

Interestingly, that diode in the symbol is redundant anyway. A lot of people forget about this bit of the MOSFET symbol... The arrow represents, as it always does in semiconductor symbols, the junction formed between the body and channel.

[c4757p]

More on the arrow thing.

[codeboy2k]

More on the arrow thing.

I was waiting for someone to post that point, that many engineers don't realize..

The arrow in the diagrams ALWAYS points to the N doped silicon..

It's an easy way to remember all the symbols... and how to draw and N-channel FET vs a PFET, NPN, PNP, etc...

[sonnytiger]

Ok cool, so basically if a reverse voltage got on it it would let current flow like a diode, ie. like a short circuit which doesn't seem good. this wouldn't be a problem if arranged in an h-bridge would it?

[mark-r]

Splitting hairs but I guess it could be argued that the arrowhead represents the junction between the substrate and the n-type channel, while the diode symbol represents the junction between the substrate and the n+ land that forms the drain terminal. The separate diode symbol makes it explicit that this junction plays no part in the FET action and is just a by product of the construction.

[c4757p]

Ok cool, so basically if a reverse voltage got on it it would let current flow like a diode, ie. like a short circuit which doesn't seem good.

Right. There are a few applications for the diode. The attached circuit is a reverse polarity protection circuit with a P-channel MOSFET. The reverse diode allows a some current to flow at startup (with the inherent voltage drop of a diode), which raises V_GS, turning the MOSFET itself on and eliminating the voltage drop. Without the diode, it would never turn on. Unfortunately, that diode is usually just a pain in the ass.

this wouldn't be a problem if arranged in an h-bridge would it?

Nope.

[c4757p]

Splitting hairs but I guess it could be argued that the arrowhead represents the junction between the substrate and the n-type channel, while the diode symbol represents the junction between the substrate and the n+ land that forms the drain terminal.

Doesn't matter. The diode connects to the channel, not the drain. If you had a MOSFET with a body terminal, you could bias B-D, B-S or both to conduct current (or not to conduct current - that is the reason for my earlier example of analog switches not connecting body to source. They bias the body even lower than the source so that the MOSFET can block current in both directions.) The only reason the diode only conducts to the drain is because its anode is already tied to the source.

[sonnytiger]

OK so for basic switching uses it can be ignored?

[c4757p]

For basic switching it doesn't make a difference. Don't ignore it, though, or you'll miss that one time when it ruins your circuit. It's just a diode, it's not hard to imagine a diode across your MOSFET and consider per use whether it will have an effect.

[amspire]

The other thing about the mosfet diode and reverse polarities on the drain is that the MOSFET channel still works with reverse polarity on the drain.

So if there is a reverse polarity and you switch on the mosfet, the mosfet will conduct rather then the reverse diode turning on. This can be useful as the mosfet channel often has a much lower voltage drop then the reverse diode, and if you stop the diode from conducting, then there is no diode reverse recovery time needed when the drain voltages reverse again. Both of these steps can save a lot of power in a high speed switching circuit.

Many modern Buck converter IC's use a mosfet instead of a diode from the switching inductor to the negative rail and use it to switch on when the inductor swings negative below the negative supply rail. This results in more efficiency then the conventional high speed switching diode or Schottky diode.

Richard.

[sonnytiger]

thanks richard that is rather interesting, okay now that i have realized it can be imagined as a diode across the mosfet it wont be hard to consider its effect in a circuit as said so i will do this. thanks for the answers!