HOT ELECTRON
Drain-source current results from accelerating electrons via the source-drain voltage, which results in horizontal electric field in the channel. As these electrons are accelerated they gain Kinetic energy and if they gain sufficient Kinetic energy they may tunnel through the oxide and a vertical electric field is also produced in the oxide due to high voltage.
So electrons that are supposed to transverse the channel and reach the drain may because of their high Kinetic energy, instead tunnel through the oxide and contribute to a undesirable gate current.This is called Hot Electron Effect since the electrons produced are highly energetic and are considered “HOT“
Source – Fundamentals of Nanoelectronics By Hanson