| Silicon carbide(SiC)gate turn-off thyristor(GTO)is currently a wide bandgap power device with excellent performance and more widely used SiC-based power devices.Compared with silicon(Si)thyristors,SiC GTO thyristors have high blocking voltage,lower leakage current,higher operating temperature and faster switching speed.As a result,SiC GTOs are very suitable for high-temperature,high-voltage and high-current application areas,such as UHVDC power transmission,smart grid and pulsed power.However,the impurities in 4H-SiC are incompletely ionized at room temperature.Therefore,the injection efficiency of the p+n emitter junction of SiC GTO is far less than unity,resulting in a limitation of device performances.Meanwhile,SiC GTOs in high-voltage and high-current applications will inevitably lead to the risk of device failure due to dV/dit,and related investigations of SiC GTOs are rarely reported.Therefore,in order to improve the performance of SiC GTO,a variety of device structures are proposed to improve the performance,dV/dt imunitity,high voltage termination strcture tolerance and experiment is implemented.The main research contents and results in this article are as follows:1.For conventional SiC GTO with p blocking layer,the effects of temperature,minority carrier lifetime of p+emitter,thin n base,p blocking base and p+buffer layer on the electrical characteristics of SiC GTO is investigated.The results show that as the temperature increases from 300K to 600K,the forward voltage drop(at JK=-100A/cm2)of the SiC GTO is reduced by 20%,which illustrates the forward voltage drop has a negative temperature coefficient.Besides,the turn-on time is shortened by 52%,and the turn-off Time increased by 24%.The minority carrier lifetime of the p+emitter is increased from 3ns to 10ns,the turn-on time is shortened by about 64%;With the thin n base minority carrier lifetime is increased from 30ns to 400ns,the turn-on time of the SiC GTO is reduced by 24%.And the carrier lifetime of p blocking base and p+buffer layer are 5.0μs and 1.0μs,respectively.2.Two new GTO thyristor structures with an n-n-base and n--n-n-base are proposed.The former mainly improves the transport coefficient of the thin base region and improves the turn on of SiC GTO;the latter increases the injection efficiency of the top p+n emitter junction on the basis of improving the base transport coefficient,and further shortens the turn-on time of SiC GTO.Compared with traditional structure,the n-n-SiC GTO due to the existence of the high-low junction,an electric field directed from the high-doped region to the low-doped region will be generated.This electric field will promote the minority holes cross over the thin base by drift action and thus reduce the carrier recombination.As a result,the transport coefficient of the n base region is increased,which increases the current amplification factor of the top p+np transistor,resulting in a significant improvement in the turn-on characteristics of the SiC thyristor.The simulation results show that by optimizing the structure parameters of the n-nbase region,when the thickness of the n-lightly doped region 1.4μm and the doping concentration 2x1015cm-3,the turn-on time is shortened to one-third of the turn-on time of traditional SiC thyristor.To further improve the injection efficiency of top p+n emitter,based on n-n-base,the n base region of the device structure is composed of low-high-low-doping(n--n-n-base).The introduction of the upper low-doped region increases the concentration gradient between the p+emitter region and the n region of the SiC GTO thyristor,thereby improving the injection efficiency of the p+n junction.Finally,the injection efficiency and the transport coefficient of the base area are both affected.The improvement increases the current amplification factor of the top p+np transistor,resulting in a significantly shorter turn-on time,especially the rise time.Therefore,the on-current rise rate di/dt of the SiC GTO thyristor is improved.The results show that through optimization,the turn-on time of the improved structure can be reduced to 145ns,and the corresponding turn-on current rise rate di/dt is 948A/cm2/μs.Compared with the traditional SiC GTO thyristor,the turn-on time is reduced by 72%,and the turn-on current rise rate di/dt is increased by 400%.On the other hand,combined with the forward blocking and turning off characteristics of the device,the results show that the improved SiC GTO can significantly improve the turn-on performance of the device without sacrificing other performance.3.An n blocking base SiC GTO structure with n+p reverse bias junction on the anode side is proposed to enhance the switching speed of the device.During turn on,the transport coefficient of thin base is improved and increase the turn on speed of SiC GTO.During turn off,the reverse biased n+p junction promotes the extraction of carriers,resulting in the turn off time is reduced.Compared with the traditional n blocking base SiC GTO thyristor,the n base has a high-low doping structure,which will generate an electric field to promote the transit of minority electrons over the short base,reduce recombination,and improve the transport coefficient of the base.Thereby improving the current magnification of the top n+pn transistor and increasing the turn-on speed of the device.Additionally,the back side introduces alternating n+ and p+ regions,the p+ region on the back side is replaced by the n+ region reduces the injection efficiency and weakens the conductance modulation effect of the long base region.More importantly,the n+ region on the back and the upper p-type region form a reverse-biased pn+junction,which will accelerate the extraction of electrons during the turn-off period.Therefore,the turn-off speed of the device is greatly improved.The results show that the turn-on time and turn-off time of the improved structure are 37ns and 783.1ns,respectively.4.The effect of an n blocking base SiC thyristor with cathode short-circuit unit on the improvement of dV/dt tolerance is explored.Firstly,the theoretical analysis of dV/dt triggered on SiC thyristor is carried out.Compared with the traditional structure,the introduction of the short-circuit point on the cathode side effectively reduces the injection efficiency of the n+p junction,and makes most of the displacement current generated by the J2 junction directly reach the anode electrode through the p+region,so as to prevent the injection of electrons in the n+region.Moreover,the p+region and the p base forms high-low junctions,and the generated electric field is also conducive to the collection of holes.Therefore,only when the applied voltage is very large,it is possible to generate enough displacement current to trigger the conduction of SiC thyristor.Secondly,the triggering effect of anode voltage pulse rise rate on traditional structure and new structure is studied.The results show that the dV/dt tolerance of the traditional SiC thyristor is about 700V/ns,while the dV/dt tolerance of the improved structure can be increased by about 12 times,and has little effect on the forward conduction characteristics.Therefore,the new structure has higher dV/dt reliability and is more suitable for high voltage or UHV applications.5.The terminal structure and layout are designed,as well as experiment of n-n-SiC GTO are carried out.Firstly,an improved charge modulation JTE terminal structure suitable for ultra-high voltage SiC GTO is designed.Its advantage is that on the basis of no additional process,modulation rings are introduced into the inner and outer of JTE3 of traditional 3S-JTE,which makes the transverse doping dose distribution closer to the gradual distribution,effectively reduces the surface electric field and improves the breakdown voltage.The simulation results show that compared with the traditional 3S-JTE,the dose range of is widened by 60%.In addition,it is found that the termination has low sensitivity and good robustness to interface charge density and ion implantation depth.The experimental results show that the structural parameters of each epitaxial layer are also consistent with the design through the SEM section results.The experiment test results show that:(1)at room temperature,the knee voltage of SiC GTO thyristor is about 3.0V,and the specific on resistance during on-state is 6.08mΩ·cm2,which is close to the reported results,(2)Through the forward and reverse breakdown characteristics test,the forward blocking voltage of the device is about 7500V(corresponding to leakage current of about lμA),reaching 93%of the ideal value,and the reverse breakdown voltage is about-60V. |
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