why does a mos transistor saturate?

Ids remains (nearly) the same, even though the drain voltage is increasing with respect to source. Why? What is the underlying physics?

thanks in advance

Added after 18 minutes:

i need a somewhat detailed explanation.

an answer like "because the drain side is pinched-off" will not count.. since i am looking forward to know "why the pinching-off implies a constant current?"

You can consult this link:

**broken link removed**

good luck

pinch off means that the Resistance is constant "it is not really due to Channel length modulation" but if u neglect the increase in the current due to CLM then think of the last point on the channel "pinch off point" it has a constant voltage Vp , and a constant length "which is not really true agaian due to CLM" then u get a constant resistance which gives constant current .

the idea about CLM neglection is valid, WHY? cause if the length of the channnel is already large then the decrease of the channel length due to the CLM can be neglected, unless in new technologies where the channel length is already small and the decrease of the chennnel length is effective and hence u can note clearly the small o/p resistance c/c's

safwatonline said:

pinch off means that the Resistance is constant "it is not really due to Channel length modulation" but if u neglect the increase in the current due to CLM then think of the last point on the channel "pinch off point" it has a constant voltage Vp , and a constant length "which is not really true agaian due to CLM" then u get a constant resistance which gives constant current .

the idea about CLM neglection is valid, WHY? cause if the length of the channnel is already large then the decrease of the channel length due to the CLM can be neglected, unless in new technologies where the channel length is already small and the decrease of the chennnel length is effective and hence u can note clearly the small o/p resistance c/c's

Click to expand...

nice!

however.. why the pinch off point has a constant voltage Vp?? What is Vp?

Vpinch off , the voltage where the channel becomes pinched off , Vp=Vd @Vds=Vgs-Vth

safwatonline said:

Vpinch off , the voltage where the channel becomes pinched off , Vp=Vd @Vds=Vgs-Vth

Click to expand...

what happens to Vp @ Vds>Vgs-Vt ?? doesnt Vp increase as we keep increasing Vds?

if it does increase, how do you explain the constant current.
if it does not incrase, why?

thanks.

it doesnt increase the pinch off point has the same voltage , but the channel length itself is decreased and a small depletion region is formed , this region will take the extra voltage drop. and since the Vp is still constant the current is still constant.
as i said before if u want to take the effect of the CLM "the decrease in the channel length" then u should consider the MOS o/p resistance where the current doesnt "saturate" but have a slope.


p.s. after pinch off the channel cannot do anything else as it is already tappered so it just shrink. alwoing the extra voltage to be dropped on the depletion region "very smalll region" which in this case have a very large electric field which sweeps the electrons through the depleted region to the drain this mechanism is called (as far as i could remember) space charge limited current

safwatonline said:

it doesnt increase the pinch off point has the same voltage , but the channel length itself is decreased and a small depletion region is formed , this region will take the extra voltage drop. and since the Vp is still constant the current is still constant.

Click to expand...

hey, thank you for that!

once the carriers velocity reaches the thermal velocity even though the applied field increased the velocity of the carriers wont increase,but reaches saturation velocity..giving almost saturation condition..........