3.1 Tunneling Mechanisms

In the silicon-dielectric-silicon structure sketched in Fig. 3.1 a variety of tunneling processes can be identified. Considering the shape of the energy barrier alone, FOWLER-NORDHEIM (FN) tunneling and direct tunneling can be distinguished. However, a more rigorous classification distinguishes between ECB (electrons from the conduction band), EVB (electrons from the valence band), HVB (holes from the valence band), and TAT (trap-assisted tunneling) processes. The EVB process is caused by electrons tunneling from the valence band to the conduction band. It thus creates free carriers at both sides of the dielectric, which, for MOS transistors, gives rise to increased substrate current. The TAT process can either be elastic, which means that the energy of the carrier is conserved, or inelastic, where the carrier loses energy due to the emission of phonons. Furthermore, in dielectrics with a very high defect density, hopping conduction via multiple defects may occur.

Figure 3.1: Schematic of the tunneling processes in a silicon-dielectric-silicon structure. The different tunneling processes are indicated by arrows and described in the text. The abbreviations EED and HED denote the electron and hole energy distribution function.

A. Gehring: Simulation of Tunneling in Semiconductor Devices