Lateral High Voltage DMOS Bulk Electric Field Optimum And New Structures

High voltage DMOS is one of the key devices of Power Integrated Circuit (PIC).As in case of power semiconductor devices,the most important feature is blockingvoltage capability.For lateral power device,two-dimensional blocking voltage orelectric field optimum is its instinct point.Researchers have done enormous andeffective work on improvement lateral blocking voltage or lateral electric field optimum.However,further research should be done in question on longitudinal blockingcapability or longitudinal electric field.To optimize two-dimensional electric field,the model of bulk or oftwo-dimensional electric field optimization is proposed in this dissertation.The model isdefined as enhancing device's holding-up capability and optimizing bulk electric fieldas the same as surface field,that is reducing high field and decreasing low field by fieldmodulation of 2D and 3D affixation field through new structure.Under the instructionof the model,the longitudinal junction terminate technique is presented.The techniquecan be described as,to improve the breakdown voltage by field modulation of the spacecharge in the depleted multi-buried-layer,the field peak is reduced and the breakdown ispostponed.In this dissertation,the bulk field optimization mechanism is applied inRESURF LDMOS and Super Junction LDMOS (SJ-LDMOS),then,three kinds oflongitudinal devices are investigated in detail.(1) The LDMOS with longitudinal floating buried-layer (FBL-LDMOS):A novelstructure with floating buried-layer embedded in p-type substrate is proposed.Bulk andsurface electric field is optimized because the field modulation of the buried-layer.Thesimulation results indicate that the breakdown voltage and the figure of merit of the newstructure are increased by 79% and 136% respectively when length of drift region,driftthickness and doping concentration of substrate are 100μm,2μm and 1.5×10¹⁴cm^-3.Based on the simulation results,the practical implementation of the 850V breakdownvoltage FBL-LDMOS with 80Ω·cm resistivity substrate and 4μm drift layer thickness isachieved,at the same time,the breakdown voltage of the conventional LDMOS is 650Vand the increasement rate is 31%. (2) The LDMOS with longitudinal uniform multi-buried-layer (LJT-LDMOS):Anew LDMOS with longitudinal uniform multi-buried-layer is presented.Based on thelongitudinal junction termination technique,the new field generated by theburied-layers is reversed to the high field around the drain side,as a result,the highfield is reduced.Simulation results show that the breakdown voltage of the novelstructure is improved by 111%.To gain more uniform electric field distribution,theLDMOS with longitudinal nonuniform multi-buried-layer (NUMF-LDMOS) isproposed.The novel LDMOS with more uniform field,higher reliability,the higherbreakdown-voltage is achieved because of the field modulation of the different lengthand width multi-buried-layer.Simulation results indicate that the breakdown voltage ofthe novel structure is improved by 133%.(3) The SJ-LDMOS with stepped doping buffered-layer (SSJ-LDMOS):A novelSJ-LDMOS with stepped doping buffered-layer is proposed.The nonuniformsubstrate-assisted-depletion effect in lateral super junction DMOS is eliminated by thestepped doping buffered-layer.A series of field peaks are generated and the bulk field isfurther optimized because of the doping concentration difference between the neighborsteps.Simulation results show that,contrasting with the conventional structure,thebreakdown voltage of the proposed structure (The step number is 3.) is improved from121.7V to 644.9V with same 1×10¹⁴cm^-3 substrate doping concentration,6×10¹⁶cm^-3 SJregion concentration and 48μm drift length.Except the SSJ-LDMOS,a SJ-LDMOSwith a floating buried-layer (FBSJ-LDMOS) is also presented.The bulk fieldoptimization mechanism is applied into the proposed 3D super junction lateral DMOS.On one hand,the n-type floating buried-layer is embedded into substrate,the bulk fieldis optimized and a new reverse PN junction sustains most of vertical voltage,then highvertical breakdown voltage is achieved.On the other hand,thesubstrate-assisted-depletion effect is eliminated by the buried-layer,the high breakdownvoltage and low on-resistance advantage of the super junction is realized.Simulationresults indicated that,on contrary to the buffered SJ-LDMOS,the breakdown voltage ofthe FBSJ-LDMOS is increased from 604V to 932V,by 54% with same 60μm driftlength.The experiment of the presented FBSJ-LDMOS shows that the breakdownvoltage is 580V with the 40μm drift length.Furthermore,contrasting with that of theSJ-LDMOS fabricated in conventional technology,the figure of merit of the new structure is 5.84×10⁵V²/(Ω·cm²),and increased by 77.5%.