TUNNELING IN THIN GATE OXIDE MOS STRUCTURES (SILICON IMPACT IONIZATION, VALENCE-BAND, ELECTRON)

A comprehensive study has been carried out to investigate the carrier tunneling related phenomena in thin gate oxide MOS structures. Specifically, MOS capacitors with oxide thickness ranging from 3 nm to 14 nm of Al-gate and n('+) poly-gate on p-type and n-type Si-substrates are used to characterize electron tunneling in Fowler-Nordheim(F-N) and direct regimes. An oxide electronic effective mass of 0.39 m(,e) is obtained for an assumed barrier height of 3.2 eV, using the parabolic 1-band interpretation for the dispersion relation in the oxide band-gap. Also, the parameters A and (beta) associated with the F-N equation are found to be slightly dependent on the oxide thickness. Theoretical analysis has shown that the same current vs. oxide-field characteristics expressed by the F-N equation also holds for the Franz-type 2-band dispersion relation. In fact, a Franz-type dispersion relation of E(,gox) = 9 eV and m(,ox) = 0.5 m(,e), is found to be equivalent to the above-mentioned parabolic relation in the F-N regime. These two dispersion relations are tested in the direct tunneling regime and it is found that the Franz-type expression yields better agreement with the experimentally obtained tunneling current vs. voltage relationships. N('+) poly-gate thin-oxide n-channel MOSFET's are used to characterize the substrate valence-band electron tunneling currents. The experimental studies on valence-band electron tunneling give further support of the 2-band nature of the oxide dispersion relations. In addition, p-channel n('+) poly-gate MOSFET's of very thin-oxides are used to study quantum yield of electron impact ionization in silicon as well as hole-tunneling phenomenon. In the impact ionization study, electrons are injected into silicon from the gate by tunneling. Using the carrier-separation properties of the induced junction and since the energy of the injected electrons is preserved when they pass through the oxides by direct tunneling, we are able to experimentally measure the number of generated electron-hole pairs as a function of the incident electron energy, at least up to 5 eV. Our results are found to be in excellent agreement with recent theoretical calculations of quantum yield. On the other hand, hole tunneling currents are also observed in ultrathin-oxide samples and they become negligibly small for oxide thickness greater than 4 nm.