sheldon
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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
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