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Because Drain and Souce same type semiconductor, substrate is opposite type.
Substrate produce diode with source and drain and we can bloke this diode by Source-Substrate connection else this diode conduct and mos dont work.
We don't try to reverse bias the substrate,body or bulk or channel.But,the junctions at source and drain reverse biased with respect to substrate such their is no loss of current into substrate that is the reason why we connect NMOS to ground and PMOS to vdd.
Same as above, but will try to present differently to get more understanding. For NMOS device, source and drain are n+ type and substrate is p type . There will be certain potential in the source and the drain. However if you connect p type to VDD, and if any of your source or drain is lower than VDD, then you have forward bias diode between source/drain and substrate. The diode will start conducting current which is basically power leakage. Now instead of connecting substrate to VDD , connect it to VSS. The same diode will be reverse bias and prevent current flow.
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