All the previous answers are okay but i want to add some comment which may complete the picture. Nitish asked: What happens to the electrons flowing in the channel when the drain to source voltage is greater than (Vgs - Vth) in n-MOS. This voltage is the pinch off voltage of the channel. At the onset of this voltage, the drain current begins to saturate and it is termed VDSat. Increasing the drain to source voltage beyond VDSat The channel length decreases and the space charge region between the end of the channel and drain increases. The electrons in the channel flows by drift under the electric field in the channel associated with the voltage drop in the channel VDSat=(Vgs - Vth). On leaving the channel, the electrons will be swept by the electric field in the drain space charge region towards the drain where they will be collected affecting the drain current.

The decrease of the channel length with the drain voltage greater than VDSat causes a slight increase in the drain current.This effect is called channel length modulation.

I wanted only to bring the parts of the picture together.

Thank you.

after leaving the inversion layer the electron reaches the so called drift region.

In this region the electrons follows the electrical field between drain and source

and will be collected by the n-doped drain region

Electrons move with saturation velocity while entering the drain region.

After pinch off , the electrons in the channel enters into the depletion region which exists between the drain region and the substrate . Due to the electric field present in this region with drain to substrate junction reverse biased electrons get attracted towards the drain and hence current flows even after pinching off.

All the previous answers are okay but i want to add some comment which may complete the picture. Nitish asked: What happens to the electrons flowing in the channel when the drain to source voltage is greater than (Vgs - Vth) in n-MOS. This voltage is the pinch off voltage of the channel. At the onset of this voltage, the drain current begins to saturate and it is termed VDSat. Increasing the drain to source voltage beyond VDSat The channel length decreases and the space charge region between the end of the channel and drain increases. The electrons in the channel flows by drift under the electric field in the channel associated with the voltage drop in the channel VDSat=(Vgs - Vth). On leaving the channel, the electrons will be swept by the electric field in the drain space charge region towards the drain where they will be collected affecting the drain current.

The decrease of the channel length with the drain voltage greater than VDSat causes a slight increase in the drain current.This effect is called channel length modulation.

I wanted only to bring the parts of the picture together.

Thank you.

Hi all,

everything above mentioned is very much ok...but i want to bring into account the quantum behaviour of electron when it is travelling from source to drain..

When the channel is pinched off, as Sir zekry said, Id is not only function of Vgs but also function of Vds, even though it has been saturated..this phenomena we call it as channel length modulation and hence there is some slope in the saturation region of the output characteristic of the mosfet.

Now when the channel pinch off , it pinches in drain side, the width of the depletion region is high on drain side..channel is present for some portion from source region and then it is covered with depletion region..

When the electron is flowing through channel it will behave like a particle, but when it is crossing the depletion region it behaves like a wave packet.

The above explanations are correct in terms of classical physics, but if u want to explain it in terms of quantum physics, this is the explanation. This phenomena is called as tunneling of electrons. And it happens even though the energy of the electron is less than energy of the barrier.

Hope that helps...

Thank you all for your contribution.

Regards,

Nitish