There are four basic types of charges/ defects that are associated with gate oxide.

1. Fixed charges:

   It is a charge in Si-SiO2 structure, located in the oxide in the immediate vicinity of Si surface. It does not move and exchange charge with Si (hence, "fixed"). It associated with incompletely oxidized silicon, or in other words, with excess silicon. It alters characteristics of Si-SiO2 based MOS gate stacks.

   Source – Incomplete oxidation.

   Effect – pushes VT in negative direction.

2. Interfaced Trapped charges:

   Similar in origin as fixed oxide charges but are located at Si/SiO2 interface. The charge associated may be positive, negative or neutral and can change during normal device operation since it traps electrons and holes.

   Source – Mechanical damage in Si wafer, Dangling Si bonds left un-reacted after oxidation.

   Effect - Trap and de-trap electrons affecting MOS device performance.

3. Oxide Trapped charges:

   They are associated with defects in the SiO2. It is usually repaired with high temperature annealing.

   Source - Exposure to radiation environment. Hot electron effect in short channel MOSFET devices.

   Effect - Interferes with electronic activity.

4. Mobile Ionic charges:

   These are electrically charged species which can move in the MOS gate oxide under the influence of electric field. They cause severe instabilities of MOSFET characteristics, e.g. fluctuations of the threshold voltage VT. Na+ ions are the most common mobile charges in SiO2.

   Source - Mostly humans. Contaminated water or if it is not fully deionized.

   Effect - Causes disorder on transistor characteristics.