How does MOSFET become a current source?

When VGS>VTH, MOSFET behaves as a voltage controlled resistor; However, making MOSFETs the leader in integrated circuits today is definitely not just about voltage controlled resistors.

When the drain voltage gradually increases, the MOS transistor will behave as a voltage controlled current source. This allows MOSFETs to become amplifiers.

What exactly makes MOSFET a voltage controlled current source? I think it's the channel pinch off effect of MOSFETs.

 What is the channel clamping effect?

(1) When the potential difference between the gate and the P-type substrate contact surface is greater than VTH, a channel begins to form. If VD=VS=0, although a channel is formed, there is still no current, so the potential along the length of the channel is equal.

(2) Increasing the voltage of VD so that VD>0&VG-VD>Vth, current begins to form in the channel. Due to the presence of channel resistance, the potential along the channel length is unequal, with 0 near the source and VD near the drain. So at this point, the potential difference between the gate and the P-type substrate contact surface is also unequal, with VG near the source and VG-VD near the drain.

(3) Continue to increase the voltage of VD so that VG-VD=Vth, and at x=L, the channel is cut off at the drain. This will result in channel clamping effect.

(4) Continue to increase the voltage of VD to make VG-VD

 If there is no channel between L1 and L, will there be no current?

Of course not, otherwise MOSFETs wouldn't be as popular as they are today.

Because the drain is n-type doped and the substrate is p-type doped, there is a PN junction between the drain and the substrate, and VD>0, VSUB=0， The potential of the n region is higher than that of the P region, indicating that the PN junction is in a reverse biased state, hence the existence of a depletion region. The electric field within the depletion region will attract electrons that reach the channel cutoff, ensuring that electrons can continue to pass through.

However, at this point, VD will not have a significant impact on the magnitude of the current, and MOSFET will behave as a constant current source.