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working at modelling other SiC processes, including oxidation, where different growth rates and interfacial fluxes occur for the silicon and carbon faces, and dopant diffusion takes place at very high temperatures. We detailed our calibration of SiC physical models applied to TCAD simulations three years ago in this magazine (see Compound Semiconductor, October 2008, p 31).


Figure 2. A cross-


section of the simulated HEMT structure (gate region)


Catering for crystallinity…


Simulations of wide bandgap power devices must also cater for intrinsic properties specific to each material. Except for its cubic 3C polytype that has limited commercial application, all polytypes of SiC have hexagonal crystal structures. Consequently, anisotropy features in many important physical parameters – mobility, impact ionization, thermal conductivity, effective mass and electrical permittivity.


What’s more, doping techniques employed for SiC can also differ from those used for other semiconductors. In- situ doping of epitaxial layers is common, especially for low-doped regions, but ion implantation is gaining popularity in modern SiC devices. Here, too, the technology differs from silicon.


In SiC, ion implantation tends to be carried out at a high temperature to activate the dopants during the implant. Fortunately, models for ion implantation in SiC have substantially improved over the years, and our latest simulation tool accounts for wafer off-axis angle and temperature. Thanks to these refinements, our simulations of dopant concentration are a good match to real data (see Figure 1). We are also currently


…and polarization In GaN devices – particularly HFETs, where a two- dimensional electron gas functions as the conduction channel – it is paramount that models handle polarization effects. The polarized wurtzite crystal structures of AlGaN, InGaN and GaN have dipoles across the crystal in the [0001] direction that lead to spontaneous (pyroelectric) polarization. In addition, there is strain-induced (piezoelectric) polarization in III-N devices incorporating pseudomorphic heterostructures. The primary effect of these sources of polarization is the creation of an interface charge, which is due to abrupt variations in the polarization at the AlGaN-GaN heterointerface and at the AlGaN surface. We compute this interface charge with a built-in polarization model that accounts for spontaneous and piezoelectric components.


More recently, Jesús del Alamo’s group from MIT has reported degradation in GaN-based HFETs and postulated a link with the converse piezoelectric effect. High electric fields develop near the drain side of the gate, leading to strain relaxation through formation of mechanical defects and, consequently, the generation of electrical traps. When negatively charged, these traps cut drive currents, shift the threshold voltage positively, and increase drain access resistance. We believe that simulations can help to optimise the device design to mitigate these deleterious effects because they enable visualization of the spatial distribution of converse piezoelectric fields when the transistor is stressed.


Figure 3. Off-state leakage curve for device with


8 µm field plate and


18µm gate- drain spacing


22 www.compoundsemiconductor.net October 2011


Early efforts to develop GaN HEMTs focussed on physical characterization of trapping effects and ways to mitigate them. These traps hampered device commercialisation for many years, so it is of no surprise that early TCAD simulations focused on providing insights into the bulk and surface trapping behaviour to guide process improvements. Such efforts paid dividends, revealing the benefits of limiting carbon impurities in buffer layers and showing ways to optimise the device structure to suppress operational conditions conducive to trapping.


An important illustration of these early efforts was a simulation by us of the impact of field plates on the electron temperature in the channel. This study, which we detailed in Compound Semiconductor in 2006 (July edition, p 17), showed that field plates can reduce the


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