FOWLER-NORDHEIM TUNNELING IN IMPLANTED MOS DEVICES

Current-voltage curves of thermally oxidized silicon with polysilicon gates have been measured and interpreted with the Fowler-Nordheim model of tunneling of electrons through the energy barrier at one of the interfaces of the dielectric. The measurements were done on wafers which have been implanted with various doses As** plus ions. From the F-N curves the barrier height and the pre-exponential term have been extracted. The barrier height and the pre-exponential term decrease substantially if the dose was larger than 10**1**4/cm**2. Although the Fowler-Nordheim plots shifted and had different slopes, it is shown that the expression following from the model is still applicable. It was verified whether the current density was homogeneous over the area. When however the As** plus implantation was performed with the gate as a mask, the Fowler-Nordheim curves turned out to be kinked. This kinked curve suggests that it is composed of two straight lines. The steep part of the curve is thought to correspond to the unimplanted region under the gate and the flat part to the implanted region at the side-wall. This phenomenon can have important implications for self-aligned source and drain implantation techniques. The lowering of the barrier and the pre-exponential term by implantation is interpreted in terms of trap-assisted tunneling.