Charge trapping on defects in AlGaN/GaN field effect transistors

The presence of electronic traps in GaN-based devices limits device performance and reliability. Crystallographic defects in the bulk and electronic states on the surface act as trapping centers. We review the trapping phenomena in GaN-based high electron mobility transistors and discuss a characterization method, current transient spectroscopy, applied for trap identification. Probing the charge trapping mechanisms allows us to extract the trap characteristics including the trapping potential, the binding energy of an electron on the trap, and the physical location of the active centers in the device. INTRODUCTION GaN-based electronic devices have recently demonstrated excellent performance at microwave frequencies and tremendous potential in a variety of applications. However, quality of GaN material is still lacking that of mature technologies. Presence of dislocations and defects in large densities is one of the factors. In devices, defects act as charge trapping centers that limit the device performance. In AlGaN/GaN high electron mobility transistors (HEMTs), the parasitic charge moving in and out of the traps on the surface and/or in the bulk of the heterostructure affects the density of the two dimensional electron gas (2DEG) in the channel, causing effects such as current collapse, and transconductance frequency dispersion. Understanding the origin of the traps in GaN-based transistors, their location, and the physical mechanisms involved in the trapping is essential in device development. While the majority of the trapping effects result in similar degradation of the transistor characteristics at high frequencies, the dominating trapping mechanisms could vary in devices grown by different methods or subjected to different processing procedures. Transient spectroscopy allows extraction of the fundamental characteristics of the traps: the activation energy and the trap cross-section. A trapping center can be identified in different devices through these parameters. Extraction of the trap characteristics from the experimental data requires a theoretical understanding of the trapping process. Characteristics of the capture and emission process depend typically on the local temperature and the electric field. As a result, the apparent activation energy often significantly differs from the binding energy of the trap. We discuss in detail one of the most commonly encountered manifestation of trapping in AlGaN/GaN transistors: gate lag. We will focus on the physical properties of the traps, the mechanisms of the charge transfer, and the spatial location of the active traps in GaN transistors.