NMOS pass characteristics

[the_pro]

hello... every one.
i'm newer in this field.

i have a small doubt about pass characteristic of nmos transistor.

if i will give Vdd=1.8 v (logic ' 1' ) to the gate terminal and its drain terminal has also 1.8 v then source will go up to maximum voltage level Vdd-Vtn.
now if i change the transistor width up to 30 times greater then its length then will my source exceed Vdd-Vtn level and reach nearer to Vdd ?

thanks in advance...

 

[nitishn5]

Think of it in this way.

If the gate is at Vgate and the drain is at Vdrain, The source terminal can go up to Vgate-Vth beyond which the transistor would turn off.

If the source is above the Vgate-Vth level, the transistor is OFF and the the switch is open.
This is why a NMOS is a better pass device for logic 0 and a PMOS is a better device for logic 1.

You can use a complementary pass switch using both NMOS and PMOS to solve this problem.

 

[abhiverma812]

Well width and length does not matter in this case and you can never go beyond Vdd-Vtn. You need to understand the working of NMOS and various regions it can work in. For NMOS to pass current and act as switch, a minimum difference in voltage of Vtn should exist between gate and source, i.e. Vgs => Vtn.
When you try to increase source voltage > Vdd-Vtn, this condition is violated and NMOS enters into cut off state.

 

[the_pro]

thank you for information.