Study On The Structure Of High-Voltage LDMOS

With the development of electronics devices,high voltage power MOS devices are widely used in circuit design.Among many devices,the research on high voltage Lateral Double-diffused Metal-Oxide-Semiconductor(LDMOS)is one of the main focuses.It can reduce switching loss,improve switching speed and be easily integrated.As a high voltage device,the goal of high voltage LDMOS device is to improve the BV and reduce the R_on.Thus,scholars mainly do the research of LDMOS on the structure,material and fabrication process of the device.In addition,physical models and circuit models of the device also provide strong support for IC design.Therefore,it is important and meaningful that the high voltage LDMOS is taken as the object of the study.At first the thesis explains the research significance of LDMOS,and then,illustrates the basic structure,working principle and research status.And,it analyzes the structure characteristics,material properties and operating mechanisms of Silicon-on-Insulator(SOI),Trench,field plate and Super Junction(SJ),when they are used in the LDMOS design.Secondly,a trench SOI LDMOS with the convex buried oxide layer(CBOX SOI LDMOS)is proposed,which is characterized by an additional oxide area on the buried oxide layer(BOX).The electric field strength is higher because of the CBOX.Moreover,due the shortened electronic flow path and the deeper depletion of the N-drift region,the R_on of the device is reduced.In contrast to the conventional trench SOI device,the Figure-of-merit value(FOM,=BV²/R_on)increased by 64.6%for the proposed structure.Then,a trench SOI LDMOS with CBOX and P-type pillar is proposed.Compared with CBOX SOI LDMOS,the proposed device can increase the BV by 23.6%and the R_onby 61.1%.Next,a trench SOI LDMOS with CBOX and vertical field plate(VFP)is proposed.The device performance was not obvious improved,which means that the combination of CBOX and VFP is not appropriate.Thirdly,a semi-elliptic trench SOI LDMOS is proposed.The structure is characterized by a semi-elliptic trench between the gate and drain at the top of the silicon layer.Due to the semi-elliptic trench,the surface electric field of the device produces a sharply elevated peak,and at the same time,pulls down the electric field peak of the source and drain.Moreover,the concentration of holes on the surface of BOX is also increased,so the higher electric field can be introduced into the BOX layer.As a result,the BV is improved from 432V of the conventional structure to 526V of the proposed device.The FOM is improved by 16.8%for the novel structure compared with the conventional trench SOI LDMOS.Finally,the thesis summarizes the all the work during the graduate stage and point out the problems.Moreover,the research trends is also planned as LDMOS with novel material,new fabrication process and in high frequency field.