Question on TCAD example

[prescott2006]

Hi, I want to create a NMOS using Silvaco and alter some parameters to see the changes in I-V characteristic, so I open the mos1ex01 to edit, but I found that there is a line "init orientation=100 c.phos=1e14 space.mul=2", I am curious why the initial substrate is doped by phosphorus, isn't it should be doped by boron or other column-III elements? :?

 

[timof]

initial substrate (as well as silicon in general) can be doped with acceptors or donors, so there is no contradiction here in this example...

 

[prescott2006]

initial substrate (as well as silicon in general) can be doped with acceptors or donors, so there is no contradiction here in this example...

For a NMOS, isn't the substrate should be p-type? But if we dope with donors, it will become n-type right? I am so confused.:?

 

[erikl]

... there is no contradiction here in this example...

I understand an NMOS needs a P-bulk. However, 1e14 is a rather low value for a MOSFET bulk doping concentration, so may be this info actually concerns the base material doping concentration of an N-substrate double or triple well process, where NMOSFETs are built into a P-well. MOSFET bulk doping concentrations (here: the P-well concentration for the NMOS) to be found in literature are more in the order of 4e15 .. 3e17/cm³ (the latter value found for a 130nm process).

 

[oke2005]

For a NMOS, isn't the substrate should be p-type? But if we dope with donors, it will become n-type right? I am so confused.:?

Yes, the NMOS need a p-type substrate/body, and the initial substrate is n-type.

But in the example (mos01ex01), just after the init line, you'll find that it creates the P-well implant as the 'substrate' or 'body' for your NMOS, and later the NMOS is built on top of this P-well.
This example is very useful for the introduction to CMOS processing simulation; you need to be familiar with the use of well, which will give the appropriate 'substrate' in a single wafer for two different types (n/p) of device.

 

[Colbhaidh]

mos1ex01 is a very simplistic model. N doped substrates are very common on power devices. However you can easily change the init statement to a P substrate using
init orientation=100 c.boron=2e15
which is a common P tyype substrate doping. All modern IC processes use P-Wells and N-Wells and do not rely on the substrate doping