Chapter 2 - GaN-Based Modulation Doped FETs and Heterojunction Bipolar Transistors

Abstract

This chapter focuses on modulation doped field-effect transistors (MODFETs) based on the gallium nitride (GaN) material system. With its reduced impurity scattering and unique gate capacitance-voltage characteristics, the MODFET has become the dominant high-frequency device. Among the MODFET's most attractive attributes are close proximity of the mobile charge to the gate electrode and high drain efficiency. In MODFETs, the carriers that form the channel in the smaller bandgap material are donated by the larger bandgap material, and ohmic contacts or both. Unlike the GaAs-based MODFETs on surfaces, polarization-induced charge in the GaN-based devices on the polar surfaces is quite large. An in-depth discussion of polarization effects for normal and inverted MODFETs is presented and experimental data and theoretical results is provided on the particulars of the interface charge in relation to parameters such as the AlGaN mole fraction and thickness. In addition, calculation results for electron distribution and current-voltage characteristics of MODFETs are presented.