Chapter 5

Chapter 5

Interface States in MOSFETs

5.1 Introduction

The SiSiO2 interface is the only known interface that is good enough to enable operation of MOSFETs to industrial standards. Thus, the properties of silicon dioxide are fundamental to the success of silicon integrated circuit technologies [16].

5.2 Properties of SiSiO2 interface

Although the oxide is not a crystal, the silicon and oxygen atoms are packed in an orderly

manner, each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms [16].

As the average distance between the oxygen atoms is larger than the average distance between the silicon atoms in the silicon, this means that some of the interface atoms from the silicon will inevitably miss oxygen atoms to create SiO bonds. This is also known as dangling bond [16].

Atoms from the silicon that remain bonded only to three silicon atoms with the fourth bond unsaturated, represents interface defects. The energy levels associated with the fourth unsaturated bond of the trivalent silicon atoms do not appear in the conduction or the valance band, rather in the silicon energy band gap [16].

Electrons and holes that appear on these levels cannot move freely as there is a relatively large distance between the neighbouring interfacial trivalent silicon atoms (these levels are localized and isolated from each other) [16, 17].

As these levels can effectively trap the mobile electrons and holes (from the conduction and valence bands respectively), these are called interface states. Impurity atoms and groups (such as H, OH and N) can be bonded to the unsaturated bonds of the interfacial trivalent silicon atoms, which result in a shift of the corresponding energy levels into the conduction and valence band. Although this process effectively neutralizes the interface states, it is not possible to enforce such a saturation of all the interfacial trivalent atoms, which means that the density of the interface states can never be reduces to zero [17].