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transconductance and mobility

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salma ali bakr

salma ali bakr

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hi guys,

in an NMOS transistor
for a constant Vds
does the transconductance increase till a certain value of Vgs then decrease again?
does that have to do with electron mobility?

does electron mobility vary as a function of channel length?

thanks,
Salma
 

The relation of transconductance is as follows (operation in saturation and ignoring channel length modulation)

gm = µn Cox (W/L)(Vgs-Vt)

as Vgs is increased it results in an electric field perpendicular to the channel (Ex) that accelerates the carriers towards the surface increasing scattering hence decreasing mobility and decreasing gm.

Also, the carrier mobility does suffer due to short channel lengths (2nd order effect). There is another component of electric field (Ey) parallel to the channel due to Vds. The mobility degradation effect due to Vds is more important than due to Vgs, because for high velocity values the velocity saturates.

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Thanks
 
i got the part about the variation with Vgs
but with channel length,Vds and the velocity, i'm not clear about that:(
can anyone elaborate more please,thanks
 

An increase in Vds increases parallel component of the electric field (Ey) that accelerates the carriers along the channel. Thus, the value of carrier velocity saturates at certain value of Vds.

vd = μE

where vd is the electron velocity. if vd becomes constant µ decreases as E increases.

Due to decreased channel length field is stronger and velocity saturates at lower value of Vds.

Hope its clear now.
 
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