The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> MOSCAP
https://designers-guide.org/forum/YaBB.pl?num=1247051566

Message started by raja.cedt on Jul 8^th, 2009, 4:12am

Title: MOSCAP
Post by raja.cedt on Jul 8^th, 2009, 4:12am

hi,
can any body please tell me why moscap is made in N substrate (i knew that we can connect nmos drain and source and it works well, in that case it will be P substrate)

Thanks,
rajasekhar.N

Title: Re: MOSCAP
Post by solidstate on Jul 8^th, 2009, 2:19pm

As far as I know, this allows for the moscap to have a dedicated ground terminal; without the nwell, the bottom plate of the moscap would be the substrate, so substrate noise (e.g. from digital logic) could then be coupled in..

Title: Re: MOSCAP
Post by raja.cedt on Jul 8^th, 2009, 9:46pm

hi,
thats fine but they could have put pwell right because then it works like nmos then we can connect Drain and Source..

Thanks,
rajasekhar.

Title: Re: MOSCAP
Post by shashi.kiran on Jul 9^th, 2009, 4:39am

if ur using NMOS -CAP it has to be p-sub and PMOS CAP it has to be n-well ..!!..i dont think we use Nmos and n-type substrate ..!!

Title: Re: MOSCAP
Post by Dipankar on Jul 9^th, 2009, 7:01am

Dear All,
N-Moscap in nwell means you need very low gate voltage to send the device into strong inversion. So the C-V does not have accumulation region . With this kind of C-V (monotonic) a VARACTOR is realized. They call it NMOSCAP-VARACTOR.

Title: Re: MOSCAP
Post by raja.cedt on Jul 9^th, 2009, 9:24am

hi,
i donno weather i am complicating this question or not, but what i thought is it is like having n diffusion's in nwell,so how inversion will come..i think it will be in accumulation only and works for +ve voltage

Thanks,
Rajasekhar.

Title: Re: MOSCAP
Post by shashi.kiran on Jul 9^th, 2009, 9:40pm

@ raja

if you apply +ve gate voltage ...how can u say its in accumulation ??...infact it will be inverted ....i guess dipankar got it right ...it should be in inversion with less gate voltage.

Title: Re: MOSCAP
Post by raja.cedt on Jul 9^th, 2009, 10:18pm

if you apply +ve voltage it will attract all electrons from nwell..so this is accumulation..how inversion will come?

Title: Re: MOSCAP
Post by Dipankar on Jul 10^th, 2009, 7:00pm

Dear Raja,

The crux is nmoscap in nwell gives a monotonic CV curve. That's the motivation behind the structure - creating a mos-varactor.