How to change Vt of a specific MOSFET

[idan.reller]

How can I change the threshold voltage of a specific MOSFET?

If I change the model parameters, then all transistors referencing this model will be affected.
I can create a new model with the altered Vt, and then reference only the specific MOS transistor to that model, yet I'm trying to avoid that.


Thanks.

 

[dick_freebird]

For simulation purposes, why not just a series gate voltage source
on the one (?) you want to mess with?

 

[dick_freebird]

Also, have you explored the .ALTER command to see if your
particular simulator will touch individual devices' model
parameters that way? Many attributes are not so simple
to emulate, as VT offset.

 

[idan.reller]

Hey, thanks for the reply.

I tried to use the .alter command, but didn't find anything useful.

I'll try to add a voltage source for the Vt offset. Since it is a pass transistor which conducts current in both ways, perhaps I should use a controlled voltage source, with its polarity determined by the transistor's current direction.

Thanks.

 

[erikl]

... it is a pass transistor which conducts current in both ways ...

So don't forget to switch the bulk voltage, too.

 

[idan.reller]

Hey erikl,

I'm not sure I understand what you mean by switching the bulk voltage. The bulk is connected to VDD/VSS, according to the MOS type.

Are you talking about Vbs?
If so, perhaps I should use two controlled sources, one of each side of the transistor. Only one source should be active at a given time. The active one should be on the drain side of the transistor, according to the current flow direction, in order to avoid bulk effects (avoid changing Vbs).

Am I right?

Anyway, as I understand, in order to get good transistor behavior I should create a new model with its new threshold voltage...

 

[erikl]

Hi idan,

I'm not sure I understand what you mean by switching the bulk voltage. The bulk is connected to VDD/VSS, according to the MOS type.
Are you talking about Vbs?

Yes and no. Be aware that a 4-terminal MOSFET inherently is symmetric, i.e. drain and source can be interchanged. If you want a specific MOSFET to change direction of the current flow (and if you want to control this current), you should also exchange the drain against the source, otherwise the parasitic drain-bulk diode conducts the current flow. If originally the bulk was connected to the source (Vbs=0), and you want to change the current direction through the MOSFET, you should also reconnect the bulk to the new source (which was the former drain) in order to avoid the current flowing through the parasitic diode and to be able to control the current by the gate.

Anyway, as I understand, in order to get good transistor behavior I should create a new model with its new threshold voltage...

Good idea!

 

[idan.reller]

Hi all,

My last two posts are complete nonsense... Sorry about that. Please disregard...
For some reason, I got confused with the voltage source.

Anyway, to conclude:
1. If the simulator supports changing the Vt of a specific MOSFET - that's the best (Mine doesn't support it...)
2. You can create a new model with a new Vt (cumbersome, but works)
3. You can add a series voltage source to the specific MOSFET for which you want to simulate a different Vt.