| VDMOS is vertical double diffusion metal oxide semiconductor field effect transistor,which has stable circuit characteristics.In structure,it is a voltage controlled gate and current path source and drain on the silicon chip.From the name,it can be seen that VDMOS is a voltage controlled device,and its working principle is to apply bias on the gate electrode,so that the P-body forms an inversion layer as a conductive channel.Because of its many excellent characteristics,such as high input impedance,wide safe working range,strong self-regulation ability,good high frequency response and no secondary breakdown phenomenon,it has a wide range of applications,broad market demand and good development prospects.Many fields have a wide range of applications,such as power switches,industrial precision control,drive motor,in modern electronic equipment plays an extremely important role.With the development of process technology,the breakdown voltage of modern VDMOS is higher and higher.Because of the thick epitaxial layer,the on resistance of conventional VDMOS increases greatly with the increase of withstand voltage.In order to reduce the on resistance of high-voltage VDMOS devices,we can only increase the chip area,but the increase of cost is a factor that can not be ignored in commercial production.At present,there are few high-voltage VDMOS products with excellent characteristics in China,and most microelectronics companies choose to import foreign VDMOS devices.With the import volume of semiconductors surpassing that of petroleum,the demand for independent research and development of high-voltage VDMOS is increasingly severe.The following is a list of the device parameters and their influencing factors that need to be considered in the design process1.Avalanche breakdown voltage bvdss and on resistance Ron: avalanche breakdown voltage and on resistance are a pair of opposite parameters.Because the breakdown of devices usually occurs at the corner of P-body region or the on breakdown of n-drift region,in order to increase the withstand voltage of devices,the usual way is to reduce the injection dose of drift region or increase the material resistivity of drift region,which will directly increase the breakdown voltage of devices Therefore,in the actual design process,these two parameters can only be simulated and optimized according to the design requirements.Finally,the appropriate drift region parameters are selected to meet the requirements of breakdown voltage and reduce the on resistance as much as possible.2.Vdnos longitudinal electric field: the distribution density of the longitudinal electric field will affect the breakdown voltage of the device.The thickness of the silicon wafer,the impurity concentration of the silicon wafer,the change of the thickness of the silicon wafer,the p-base spacing,and the junction depth of the vertical diffusion will affect the longitudinal electric field distribution of the device.The distribution structure and distribution density of the VDMOS longitudinal electric field are obtained by changing the applied bias simulation,and then the process is adjusted according to the design requirements Parameters.3.Parasitic capacitance.The factors that affect the parasitic capacitance of VDMOS devices are: the thickness of silicon wafer,the junction depth in vertical diffusion and the gate source voltage of VDMOS.One of the main indexes of this paper is the parasitic gate drain capacitance,which will be analyzed in detail in the subsequent design process.As VDMOS devices are widely used in high frequency field,the parasitic capacitance is directly related to the switching characteristics of the device.In order to reduce the switching time and the switching loss of the device,the conventional VDMOS structure is adjusted,and the structure is simulated and optimized.In this paper,the 1200 V VDMOS and its terminal structure are designed by the computer process simulation software SILVACO TCAD.The traditional design process of VDMOS is to optimize the existing VDMOS devices from the aspects of process and parameters.In this paper,a suitable new structure is selected based on the simulation of several structures.By comprehensively optimizing the process steps and cell parameters,a 1200 V power MOS with excellent performance is designed. |
