Why in MOSFET p-substrate is lightly doped?

In a MOSFET, the p-substrate is lightly doped to maintain a high mobility of charge carriers and to reduce the electric field strength near the surface. This light doping helps in creating a wider depletion region, which enhances the control of the gate over the channel, improving the device's performance. Additionally, it minimizes the impact of short-channel effects and threshold voltage variations, leading to better stability and reliability in the device operation.