Study On Modeling And Simulation For Uniaxially Strained Si NMOSFET

To improve the performance of silicon device further, in addition to using novel device structure, several new materials and technologies are applied to the design and fabrication of devices. Because of the higher mobility, adjustable energy band gap and compatibility with traditional bulk Si technology, silicon-based strain technology is regarded as an important method applied to the high speed and high performance device and circuit. Moreover, for the convenience of CMOS circuit fabrication the uniaxially strained Si technology is more attractive.Nevertheless, following with the performance improvement of the uniaxially strained Si technology the I-V model, C-V model, subthreshold region model and gate current model have new changes comparing with those of traditional bulk Si device. With the method only to change the values of existed model’s parameters, it is difficult to describe all new characteristics of the uniaxially strained Si device. And the electrical characteristics already described are not self-consistent too, so the accuracy of SPICE simulation cannot meet the application request. It is necessary to develop device model based on the mechanism of strained technology. For this purpose, this dissertation is mainly focused on the modeling and simulation for the uniaxially strained Si device and circuit. The author’s major contributions are outlined as following:1. The structure and working principle of uniaxially strained Si NMOSFET has been investigated. After analyzing the influence of uniaxially strain on effective mass and carrier scattering rate, the mobility model of uniaxially strained Si NMOSFET has been established, which can describe the physical essence and electrical characteristics of device accurately. At the same time, the relationship between threshold voltage and stress intensity is put forward, the influences of short channel effect, Drain Induced Barrier Lowering effect, narrow channel effect, substrate bias and so on are completely evaluated and fully implemented into threshold voltage model.2. Beginning with the basic Drift-Diffusion equation, the channel current model is developed for different bias conditions. Especially, for the subthreshold current model it is based on the charge of weak inversion but not the normal effective channel thickness approximation. The simulation results with the model match the experimental results well, which validates the accuracy of the model. The model can be applied in strained Si circuit’s SPICE simulation; it also provides helpful reference for the design of uniaxially strained Si device.3. Based on the charge models under different operating conditions, the 16 differential capacitance models for uniaxially strained Si NMOSFET are developed, the simulation results with those models match the experimental results well, which validates the accuracy of the models. Meanwhile the simulation results of key gate capacitance Cgg with respect to stress intensity, bias voltage,channel length and concentration of poly gate are obtained and analyzed. At last the models for parasitic components, source/drain diodes and temperature effect models are provided.4. Based on the physical process of generation of the hot carrier effect and quantum mechanics for the tunneling, a model of hot carrier gate current and tunneling gate current for uniaxially strained Si NMOSFET is developed. With that model, the simulation results of gate current against stress intensity, gate-source bias, channel doping concentration, and drain-source bias are obtained and analyzed. The relationship between life time of time-dependent dielectric break down(TDDB) and gate-source bias is simulated and analyzed too.5. In the model of direct current(DC) characteristics, transient characteristics, alternating current(AC) characteristics, subthreshold region characteristics and gate current characteristics, the stress intensity is introduced as an important model parameter, which is convenient to observe the relationship between device’s electrical characteristics and the stress intensity directly. The model containing stress intensity has clear physical meaning which is helpful for the analysis and design of devices.6. Based on the equivalent circuit, model parameters, model equations of uniaxially strained Si NMOSFET, with verilog A language the Device Model File has been created as a plug-in for SPICE simulator, which can be used as same as simulator’s built-in models. This method is more flexible to develop a new model and update one model to new version. By extracting the model parameters with tool Para Plus++ developed by the research group synchronously, comparing the simulation results and experimental results, the accuracy of the model has been validated, which solves the issue that commercial simulator can simulate uniaxially strained Si circuits accurately.