transistor as a switch - cadence problem

Hello,
i am trying to use a mosfet transistor as a switch in the attached circuit which i try to use it in an LC cross coupled oscillator circuit

such that when the gate voltage is high it connect the both capacitors and when the gate voltage is low it switched off (very high impedance) ,but when i tries to switch off the transistor by making gate voltage = 0 cadence-spectre simulator show this warning message in the netlist:
M44: Missing bulk-drain diode would be forward biased
and thus the switch still connect the capacitors and i really don't know what to do with that
Any help will be appreciated
Thanks in advance

connect the bulk to ground and try it.

actually i did and it switch off but when i try to switch it on again same problem happen ....i really don't know if this problem is related to the circuit or the simulator it self

kumite said:

M44: Missing bulk-drain diode would be forward biased

Click to expand...

"Missing" means this bulk-drain diode is just missing in your schematic, but any MOSFET has this diode, involved by its construction: . Actually there are two of them: bulk-drain and bulk-source. So if you reverse the polarity between drain & source, this drain-to-bulk diode gets conductive. As bulk is connected to source, your switch can't close any more. There are 2 possibilities if you need to reverse the polarity:

1. you must also exchange source (+bulk) and drain, e.g. put the MOSFET into the DC branch of a bridge rectifier
or
2. put 2 MOSFETs inversely phased in series (source to source). This means you have to control both gates in antiphase.

The "missing" messages pertain to the compact-model not
having properly specified diode params, and the diode thus
being not-modeled electrically; the warning just alerts you
to this issue and that results may not be realistic if that
diode would have wanted to conduct. Which may or may
not be the case - slight "forward bias" might contribute
trivial D-B current, or not, depending on how much at
what temp, etc.

Now adding an explicit external diode is a good way to
gauge this, provided you can give it reasonable values
that don't swamp the MOSFET operation, etc.

I don't believe the missing diodes account for an abnormal
switch behavior - more likely they would add abnormality
when present. Other than, they might constrain the body's
movement under charge pumping influences (switch turnon
and turnoff). But they can't do much until forward biased....

Now what you show, will act as a "MOS diode" under some
bias / switching conditions, and this config is only good for
a single-polarity use with some constraints like Vd > Vs.
Whether it's anything like a "switch" depends on that.

Thanks a lot for your replies,
actually i found that i was misunderstanding switching operation using MOS-transistor as in this configuration drain and source voltage is always the same (zero DC volt) but can any one recommend me a way to switch between both the capacitors using a MOS-transistor

kumite said:

... can any one recommend me a way to switch between both the capacitors using a MOS-transistor

Click to expand...

Just read my suggestions above!

Yes i tried, i reverse the source and drain connection ...the good news it switched off but not on .... however now i just Fixed It By A Couple Of resistors .....really Thanks All Of you ......you Makes Me understand

use TG for switch operation. it has some advantage over pass gate.

Hi,
For switch between two capacitances i usually add resistance between source/drain of nmos and gnd.