Distribution of built-in electric field in GaN(cap)/AlGaN/GaN(buffer) transistor heterostructures with various AlGaN thicknesses

A theoretical approach supported by experimental data (i.e., measurements of built-in electric field in AlGaN layer and two dimensional electron gas (2DEG) concentration at the AlGaN/GaN interface) has been proposed to determine the distribution of built-in electric field in undoped polar GaN(cap)/AlGaN/GaN(buffer) heterostructures. In this method the Schrodinger and Poisson equations are solving self consistently for various boundary conditions. It is clearly shown that the built-in electric field in AlGaN layer depends mainly on the surface boundary condition whereas the concentration of 2DEG at AlGaN/GaN interface depends on two boundary conditions (i.e., the Fermi-level position on GaN(cap) surface and inside GaN(buffer) layer). Comparing the measured built-in electric field in AlGaN layer with this one calculated for various Fermi-level position on GaN surface it is possible to determine the Fermi-level pinning on GaN(cap) surface. Next it is possible to determine the Fermi-level position in GaN(buffer) layer at a given distance from AlGaN/GaN interface. For this purpose the measured 2DEG concentration was compared with this one calculated for the fixed Fermi-level position on GaN(cap) surface and the varied Fermi-level position in GaN buffer layer. In this way the distribution of built-in electric can be determined for real heterostructures for which the built-in electric field in AlGaN layer is measured by electromodulation spectroscopy and 2DEG concentration is extracted from Hall measurements. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim