what's the trend of vth when width or length goes smaller?

[extraord]

hi guys,
I'm so confused that when i sweep the width or length of transistors and probe the
vth of them. the trendency is when length goes smaller vth goes high. and when
width goes larger vth goes high.
all of these is directly different from text book. why?

 

[qiushidaren]

How did you sweep the width and length, I mean, your method.

 

[isaacnewton]

Re: what's the trend of vth when width or length goes smalle

How did you sweep the width and length, I mean, your method.

Set channel length or width as a design variable.

 

[sengyee88]

Re: what's the trend of vth when width or length goes smalle

Is the bulk of the transistor connected to source or supply line? If Vsb≠0, Vth will change with transistor size.

 

[elbadry]

Re: what's the trend of vth when width or length goes smalle

Physically, it's possible that Vth increases with smaller length ( this is known as reverse short-channel effect )

Also, in technologies where FOX isolation is used the so called Narrow width effect causes Vth too decrease as width decreases.

 

[safwatonline]

i also noted that , but isn't the channel decreasing means more effect of Vds , i.e. the channel is formed not only by Vgs but also Vds but this will mean that Vth decrease with length decreasing !!

 

[evilguy]

Re: what's the trend of vth when width or length goes smalle

hi guys,
I'm so confused that when i sweep the width or length of transistors and probe the
vth of them. the trendency is when length goes smaller vth goes high. and when
width goes larger vth goes high.
all of these is directly different from text book. why?

how do you prove vth? are you using SPICE base simulator?

 

[extraord]

Re: what's the trend of vth when width or length goes smalle

yes, I use hspice to check that. and almost all process include
tsmc 13 18, behave like that.
And vds=0. vg=0 or vdd the trend is almost the same.

I think the ldd effects is one factor of that.
but what's the meaning of FOX effects on the width reduced?