[SOLVED] Source and bulk short

Hello everyone

I just wanted to get the concept of the shorting source and bulk terminals clear. Ive come across a couple of reasons, which are:

1. By keeping Source and Bulk at same terminal you prevent body effect.

2. To keep charges from leaking into the bulk and make sure they flow through the channel.

Which of these two reasons are valid (or if there are any other reasons) for shorting source and bulk?

Also I wanted to know what is the need for keeping a tap in the design?

Thanks

Sure, those reasons are valid. When you have a common-source diffpair, shorting the body to the source reduces body effect (and therefore your circuit still works when its power supply is lower; this also improves common-mode range). Charges leaking into the bulk is often called "hot carrier injection," and can be very undesirable. Tying the source to the body can largely avoid this.

If you tie the MOSFET's body tap to anything other than ground or the supply, though, that means that you need to generate a well specifically for that MOSFET (or for that collection of MOSFETs). Creating a new well consumes die area—much more than simply generating the MOSFET within an existing well.

Creating a separate well for devices is also helpful to isolate noise.

Hope this helps.

ZekeR said:

Charges leaking into the bulk is often called "hot carrier injection," ...

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Sorry, ZekeR, as I have to contradict here: HCI -- as defined -- isn't charge injection into the bulk, but into the gate dielectric, see e.g. here.

"keep charges from leaking into the bulk"
avoid forward bias of the bulk to source and drain junctions describes the basically simple problem more clearly, I think.

erikl said:

Sorry, ZekeR, as I have to contradict here: HCI -- as defined -- isn't charge injection into the bulk, but into the gate dielectric, see e.g. here.

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From that Wikipedia page:
"In MOSFETs, hot electrons have sufficient energy to tunnel through the thin oxide gate to show up as gate current, or as substrate leakage current. The hot electrons may jump from the channel region or from the drain, for instance, and into the gate or the substrate."

...

"This is the reason why the substrate current is monitored during HCI stress. A high substrate current means a large number of created electron-hole pairs and thus an efficient Si-H bond breakage mechanism."


So, yes HCI seems to include substrate injection. However, I was not aware of gate injection, so thank you for informing me.

I am also now in doubt whether substrate HCI has anything to do with source-body bias. (I had only ever heard of it in one context, so I am not the most knowledgeable here). Can you clarify whether I was right on that point?

To avoid further complicating the issue, I will refer to FvM's post.

ZekeR said:

So, yes HCI seems to include substrate injection. However, I was not aware of gate injection

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... and I didn't think about substrate injection, because I always saw it only in connection with gate dielectric injection and its unwanted effect on Vth variation and, possibly, gate current. Thank you, too, to make this apparent to me!

ZekeR said:

I am also now in doubt whether substrate HCI has anything to do with source-body bias. ... Can you clarify whether I was right on that point?

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I think charge carriers in semiconductors can just be accelerated to the max. available voltage, i.e. to a few single eV in our considered case. So if these essentially in direction S-D moving carriers are scattered in direction to the gate dielectric and are able to intrude or even tunnel through it, they will as well be able to be scattered into the bulk, at least penetrating the channel-bulk junction corresponding to their already available energy. As more energy they gain traveling through the channel, as higher the channel-bulk junction reverse potential gets, however. So the channel-bulk injection probability essentially doesn't rise with their increasing energy -- respectively with their travel through the channel -- in contrary to the channel-gate injection probability.

ZekeR said:

To avoid further complicating the issue, I will refer to FvM's post.

Click to expand...

Right, I think this is the best answer to the original question.

The reason is to reduce the Body effect only. Else there is variation in the Threshold Voltage of the MOSFET and which will vary with a constant called "GAMMA" in the threshold equation for the MOSFET.