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Design >> High-Power Design >> Any reason why people used Nsub for analog ..
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Message started by phawk on Jan 15^th, 2007, 11:01am

Title: Any reason why people used Nsub for analog ..
Post by phawk on Jan 15^th, 2007, 11:01am

Is there any reason why people used nsub for analog switch chip design?
Thanks!

Title: Re: Any reason why people used Nsub for analog ..
Post by sheldon on Jan 22^nd, 2007, 5:38am

PHAWK,

There are several reasons why for reasons using n-substrate. First
some background,

1) MOSFET transistors are majority carrier devices.
2) N-channel devices have lower rdson per unit area so n-channel
devices are preferred.
3) In the past, short-channel transistors were created by doping the
MOSFET body, p-well, and source through a single oxide window.
The boron out diffuses the Arsenic and forms the body of the n-channel
transistor.
To build this type of structure means that the drain needs to be
built into a lightly-doped region usually an epitaxial layer. However,
lightly-doped regions mean high resistance so the device needs to
built on a highly doped substrate region for example an buried
collector region or an n-wafer.
4) The benefit of creating the drain in a lightly-doped epitaxial region
is that the doping concentration of the lightly-doped region determines
the breakdown voltage of the junction. So the BVdss, the drain
source breakdown voltage is maximized.

So n-channel DMOS transistor structure is preferred for building high
breakdown devices.

5) In many applications, for example, automotive, high-sided switches
are used for safety reasons(I've heard). A low side switch is connected
from the load to ground so the load floats when the switch is open.
For a high-side switch, the switch is between the source and load so
the load is grounded when the switch is open.

If a high-side switch were built in a p-substrate, the substrate is grounded.
As a result, the breakdown is limited by the substrate to drain breakdown
voltage. In addition, the current handling capability is limited compared
to an n-substrate device. The drain's thickness and resistivity are limited
by the need to build the transistor on a p-substrate.

So to build high power density, high-voltage, high-sided switches n-substrates
are preferred to p-substrates.

Best Regards,

Sheldon

Title: Re: Any reason why people used Nsub for analog ..
Post by phawk on Jan 30^th, 2007, 2:00pm

Hi sheldon,
Thanks a lot!

-phawk