Research On Verilog-A-Based Modeling Of LDMOS Device

As it has the advantages of withstanding high voltage, great drive power, and easily compatible with CMOS process technology, LDMOS(Lateral double-Diffused MOSFET) is widely used in the field of switching power supply, automotive electronics, industrial control, household appliances, etc. In recent years, there are more and more studies of LDMOS models, but compared with conventional MOS devices, some special physical phenomena observed in the application of LDMOS makes LDMOS modeling relatively difficult. So, it is of importance to accurately model LDMOS devices.The subject of this thesis is to analyze the LDMOS device structure and its physical characteristics and adopt approximate method based on physical derivation to build a semi-empirical compact model. To concentrated more energy into modeling itself rather than focus too much on the problem of the access to the simulator, and shorten the modeling development period, this thesis adopt Verilog-A language to implement LDMOS model.This thesis is mainly about the following tasks:1. The general situation of LDMOS modeling, the history of device modeling language, and the advantages of modeling by Verilog-A are introduced.2. Considering the model implementation method, the structure and the implementation procedure of device modeling language Verilog-A are analyzed, including grammar structure, loading model, processing input data, calculating intermediate data and model operation. The LDMOS device structure and process flow are expounded by taking a 0.18 um 25V BCD process for example. The double diffused channel and the drift structure of LDMOS which are different from conventional MOS device are especially introduced, based on these, some special physical phenomena observed when LDMOS works under high voltage, such as quasi-saturation effect and self-heating effect, are explained in detail.3. Based on the analysis of some special effects of LDMOS, semi-empirical LDMOS model is built by physical derivation and approximate treatment, including quasi-saturation effect model and self-heating effect model. The difficulty of LDMOS modeling is its drift region modeling, the drift region resistance keeps almost the same when the applied gate voltage and drain voltage are very low, so it can be regarded as a constant resistance. With the increase of gate voltage and drain voltage, the drift region resistance will be changed, thus the drift region can be equivalent to a controlled resistor controlled by gate voltage and drain voltage. As to self-heating effect, the thermal resistance and thermal capacitance network is adopted, and simplification is applied based on it, mainly considering the effect of thermal resistance on current. For the built LDMOS model, based on the Verilog-A model of BSIM4, the source code is modified, adding the quasi-saturation model and self-heating model. At last, the parameters of LDMOS model are extracted by using parameter extraction software MBP(Model Builder Program) and the simulation curves that match the measured data are obtained.