Engineering and impact of surface states on AlGaN/GaN-based hetero field effect transistors

Abstract

This work investigates the impact and origin as well as the electrical behaviour of surface states of AlGaN/GaN-based heterostructure field effect transistors (HFETs). Different GaN-cap layers were grown on top of AlGaN-barrier layers, and the active transistor surface areas were treated chemically differently. The investigations show that one relevant surface state at the AlGaN/GaN surface is that of donor-like N-vacancies. Surface plasma treatments with O2- and N2-plasmas on structures with and without GaN-cap layers reveal very different electrical dc behaviour of AlGaN/GaN-based HFETs. Band structure simulations and considerations explain the different character of surface states and the impact on the dc channel current as well as the influence on threshold voltage and barrier height by Fermi-level pinning. N-vacancies on the surface of AlGaN/GaN HFETs are ionized and therefore very sensitive to N2-treatment, whereas on GaN/AlGaN/GaN surfaces they remain neutral and do not affect nS and ID significantly. Oxidation, however, also introduces acceptor-like surface states, which become ionized only in the case of GaN capping, leading to an additional positive charge at the upper heterojunction and increasing negative charge in the channel. It is further shown that 30 nm low-temperature grown GaN capping drastically reduces the gate leakage, leading to a transistor with insulated gate characteristics. © 2005 IOP Publishing Ltd.