Why is the substrate in NMOS connected to Ground and in PMOS to VDD?

[Narveer Yadav]

Why is the substrate in NMOS connected to Ground and in PMOS to VDD?

[sarah borrison]

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.

[sarah borrison]

Why is the substrate in NMOS connected to Ground and in PMOS to VDD?

we try to reverse bias not the channel and the substrate but we try to maintain the drain,source junctions reverse biased with respect to the substrate so that we dont loose our current into the substrate.