Theoretical investigation of electrical characteristics of AlGaN/GaN modulation doped field-effect transistors

Abstract

A theoretical investigation of the electrical characteristics of GaN/AlxGa1−xN (x is the Al mole fraction in AlGaN) modulation doped field-effect transistors (MODFETs) is carried out. Using a self-consistent solution of Schrödinger’s equation and Poisson’s equation, relations between the concentration of two-dimensional electron gas (2DEG), the Fermi level in GaN, and the average distance of the electrons from the heterointerface are calculated. A relation between the gate bias and the 2DEG concentration is obtained for a flat quasi Fermi level in AlxGa1−xN. Based on the relation between the 2DEG concentration and the applied gate bias, a model for the drain current and the transconductance of the device is developed. The effects of the n-AlxGa1−xN layer thickness, the spacer i-AlxGa1−xN layer thickness, the n-AlxGa1−xN doping level, the aluminum mole fraction x, and the channel length L on the electrical characteristics of MODFETs are presented. Theoretical results are compared with the recent experimental data, which show striking agreement. The 2DEG concentration is found to be as high as 1013 cm−2, and transconductance as high as 1000 mS/mm. Finally, the effect of the difference in the properties of AlN and GaN, and a device structure that would take advantage of the bending of the quasi Fermi level for electrons in n-AlxGa1−xN, are discussed.