what is hot electron effect

The hot electron (or short channel) effect is described in as occurring when a high voltage is applied across the source and drain of a device, the electric field is high, and the electrons are accelerated in the channel. The fastest electrons may damage the oxide and the interface near the drain, thus inducing transistor threshold shift and mobility change over the life of the part. As the gate is always positive in an N-channel MOSFET, the shift is always in the same direction. Thus, as time goes by, the threshold eventually reaches a point where the device no longer operates as required in the design.
The problem of hot electron has increased as technologies scale down due to the fact that device features have scaled proportionally faster than voltage. This leads to the devices having higher field strengths and thinner gate oxides.