Question

in mosfet operation,why does the substrate not have any effect on device operation? why are the two p-n junctions assumed to be cutoff by connecting the substrate to source.?

Answer 1

please refer to link for ans of
this question
http://openstudy.com/updates/51179f19e4b09e16c5c8b579

Answer 2

actually when we talk about a single mos circuit then effect of substrate is negligible but when we fabricate ic's then a very large number of mosfet's are used in that case we can not neglect the effect of substrate so in that case we consider the effect of substrate.
there is two back to back pn junction formed between substrate drain and source since we measure all the voltages with respect to source and assume source is at zero potential.we connect substrate terminal to source.and when we apply higher gate voltage then inversion layer is formed below the gate where two back to back diodes eist due to opposite polarities it will be in reverse bias so it is considered as cut off when device conducts