Is it possible to implement a MOSCAP with the D-S diffusion ?

[hrkhari]

Dear Guys:

Since a capacitor is a two terminal device, is it possible to implement a MOSCAP with the D-S diffussion tied together with the tuning voltage, compared to the typical implementation where the tuning voltage is applied at the gate. Thanks in advance

Rgds

 

[pi331133]

moscap

if you want to use a normal NMOS as the varactor, you can connect the source and drain together in layout as you said. In fact, MOSCAP is a three terminals device, the gate, the source/drain and the p-sub. Normally the gate voltage is fixed the tuning voltage is applied to source/drain, this can reduce the effect of the parasitic cap from source/drain to p-sub. Thanks.

 

[sharas]

Re: MOSCAP

you must apply voltage at the gate in order to form an inversion layer. Therefore, the S/D and bulk should be tied together - and you can consider the transistor as a 2 terminal device.

 

[hrkhari]

Re: MOSCAP

Hi Guys:

Which is better tying the D-S-B together or tying the D-S together while grounding the B for nMOS or pulling to Vdd for pMOS in realizing a MOSCAP. Please do clarify whether pMOS is better off in realizing a MOS varactor rather than a nMOS. Thanks in advance

Rgds

 

[sharas]

Re: MOSCAP

For nmos, you should ground S+D+bulk. For pmos you must connect all 3 of them to Vdd. I don't see any afvantage for on over the other.

 

[goodboy_pl]

Re: MOSCAP

float moscaps is simply an pmos or even nmos in nwell with drain and source tied together to form one pin and gate the second pin.
but the accurate capacitance is not modeled in libraries unfortunately!

BEST!

 

[hrkhari]

Re: MOSCAP

goodboy_pl wrote:

float moscaps is simply an pmos or even nmos in nwell

What do you mean nmos in nwell, is it equivalent to a AMOS with drain and source floating and the bulk together with the gate forms a two terminal MOSCAP, kindly confirm. Does the typical silicon process supports this?, although there's no any extra process step involved in this type of design. Thanks in advance

Rgds