Q. What is the significance of source gate cutoff voltage?

The source gate cutoff voltage is also called pinch off voltage. It is obtained when VDS increases and approaches a level referred to as VP, the depletion region widens , causing the resistance to increase. If VDS is increased to a level where the it appears that the two depletion regions would touch a condition known as pinch off voltage is obtained. When plotted against IDSS the region to the right of the pinch off locus is the region typically employed in linear amplifiers (amplifiers with minimum distortion) and is commonly referred to as constant-current, saturation, or linear amplification region.