How do you size NMOS and PMOS transistors to increase the threshold voltage?

• Dec 13, 2006

• #1

gafsos

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Hi,

How do you size NMOS and PMOS transistors to increase the threshold voltage?

THX

 

• Dec 13, 2006

• #2

P

pequeniño

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Re: threshold voltage

the threshold voltage in a MOS device follow the next expression:
Vth=Φms +2Φf + Qdep/Cox
where:
Φms=difference between the work functions of the polysilicon gate and the silicon substrate
Φf=(KT/q)ln(Nsub/ni), where q is the electron charge, Nsub is the doping concentration of the substrate, Qdep is the charge in the depletion region and Cox is the gate oxide capacitance per unit area. For typical dimensions of the oxide thin layer
the value of the Cox parameter is Cox≈6.9fF/um².
So, you must state the influence of the MOS device dimensions in the mentioned parameters (mainly Cox) and size it properly.

 

• Dec 13, 2006

• #3

G

gevy

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Re: threshold voltage

Really threshold voltage is increased for long channel transistors.

 

• Dec 13, 2006

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nassim_el85

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threshold voltage

Hi

I think the better solution for increasing the threshold voltage is using the body effect and increase the source voltage in NMOS or Decrease it in PMOS if it can be possible.

 

• Dec 13, 2006

• #5

N

nxing

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threshold voltage

you can do the simulation and find out the relationship, this way, you will find much more information.

 

• Dec 14, 2006

• #6

L

liuyonggen_1

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Re: threshold voltage

some approaches to increase the threshold voltage:
1: the simplest way: connect the substrate with GND for NMOS transistor and VDD for PMOS transistor.
2:increase the doping level of the substrate.
3:length device can neglect the drain-induced barrier low effect to increase the threshold voltage.

 

• Dec 14, 2006

• #7

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moh_kholy83

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Re: threshold voltage

for short channel devices
decreasing L will increase Vth due to reverse short channel effect
also as Vsb increases Vth increases
use large W

 

• Dec 15, 2006

• #8

G

Guest

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Re: threshold voltage

Hi
I suggest to use Reverse Body Bias (RBB) by body effect in threshold voltage (reverse biasing of body-source junction) to increase threshold voltage rather than enlarging transistor size; you can find more infromation in bacis microelectronics books or searching in papers!

Regards,
SAZ

 

• Dec 16, 2006

• #9

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jttzeng

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Re: threshold voltage

gafsos said:

Hi,

How do you size NMOS and PMOS transistors to increase the threshold voltage?

THX

Click to expand...

Dear Friend
Do your know what kind of process (technology) you used for your design? The threshold voltage is usually depend with device width and length, but shows different behavior by technology. In general, we know the shrot channel effect and narrow wdith effect from text book. but for advance technology, device was designed with reverse short channel (RSCE) and inverse narrow width (INWE) effect. The Vth will increase with shorter length(RSC) and wider width (INWE). We can check model to see the Vth W/L dependence.
B/R
JT

 

• Dec 16, 2006

• #10

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rakesh1234

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Re: threshold voltage

threshold voltage can be varied by decrease the width of the transistor and increase the lenth of the transistor

 

• Dec 16, 2006

• #11

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MikeR

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threshold voltage

Very interesting question. But by definition the threshold voltage is a voltage of a gate at which through the channel the current of a drain starts to proceed. In first approach Vth independent to width and length of channel. Exist only one practical way to change Vth – change Vbs (bulk –source voltage). For most detailed definition you can read BSIM4.5.0 Manual chapter 2.
If the question about change of a Vds on MOS connected in diode mode, that it is very simply defined by the equation:
Vds = Vth+sqrt(2*Id/(K’*W/L))

Interesting, for what you want increase Vth ?

 

• Dec 17, 2006

• #12

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caplash_vlsi

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Re: threshold voltage

i think that increasing w or l increases Cox....now threshold voltage is vol. at which strong inversion of channel takes place.....i.e. sufficient charge gets accumulated on channel....now with increase in Cox as Q=CV....more charge will be acc. for given vol. so in effect threshold vol. decreases with increasing w or l.....opposite is also true....and hence gives explanation to ur ques.

 

• Dec 17, 2006

• #13

A

analog_guru

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Re: threshold voltage

The "effective threshold voltage" decreases as the channel length decreases (assuming there is no reverse-short-channel effect).
The "effective threshold voltage" decreases or increases depending on the isolation technology.
I think this questin falls in the catagory of those where their answers are best found by simulating different transistors from the process of choice and plot Vth against W and L.

Added after 3 minutes:

I am sorry, in my previous post. The second point was with respect to the width of the transistor. Here is what I meant.
The "effective threshold voltage" decreases when the channel width increases or decreases depending on the isolation technology.

 

• Sep 17, 2010

• #14

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kishram4u

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Re: threshold voltage

dude......itz does depend on sizes .

the threshold voltage changes with the ratio Wp/Wn.The reason for this is that the more mobile electrons in the
NMOSFET enable the NMOSFET to operate at a lower VDS than the PMOSFET for the same
IDS, hence pulling VOUT (= VIN at threshold) low. The threshold voltage can be increased by
increasing the width of the PMOSFET (hence Wp/Wn) so that the VDS of the PMOSFET falls.
In fact, if the ratio Wp/Wn = μn/μp = 2.7, then a threshold voltage of VDD/2 is achieved