15 20 25 30 35
0 25 50 75 100
Uniaxial Tensile-cap compared to Control Si
N-MOSFET
Enhancement (%)
Fig. 5.11 The current enhancement rate of uniaxial tensile-cap compared to conventional structure.
Chapter 6 Conclusions
The carrier transport mechanism in the Schottky-barrier MOSFETs is a confusing issue in the past years. In recent years, we have confirmed the carrier transport mechanism by effective Schottky-barrier height variation. In this thesis, the effective Schottky-barrier height in the DSI-SBMOS has been investigated. When the SBMOS operates in OFF-state, the carriers move from source into channel by minority carriers tunneling mechanism. In the subthreshold region, the carriers thermally emit over the Schottky barrier due to its high effective Schottky-barrier height in both conduction and valance band. In ON-state, the most of the carriers tunnel through the Schottky barrier that leads to negative effective Schottky-barrier height.
On the other hand, some of the DSI-SBMOS with the different dopant-segregation -implantation energy have performed that the effective Schottky-barrier height is contributed by the channel barrier. Some characteristics of the DSI-SBMOS, such as threshold voltage and OFF-state leakage, are determined by the channel barrier. We may conclude that the Fermi-level-pining position is transferred from source-channel interface to the interface between DSI-layer and substrate.
Besides, the ballistic transport characteristic in the channel is performed by effective ballistic mobility successfully. We concluded that the carrier transport of DSI-SBMOS in the channel in low field region has larger backscattering probability than conventional MOSFET as a result of its complicated barrier which leads to large subthreshold swing. In addition, the backscattering probability in the high field region is from the ratio of the thermal injection velocity and carrier average velocity. We found that probability is smaller in SBMOS than that in the conventional device due to carrier non-local tunneling mechanism. By non-local tunneling mechanism, carriers have higher transport energy that contributed to larger transmission rate from source to channel.
In addition, we compared the backscattering characteristic in the DSI-SBMOS and
strained-Si devices. For strained-Si devices, strained-Si technology will enhance the thermal injection velocity, while it affects the backscattering coefficient lightly. For DSI-SBMOS, it enhances the backscattering coefficient instead of thermal injection velocity. If we combine these two technologies, we can get better backscattering characteristic that brings the devices close to carrier transport limitation.
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