The piezoelectric polarization source term is implemented in a manner similar to the spontaneous polarizationa Gaussian function is used to approximate the interfacial charge density and rotation matrices are used to place the charge density at each of the three GaN/AlGaN interfaces. The only difference is that the charge density due to the piezoelectric polarization is given by the analytic expression for σ p discussed in Section 2. The analytic expression and charge density at each of the three GaN/AlGaN interfaces is computed by the piezo_analytic.m and rho_strain_analytic.m m-files, respectively.