Question

what does the following line mean
'since the drain will be at positive voltage relative to the source,the two p-n junction can be effectively cutoff by simply connecting substrate terminal to source terminal'
this is regarding a mosfet and how can it be used as three terminal device instead of 4

Answer 1

I hope you are talking about N-channel mosfet, whose drain terminal is named to the terminal which is at higher potential and other is source. In N-channel mosfet, substrate is p-type, so the two parasitic diodes are n(drain)-p(substrate) and n(source)-p(substrate). Since drain is positive than source and you have connected bulk to source , so bulk is also at lower potential than drain, so you have reversed biased n-p (drain -substrate) junction and also because source-bulk terminals are connected together, its n-p junction is shorted. In this way you are using a 4 terminal device as a 3 terminal one. This configuration is mostly used. But you can play with bulk voltage to control Vth of device , but try to keep bulk potential lower that drain voltage, otherwise there will be a current leakage path from n( drain) to p( bulk), if this junction gets forward biased.

Answer 2

It's not being used as a three terminal device, actually we have to set a constant voltage to fourth one