Modeling and simulation of GaN-based high electron mobility transistors

Microwave power transistors are critical components of modern systems ranging from commercial wireless communications to military radar systems. As the requirement for faster, more powerful devices is increasing, it is clear that traditional technologies, such as GaAs MESFETs, InP DHBTs, CMOS, LDMOS, etc., cannot keep pace with the demand for improvement in these areas. GaN-based transistors have been successfully demonstrated for applications in high power, high temperature and high speed circuitry and promise to become the dominant family of RF devices into the near future. However, efforts to model GaN-based transistors have not kept pace with fabrication. Operation of GaN-based devices, such as AlGaN/GaN HEMTs, is critically dependent on the existence of surface charges as well as polarization charges. To date, calculation of the piezoelectric polarization in such a HEMT has been limited by a restrictive assumption of a stress free AlGaN-barrier layer. This dissertation provides the complete formulation for the piezoelectric polarization concomitant with a triaxial stress formed in the AlGaN-barrier layer. This is extended to a more complete formulation of electro-mechanical coupling which allows the piezoelectric polarization to be subject to modification by electric-fields. Moreover, this improvement in the formulation of electro-mechanical coupling provides a fundamental method to include the pressure dependence of various parameters due to the physical deformation of the crystal lattice in the presence of the large built-in electric fields. To illustrate this, the pressure dependence of the band gap in the AlGaN-barrier layer, due to the triaxial stress resulting from electro-mechanical coupling, is demonstrated.^ It is also highly desirable to include the most up to date model accurately in simulations carried out in commercially available platforms. To date, reports of such simulations have been few, while all have neglected to properly account for surface and polarization charges both in their magnitudes as well as locations. This dissertation also focuses on the development of 2D simulations of GaN-based HEMTs properly accounting for the theoretical surface and polarization charge distributions. This includes the modification of the traditionally predicated piezoelectric polarization due to the inclusion of triaxial stress and complete electro-mechanical coupling.