High Energetic Carriers

Abstract

High energetic carriers are electrons (holes) that occupy energetic states far above (below) the conduction (valence) band edge. An example of the characteristic energy is the threshold energy for impact ionization ε _imp which is of the order of the bandgap E _G . A carrier possessing this energy can transfer its energy via collision to an electron in the valence band and move it into the conduction band. In this process, which is called impact ionization, an electron-hole pair is created and the original high energetic carrier is scattered into a state of considerably lower energy. Another energy range of interest is the conduction and valence-band barriers between bulk silicon (E _G = 1.12eV) and the large-gap oxide SiO₂ (E _G ≈ 9 eV). These range from 3.2 eV to about 5 eV for the conduction and valence bands, respectively. Carriers with comparable energies can enter the oxide and accumulate electrical charge in traps. This eventually leads to a failure of the device. To minimize this oxide charging is a major concern of device design. We will treat it in the next chapter. We use the substrate and gate currents of a MOSFET for the experimental verification of high energetic carriers. Both are very indirect measures of device degradation due to oxide damage because the damage is caused by filling up traps in the oxide, which requires an understanding of the trap mechanisms. Furthermore, the substrate and gate currents are very sensitive to uncertainties in device processing. This is particularly so in the accuracy of the doping profiles which determine the electric field in the device.