The Electrical Safe Operation Area And Thermal Safe Operation Area Of SOI LDMOS

The thermal failure of device has become a critical problem with the development of thepower integrated circuit (PIC). As a result, safe operating area (SOA) is important to the designof device and the optimization of performance. The technology of Varied Lateral Thickness(VLT) is proposed recently to achieve a good trade of the breakdown voltage and on-resistance.But the SOA characteristics of power device based on the technology has not been study to thisday. This paper employ two-dimensional semiconductor device simulation software, Silvaco, toinvestigate the SOA of SOI VLT LDMOS under different bias condition. The main results are asfollows:1) The optimization of device structure. The breakdown characteristics of SOI RESURFLDMOS and SOI VLT LDMOS are investigated using Silvaco. The silicon thickness of sourceregion, the silicon thickness of drain region, the buried oxide thickness, the drift region dopingconcentration and the length of the drift region are optimized to maximize the breakdownvoltage. Then, the basic characteristics of the two structures are simulated, including thethreshold voltage, subthreshold characteristics, output characteristics, on-resistance, and etc. Theresults show that the VLT LDMOS presents the high breakdown voltage, the improved operatingcurrent, the reduced on-resistance , and the steep subthreshold slope compared to the RESURFdevice.2) The simulation of static electrical safe operating area. Neglecting the impact of ambienttemperature and the self-heating effect, the output characteristics of the both optimized devicesin DC steady state are investigated and the static electrical safe operating area is extracted. Theresults show that the VLT technology has a significant large safe operating area and is moresuitable for high power applications because it eliminates the Kink effect of device effectivelywhen operating in saturation region.3) The simulation of static thermal safe operating area. The software Silvaco is used tosimulate the static thermal safe operating area and investigate the impact of ambient temperaturewhen considering the electro-thermal effect. The results show that the both output curves exhibitthe obvious snapback phenomenon. The static thermal safe operating area also shrinks with theincrease of the ambient temperature. Compared to the RESURF LDMOS, however, the VLTLDMOS can reduce the phenomenon of negative resistance and weaken the self-heating effect,resulting a larger static thermal safe operating area.4) The simulation of dynamic thermal safe operating area. The software Silvaco is also usedto simulate the dynamic thermal safe operating area in pulse bias condition. The impact of ambient temperature and pulse width on SOA are investigated in detail. The results show that thesnapback phenomenon in output curve eliminates under the pulse gate-biased voltage for theboth devices. The dynamic thermal safe operating area of the two devices also shrinks with theincrease of the ambient temperature. Moreover, the conventional RESURF LDMOS presents anevident self-heating effect in saturation region and a sharp reduction of on-state breakdownvoltage with the increase of gate voltage. On the contrary, the dynamic thermal safe operatingarea characteristic of the VLT LDMOS is better under the same ambient temperature. But thesaturation current of the VLT structure reduce more in a wide pulse gate-bias condition.