transistor size for threshold voltage

[rohit_singh1]

So, one of the interview questions goes like this:

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

How ... ?

TIA

 

[lostinxlation]

https://www.cs.umbc.edu/courses/graduate/CMPE640/Spring07/cpatel2/lectures/lect12_inverter.pdf

 

[ebuddy]

In a typical inverter, NMOS pulls the output to low, while PMOS pulls the output to high. To increase the threshold, you need to increase the strength of the NMOS and/or reduce the strength of PMOS. The strengh of a MOSFET is decided by its W/L (width/length) ratio. The bigger this radio is, the stronger this MOSFET is. So now you know what to do.

 

[lostinxlation]

I guess probably the OP may not fully understand the interview question. When people hear "threshold", they always imagine the transistor threshold, but the threshold in question is a gate threshold, typically discussed with the inverter switching characteristics.

Basically, inverter threshold is determined by the shape of Vin-Vout curve. If the curve shifts toward higher Vin, the threshold increases, if the curve shifts towards lower Vin, the threshold descreses. What shifts the curve is ratio of the drive strength of P and N, so, i guess it's now clear how the threshold is affected by P and N size.

 

[phoenixpavan]

I suppose what lostinxlation explained settles this... so depending on the ratio of MOS the threshold will be determined as can be seen through the I-O char graph

 

[dhaval4987]

But if suppose its not about any gate... is it possible to change a thershold only for a transistor by changing sizes?

 

[dragonfury]

But if suppose its not about any gate... is it possible to change a thershold only for a transistor by changing sizes?

That cannot be done through any change in the transistor sizes or aspect ratios.
It can only change through the doping levels or change in the Voltage between bulk and source terminal.