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industry  wide bandgap electronics


state leakage current. Our simulations employ Schottky contacts for the source and drain. Electron tunnelling is turned on and the electron tunnelling mass is made to be arbitrarily small so the contact is essentially ohmic. Taking this approach improves convergence and avoids abrupt band bending near the contacts. The tunnelling mass can also be used to tune the contact resistance.


The simulations that we have performed reveal that a breakdown voltage in excess of 600 V is possible for a


HFET with an 8 µm field plate and gate-drain spacing


of 18 µm – such a device promises to make a


Figure 6.


The meshed n-type SiC IGBT with


n+ polysilicon gate and


50nm SiO2 gate insulator. For clarity, middle portion of the drift region is truncated


commercial impact on the power device market (see Figure 3 for a plot of the off-state leakage curve). A


similar structure with a 1.8 µm field plate had good gate control at a drain voltage of 15 V (see Figure 4), and exhibits negative output conductance due to self- heating (see Figure 5).


SiC IGBTs


In the power industry, the silicon IGBT is widely used, thanks to its combination of a very low on-state resistance and superior on-state current density at high voltages. These attributes also hold for SiC equivalents that promise to increase operating voltage range.


In reality it is still not clear whether the polarization charge is


compensated by fixed charges or interface trap states.According to our simulations,accumulated holes at the surface of the AlGaN barrier are completely compensated by deep, single-level trap states We have performed three-dimensional simulations with


a 4 µm by 4 µm domain over a trench SiC IGBT amenable to current SiC process technology (see Figure 6 for details of the device architecture). Due to the relatively low doping level of just 6 x 1014


cm-3 in this


long drift region of about 160 µm, very high breakdown voltage is expected.


Simulations re-enforce this expectation, suggesting that the breakdown voltage should exceed 12 kV (see Figure 7). To perform this simulation, we turned to extended precision arithmetic (80-bit) to resolve the extremely small off-stage leakage current that is present before the onset of avalanche breakdown.


As GaN and SiC devices gain market traction and target a growing array of applications, simulation will play an ever-increasing role in fully exploiting the excellent set of attributes of these wide bandgap materials. The design of power devices has many degrees of freedom, opening the way to radical device structures and the optimisation of current designs, all of which are well supported by simulation.


In the future, modelling of fabrication process in SiC and GaN technology will become more important, as not only structural details but also process conditions will be subject to optimisation. And since the performance of power modules is tightly coupled to the performance of the discrete power devices assembled in the module, simulation efforts allowing the co-design of devices and modules will see the use of TCAD data in the characterization of behavioural models for circuit and system-level design.


© 2011 Angel Business Communications. Permission required.


Figure 7.The n-type SiC IGBT has an incredibly high breakdown voltage.This curve also catches snapback,a sudden lowering of the device internal resistance as the collector current increases.The initiation of the avalanche breakdown at the bottom corner of the trench is captured in the inset


24 www.compoundsemiconductor.net October 2011


FURTHER READING J. Joh and J. A. del Alamo, IEDM Tech. Dig 415 (2006) O. Hilt et al., Proc. ISPSD 347 (2010)


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