The drain avalanche hot carrier (DAHC) injection is said to produce the worst device degradation under normal operating temperature range. This occurs when a high voltage applied at the drain under non-saturated conditions (VD>VG) results in very high electric fields near the drain, which accelerate channel carriers into the drain's depletion region. Studies have shown that the worst effects occur when VD = 2VG.

The acceleration of the channel carriers causes them to collide with Si lattice atoms, creating dislodged electron-hole pairs in the process.  This phenomenon is known as impact ionization, with some of the displaced e-h pairs also gaining enough energy to overcome the electric potential barrier between the silicon substrate and the gate oxide. 

Under the influence of drain-to-gate field, hot carriers that surmount the substrate-gate oxide barrier get injected into the gate oxide layer where they are sometimes trapped. This hot carrier injection process occurs mainly in a narrow injection zone at the drain end of the device where the lateral field is at its maximum.

Hot carriers can be trapped at the Si-SiO2 interface (hence referred to as 'interface states') or within the oxide itself, forming a space charge (volume charge) that increases over time as more charges are trapped. These trapped charges shift some of the characteristics of the device, such as its threshold voltage (Vth) and its conveyed conductance (gm).