Two-carrier conduction in MOS tunnel oxides—1 Experimental results

Abstract

A novel device structure incorporating a p-channel MOSFET with a metal/tunnel-oxide/ n-silicon device is proposed as a tool for separating electron and hole tunneling currents in ultra-thin silicon dioxide films. With this structure, the electron and hole tunneling currents can be independently measured at the substrate and source terminals, respectively. Furthermore, the injected minority carrier (hole) current which is supplied by the p-MOSFET can be varied independently of the tunnel-oxide bias. As expected, the injected hole current modulates the electron current and “current multiplication” was observed. By correlating experimental results for 22.5 Å SiO 2 films with theoretical calculations, the electron and hole barrier heights were determined to be 3.2 and 3.6 eV, respectively, where a trapezoidal tunneling barrier was assumed and a carrier effective mass of 0.5 m 0 was used. The tunnel-oxide quality and uniformity was evaluated by measuring I–V and C-V curves on two-terminal MOS capacitors of various areas. The results suggest that the oxide films are extremely uniform in thickness, and the measured interface trap density was determined to be less than 10 11 cm −2eV −1. For the reverse-biased tunnel-oxide, the electron/hole current ratio was found to be less than unity except for the condition when the injected hole current was very small compared to the electron current without any hole injection. In addition, this ratio was found to decrease rapidly with increasing oxide thickness and/or increasing hole injection level.