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p/n guardring and p/n tap

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shanmei

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What is the difference between guardring and tap.

I know that the ntap is actually n-well tap, and the pmos is embeded in the nwell, so ntap is tied to vdd.

ptap is actually p_substrate tap, and the nmos is p_substrate, so ptap is tied to vss.

But what is the p/n guarding, whether they are the same?

p guarding= ptap?

n guarding= ntap?

Thanks.
 

A tap is usually the connection to a layer, in case of a guardring the contact between guardring and metal1, which then connects to the required potential.

For a good ohmic contact - i.e. to avoid the creation of a Schottky contact - between a not very highly doped semiconductor region (e.g. a guardring, a well, or an active region) and the metal, below the contact window a highly doped contact region is necessary: p+ on p, n+ on n respectively.
 

A guardring serves as a tap to the region of same species
that it's in. But a tap is not a guardring. Tap is point access
as minimum (could, or not, be bigger like bar or ring).

Think of fencepost and fence.

Completely enclosing a device can eliminate surface
conduction (field leakage), provide a much lower
access resistance to the region of the device you
care about tying down, and more capably **** up
wandering minority carriers (recombination @ high
doping, pinch of epi / well cross section, depletion
sweep for counter-doped guard rings). A lonely tap,
you get only the carriers that diffuse to within its
"sphere of influence" (or half-sphere).
 

Then both p-guardring and p-tap are short to vss, and n-guardring and n-tap are short to vdd, right? Thanks.
 

Then both p-guardring and p-tap are short to vss, and n-guardring and n-tap are short to vdd, right?

A p-guardring is to be connected by a p+tap (contact to metal1) to VSS, and an n-guardring is to be connected by an n+tap (contact to metal1) to VDD .
 
Again, this is a generalization which you may find different
rules / norms / values for, in ESD and I/O latchup rules /
construction, analog design etc.

You might for example have a "flying" N-well and tie its
taps to a local PMOS source, local resistor highest-potential
(but not VDD, so as to avoid voltco) or whatever. You might
elect to have a N+ ring inside P+ ring both tied to VSS for
some block level noise control (not wanting VDD noise to
be capacitively imposed locally, so don't tie it there) etc.
 
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