Evidence of hole direct tunneling through ultrathin gate oxide using P+ Poly-SiGe gate

P+ poly-Si and poly-Si0.75Ge0.25-gated PMOS transistors with ultrathin gate oxides of 25 and 29 Angstrom were used for this study. The difference in t he gate work function was used to determine the mechanisms of gate tunnelin g current in such thin gate oxides. Under negative gate bias (inversion bia s), it was found that the source/drain terminal serves as a source of holes for small V-g value, and as gate bias increases (more negative), it become s a hole sink, These observations can be interpreted in terms of two compet ing mechanisms. For the first time, hole direct tunneling is reported. Hole direct tunneling is the dominant mechanism for -2 V< V-g < 0 V. For V-g < -2 V, electron direct tunneling is dominant. Electron-hole pair generation by the tunneling electrons starts to dominate over hole direct tunneling on ly for V-g < -4 V.