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  5 HEMT design and fabrication 

  5.2 Materials perspective

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III-N material based devices have gained more attention, due to their good thermal conductivity, high breakdown field and high saturation velocities (see Tab. 5.1). But GaN  based  devices  are  facing many problems because bulk GaN substrates are not currently available. To improve material quality and be able to dissipate heat in high-power devices the structures are hetero-epitaxially grown on foreign substrate. Presently, GaN devices are fabricated on sapphire (in spite of a significant lattice mismatch), Si(111) (which shows better thermal conductivity but larger lattice mismatches than sapphire), or SiC (with better matches, excellent thermal conductivity, but still highly defective and at prohibitive price).⁽¹⁾

Fig. 5.3 Summary of Si, SiC, and GaN relevant material properties.⁽³⁾

Even when GaN substrates become available, they will not provide as much heat removal  capability as SiC substrates (Fig. 5.3). This suggests that the most efficiently cooled GaN HEMTs will be those grown on SiC and that the defects, which arise from the heteroepitaxy of GaN on SiC, must be reduced or their influence nullifie.⁽²⁾

Tab. 5.1 Comparison between intrinsic properties of Si and WBG semiconductor materials.⁽³⁾

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