[SOLVED] Deep N-Well Connections of Core Devices in an IO domain.

[nitishn5]

Can I connect the Deep N-Well of core devices to the higher IO voltages?
Is the Deep N-Well any different in terms of a core device and an IO device?

Lets say that I have a block using 3.3V IO devices and operating under higher voltage(3.3V).
But I also have some core 1.2V devices in the design which are inside Deep N-Wells.



The question is, where do I connect these Deep N-Wells?
Since I do not have any access to any 1.2V for the core devices. Can I connect them to the 3.3V supply? Is it a problem?

Does it make any difference regarding the Deep N-Wells for a Core and an IO devices?

Thanks in advance...

 

[erikl]

Since I do not have any access to any 1.2V for the core devices. Can I connect them to the 3.3V supply?

Yes, of course you can. I guess the PMOS' sources are at 3.3V, too?

Is it a problem?

In this case the body (bulk) and the PMOS themselves are at 3.3V, so you'd need level shifters from and to the core circuits - if they have to cooperate.

Does it make any difference regarding the Deep N-Wells for a Core and an IO devices?

Actually not. Deep N-Wells just are for higher voltage than standard N-Well, so you'd also use 3.3V transistors in order to get their high-voltage characterization correctly in simulation. And they'll probably need larger min. W & L dimensions than the 1.2V core transistors.

You shouldn't use the 1.2V core transistors there.

 

[nitishn5]

Yes, of course you can. I guess the PMOS' sources are at 3.3V, too?

Lets say that I am using core 1.2V NMOS devices. With the source and body tied together. I have ensured that the Vds and Vgs does not cross 1.2V since that would be a reliability issue.

I need the deep N-Wells since I am source connecting the body and hence I need to have isolated P-Wells for each of my 1.2V device.

In this case the body (bulk) and the PMOS themselves are at 3.3V, so you'd need level shifters from and to the core circuits - if they have to cooperate.

Lets say that I have no problems with the functioning of the circuit.

You shouldn't use the 1.2V core transistors there.

The reason I want to use the 1.2V core transistors is because I need the lower Vth. I am having headroom issues.


My question is regarding the reliability.
How I see it is that the core 1.2V transistor is inside the P-Well. And it is only oxide region which is sensitive to the higher voltage, so the Deep N-Well being far off should not logically affect it just because it is at a higher voltage.
Is this statement valid?

 

[erikl]

My question is regarding the reliability.
How I see it is that the core 1.2V transistor is inside the P-Well. And it is only oxide region which is sensitive to the higher voltage, so the Deep N-Well being far off should not logically affect it just because it is at a higher voltage.
Is this statement valid?

Yes, I think this is no problem for the NMOSFETs. Just for the PMOS it could be a problem.

Only because you said above

I do not have any access to any 1.2V for the core devices.

... I didn't think that you can get along with a 1.2V voltage level.

 

[nitishn5]

Thanks erikl

Thanks for the insight.