size NMOS and PMOS to increase Vt

As a designer you cant change NMOS and PMOS size in order to change Vt. The only way you can change Vt as a designer is by changing the Body Bias.

ok so here we go cmos technology consists of a pmos sitting on top of an nmos.anything above Vdd -Vth is Vout=0 as |Vgsp| < Vth and anything below Vth is Vout=Vdd as |Vgsn| <0. anything in between lies in the transition zone. Now the edges of this transition zone are controlled by Vthn and Vthp which are in turn controlled by Vsbn and Vsbp(its an eqn consisting of square roots which can be found in any text book and the effect is known as body effect). what it also depends on is the doping densities the permittivity of the oxide layer and the capacitance per unit length of the oxide layer Cox. this Cox depends on oxide thickness Xox ----------(1) Now the impt values are Vih ------- is given by the condition (dVin/dVout)= -1 based on stability arguments Vil ------- is again given by (dVin/dVout)= -1 when you draw the transfer curve youll easily locate these conditions Voh=5V--- Vol=0V And Vi=unity gain threshold voltage where Vin=Vout now when you solve this differential eqn youll see that these values(Vih, Vil and Vi) depend upon (betan/betap) or more specifically (kp', kn') ----k' and Vt are a result of fabrication and cannot be chosen in the layout editor (W/L)p (W/L)n known as the aspect ratios----IT IS THESE RATIOS THAT WE MUST TWEAK

@ Jitu JItesh Here I think that we are talking about size of NMOS in terms of W or L. So I didn't get it how this Vt will change with changes in W or L. Tox is most dependent factor for Vt. So it is understandable. Please explain me as soon as possible. Thank you in advance.

if we decrease size the feature size of transistor(pMOS or nMOS), then it is observed that Vt(threshold volt.) to turn on transistor increases as feature size of transistor decreases or technology shrinks. Note : here reducing feature size means here we are manupulating W and not L(channel length).

in the case of NMOS , if u want to increase the Vth u could do one of the following.. 1) additional charge very near to the substrate - Sio2 layer.. V= Q/C .. so additional charge increases the inbuilt voltage.. so inorder to turn the device on, u need to apply more Vth.. 2)obviously in similar lines, decreasing C increases Vth.. as tox increases ,Vth increases.. 3) also Vth depends on the source to bulk Vsb voltage.. incase of NMOS , bulk shud be as negative as possible.. so if i start making Vbs more and more negative.. Vth increases.. this is also called the body effect..!

thus the threshold equation depend upon body effect & body effect is proportional to thickness of oxide. so by increasing tox threshold can be increase.