Modeling And Electrical Characteristics Of Short Channel Double-Gate And Surrounding-gate MOSFETs

With the development of semiconductor technology, the feature size of traditional two dimensional metal-oxide-semiconductor field effect transistors(MOSFETs) is reaching the physical limits, and many non-ideal effects appear. On the other hand, new multi-gate MOS devices are converting from the planar structures to three-dimensional ones such as double-gate(DG), fin-gate, triple-gate and surrounding-gate(SG) MOSFETs. The common advantages of these multi-gate devices are the enhanced channel control ability with increasing number of gates. The volume of inversion layer increases and the effect of drain-side electric field on the channel charges are screened effectively, leading to increased drain-source current and load-bearing capacity. Some non-ideal effects are thus suppressed. In order to better understand the working mechanisms and apply the good electrical properties of these novel devices, in this thesis, we investigate the electrical characteristics of short channel DG and SG MOSFETs. By establishing their physics models, the relations between the device parameters and electrical properties are discussed, and some modification methods are proposed to suppress typical non-ideal effects.Firstly, by adopting the potential decomposition method, the two dimensional Poisson’s equation of channel potential in the short channel intrinsic or lightly-doped DG MOSFET is solved, and the analytical potential model is obtained. The threshold voltage, the subthreshold current and the subthreshold slope models are then derived. Effects of the channel length, width and gate oxide thickness on the threshold voltage, subthreshold current and subthreshold slope are discussed. The TCAD simulation software Atlas is used for device simulation. The accuracy of relevant models is verified by comparisons, and the relations between the structure parameters and electrical characteristics are summarized. The results show that, by designing appropriate channel length, width and gate oxide thickness, the subthreshold characteristics of DG MOSFET can be improved and the short channel effect(SCE) can be suppressed.Secondly, based on the structure of reported short channel junctionless cylindrical surrounding-gate(JLCSG) MOSFET, its analytical potential model is obtained by introducing one-dimensional potential to the channel potential. By using Taylor expansion and making resonable simplifications when deriving the analytical expression of subthreshold current, the difficulty in direct integral is solved. Then, the expression of subthreshold slope is obtained. Effects of the channel length, gate oxide thickness and channel diameter on the subthreshold current and subthreshold slope are discussed. The possible reasons for errors between the subthreshold characteristics simulation results and the reported experimental results are analyzed. The results show that, a relatively thin gate oxide layer can enhance the gate control ability to the channel, and a large channel diameter can increase the subthreshold slope.Finally, based on the structure of reported short channel triple-material cylindrical surrounding-gate(TMCSG) MOSFET, its channel potential expression is obtained by using the similar potential decomposition method. Expressions of the threshold voltage, subthreshold current and subthreshold slope are then derived. Effects of the channel diameter, gate oxide thickness and triple-gate length ratio on the electrical characteristics are discussed. The possible reasons for errors between the subthreshold characteristics simulation results and the reported experimental results are also analyzed. The results show that, optimized triple-gate length ratio, gate oxide thickness and channel diameter can suppress the SCE and the drain-induced barrier lowering(DIBL) effect of short channel TMCSG MOSFET.As a summary, investigating the electrical models of short channel DG, JLCSG and TMCSG MOSFETs can help to explore the dependence of their electrical performances on the structure parameters, and provide useful guides to extend applications of these new MOS devices.