The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Modeling >> Semiconductor Devices >> How is Vth defined?
https://designers-guide.org/forum/YaBB.pl?num=1186499080

Message started by fran2k5 on Aug 7^th, 2007, 8:04am

Title: How is Vth defined?
Post by fran2k5 on Aug 7^th, 2007, 8:04am

Hi everybody,

recently I was performing some simple simulations of a single device in a 0.25um technology.
I wanted to check the variation of Vth with L keeping the same W.
I was quite surprised to notice that Vth decreases for longer transistors.
Could anybody help me out to interpret such results? I believe that the behavior should be opposite...
To check the Vth I use the "op" function of the calculator. How is this Vth defined? Does it correspond to the BSIM one?

Thank you very much,

fran2k5

Title: Re: How is Vth defined?
Post by vivkr on Aug 7^th, 2007, 11:10pm

Hi Frank,

Does the Vth first increase when you increase the channel length, and then start to fall off after a certain point?
If yes, then you are seeing reverse short channel effect. If not, then I could not say what the cause might be.

Try searching for reverse short channel effect on the web for more.

Regards
Vivek

Title: Re: How is Vth defined?
Post by fran2k5 on Aug 8^th, 2007, 5:38am

Thank you Vivek.

I believe you are right. It seems that RSCE is predominant.
The fact that the short-channel effect is evident in the sub-threshold Ids-Vds curves does not really mean that the threshold voltage has to decrease.

Regards,

frank