Why when I increase the no. of fingers in PMOS the current decreses?

[quaternion]

No. of fingers

I am sizing a PMOS to be the pass-device for a LDO.

When increasing the no. of fingers the current decreses??

 

[Audioguru]

No. of fingers

Why do your Mosfets have fingers?
Mine don't have any fingers and they don't have any toes.
Look at the datasheet. No fingers.

Why are your pictures a black negative??

 

[truebs]

Re: No. of fingers

What current are you talking about ?

 

[quaternion]

Re: No. of fingers

The source to drain current ,it decreases when increasing the no. of fingers for same Vgs and Vds?
Shouldn't it increase as W/L increases (by increasing fingers)

 

[cyberrat]

No. of fingers

I can see that in your software you are giving some attribute of "fingers" but by what exactly do you mean "fingers" ???

What electrical property is this?

It would be very "handy" to know

Captain Birds Eye might be able to help you with Fish fingers, but I'm at a loss for these sort of fingers?

 

[quaternion]

Re: No. of fingers

It seems that I was wrong when I put the topic here.May be it should be in analog design section.

The transistor fingers are the transistor gates that are connected to each others to form large W/L transistor in a smaller area and lower capacitance rather than making long stick gate that also has large distributed resistance.

I put that question here sinking that it is an elementary question?

 

[Audioguru]

No. of fingers

Oh, you are making your own Mosfets?

 

[mbyoussef]

Re: No. of fingers

How much is your vgs1? Are you sure your transistor is ON?

 

[quaternion]

Re: No. of fingers

the transistor threshold voltage ≈ 0.26V, vgs ≈0.3 or 0.36 , I don't remeber(that is because I need very small drop out voltage ≈0.1v ).

It appears from the graphs that the transistor is on and conducting a current in the range of 1mA.

 

[mbyoussef]

Re: No. of fingers

your vgs should be -0.3 or -0.36V for the transistor to turn on

 

[quaternion]

Re: No. of fingers

I know the rule of reversing the polarity for the pmos.

That is true I have just dropped the sign , I have already set Vgs = -0.36V and Vds = -100mV(these are the true no.s nothing dropped)

By the way W(for 1 finger) = 100u, and the multiplier = 18 , and l =340n.


Thanks

 

[safwatonline]

No. of fingers

Some PDK's are set that it preserves the total width when u change then number of fingers, i.e. if Wt=x and f=1 then W(finger)=x and if f=2 Wt=x but W(finger)=x/2 i.e. Wt/L is the same but since the c/c's of MOSTs with small W not the same as large W you see current difference.
i think this is ur case.

 

[quaternion]

Re: No. of fingers

Thanks safwat.

I hope that is true; this may explain the non-linearity in the current decrease.
And that means when I have W/L ==> I shall put it as it is(or slightly more) and then change no. of fingers in order to get lower capacitance.