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wide bandgap electronics  industry


vehicles is spurring the development of a new breed of power electronic system. In turn, this is creating new opportunities for the semiconductor power device technologies lying at the heart of most of these systems.


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Examples of such activities include increased efforts to harness the power of the sun and wind, which have motivated the development of more efficient inverters. Similarly, future power grids will require new power- switching and flow-control technologies to manage and distribute increasingly diverse energy sources, including renewables with seasonal and diurnal variation. And in transportation, it is possible to increase the driving range of hybrid and electric vehicles with more efficient inverters and converters that can operate at a higher temperature, require less cooling, and enable a reduction in the weight of the car.


Silicon power devices are dominant in today’s power electronics and they will continue to evolve. However, wide bandgap alternatives based on SiC and GaN are now starting to fulfil their long-held promise for high- power, high-temperature applications, and they are gaining traction in applications beyond the performance envelope of silicon.


At a given breakdown voltage, silicon is inferior to both SiC and GaN in terms of on-resistance, which is a key figure of merit in power switching applications. In the case of SiC, whether used as a device technology or as a substrate for GaN devices, its higher thermal conductivity improves heat dissipation. What’s more, in SiC and GaN, the low intrinsic carrier concentration resulting from the large energy gap allows device operation at higher junction temperatures. Both effects simplify heat sink design and cooling systems and could unleash a range of products setting new benchmarks for affordability, size and weight.


Recently, an increasing interest in GaN and SiC has spawned many new device designs that – when combined with improved processing techniques, higher quality SiC substrates, and lower-defect GaN heteroepitaxy – have led to promising demonstration devices. But there remains a strong impetus for further optimisation of device design and the tailoring of device characteristics to a wide range of applications.


One tool that engineers can use to develop novel device structures and exploit the benefits of wide bandgap semiconductors to the full is technology


rowing interest in greener forms of electricity generation and fuel efficient


computer-aided design (TCAD). At Synopsys, which is based in Mountain View, CA, we have developed software capable of doing precisely that – the Sentaurus Device simulator. In this article we illustrate the capability of this tool through simulations of a normally off GaN HFET and a SiC insulated-gate bipolar transistor (IGBT) designed to meet low loss power switching applications.


Simulating GaN and SiC devices presents a set of challenges that are not faced when working with more common semiconductors, such as silicon and GaAs. One of these is the vast range of values for some of the characteristics associated with wide bandgap materials. For example, the intrinsic carrier concentration innate in GaN and SiC is incredibly low, but the doping levels of contact and cap layers can be very high. For accurate simulation of leakage currents and the onset of avalanche breakdown, the simulator must be capable of numerically resolving 25 or more orders of magnitude.


Recent versions of Sentaurus Device address this issue through extended precision arithmetic, which improves the relative accuracy of the numerical resolution.


Normal 64-bit floating-point representation has a relative accuracy of 2.22 x 10-16


. 80-bit extended precision


arithmetic, which is supported in hardware with no noticeable degradation in performance, has a relative accuracy of 1.08 x 10-19


. In contrast, moving up to


128-bit and 256-bit improves the relative accuracy to 4.93 x 10-32


and 1.22 x 10-63 , respectively, but at the expense of longer simulation time.


Figure 1.4H-SiC on-axis,60 keV doses (0.63,1.3,3.4,4.9) x 1012


,aluminium implant at cm-2


.Since the


implant is performed on an on-axis wafer,deep channelling tails are created - their close match to the experimental SIMS profile attests to the accuracy of the Monte Carlo implant model


October 2011 www.compoundsemiconductor.net 21


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