Question about switching MOSFET

[EDA_hg81]

bds22 mosfet

The MOSFET such as BDS22 has separate substrate control pin.
How should we use this pin?
Thanks

 

[FvM]

I don't know BDS22 (what's the manufacturer?) but Vishay SD211 et al have also separate substrate pin. They are used e. g. as switches with floating source. S is typically connected to negative supply, but may be also driven by an active guard potential in some applications. In a common source circuit, S can be connected to source as with 3 terminal FET.

 

[A.Anand Srinivasan]

that is used to modify the threshold voltage of the MOSFET and it uses backgate effect... i.e it behaves just like another gate terminal....

 

[raptor1981]

Switching scheme of mosfet is dependents on the application where it is used. So consider two example such as sampling purpose and circuit emulation. consider the first case of sampling purpose, here we are controlling the threshold voltage by giving suitable amount of bias to the circuit. This bias is adjusted in such a way that the total signal is transmitted to the output of the circuit. In the second case we are extracting a resistance
behavior from a sequence of switches. So here we
are interested in the on - off ( Charge transfer ) mechanism of MOSFET.

Remember in case of particular application (SR)
the bias (body) is varied dynamically depending on the input signal

 

[A.Anand Srinivasan]

raptor 1981 can u please explain the threshold variation utilised in sampling purpose... i didnt understand that....

 

[butterfish]

The MOSFET such as BDS22 has separate substrate control pin.
How should we use this pin?
Thanks

For PFET, keep Vsubstrate >= Vsource
For NFET, keep Vsubstrate <= Vsource
If not so, the switch maybe have large leakage current.

 

[santom]

Hi

Can anyone explain more clearly the point which has been posted by the user raptor1981.

Regards
santom

 

[troybtj]

There are some circuits out there (mostly Solid State Relays) that use the pin, otherwise it is generally connected to the Drain (N Channel) or Source (P Channel), maintaining good gate impedance. I'd save them for finding a 4 way use for them, hard to find just laying around. It is the "extra grid line" in a standard MOSFET Schematic Diagram, it shown connected internally with the 3rd "grid".

Added after 5 minutes:

Hi

Can anyone explain more clearly the point which has been posted by the user raptor1981.

Regards
santom

Actually, WikiPedia has some ok info that shows you the concepts:

https://en.wikipedia.org/wiki/Field-effect_transistor

Shows depletion and enhancement modes with both P and N substrates, and the inputs involved (+ or -, no actual levels). Graphic is about 1/3 down on right hand side.