Q. What is Idss. What are the conditions for its maximum value?
The term 'Idss' refers to saturation condition of the Drain current. From the above graph it's clear that as the value of Vgs goes on decreasing, the value of Id also decreases, hence to obtain maximum Idss the primary condition should be that Vgs must be equal to zero. Also the applied Vds should be greater than Vp so that the saturation level is attained.
Hence the conditions are
1. Vgs = 0 V
2. Vds > Vp