Publikationsserver der RWTH Aachen University
Investigation of 'high-k' materials as alternative dielectrics for AlGaN/GaN-based metal-insulator-semiconductor heterostructure field effect transistors (MISHFET)
Abstract
dc:descriptionGallium Nitride (GaN) based heterostructure field-effect transistors (HFET) are promising devices for high-power applications at high frequencies, as to be found in communication technology. The devices benefit from the properties GaN has to offer, namely a wide bandgap, a high carrier mobility and a high saturation velocity of carriers. In recent years, improvements of the device performance were achieved by insulating the gate electrode by means of a dielectric layer (MISHFETs). This study contributes to the further improvement of GaN-based MISHFETs by insulating the gate with high-k material, taking up latest advances with Gadolinium Scandate and Hafnium Dioxide - materials which are primarily intended to serve as a substitute for SiO2 in CMOS technology. To perform experiments with this novel kind of MISHFETs, the processing technology for the fabrication of HFETs, which has been well established at the IBN, was modified and extended. In particular, deposition techniques for the high-k materials - e.g. pulsed-laser deposition (PLD) and electron-beam evaporation (EBE) - were investigated and partly modified. Also, methods for the electrical characterization of the processed devices were optimized or established, in particular means to investigate currents through insulators were established. The first experiments conducted aimed at the fabrication and characterization of SiO2 MISHFETs, which served as a reference for the subsequent studies with high-k material. The SiO2 devices proved to be superior to conventional HFETs in terms of a low gate leakage, a high drain current, and improved RF power performance. Furthermore, insights into the role of surface passivation in MISHFETs were gained and the gate current mechanisms were studied. In parallel, first attempts to incorporate GdScO3 and HfO2 insulation layers into diodes and transistors were performed. In particular, annealing in O2 atmosphere was identified to be a key element to gain electrically dense GdScO3 material. On the other hand, HfO2 was found to be an unsuitable material to serve in GaN-based MISHFETs, which was also confirmed by theoretical considerations regarding the GaN/HfO2 energy band offsets. The subsequent experiments focused on GdScO3 to be used as gate insulation. First MISHFETs and MIS diodes were fabricated successfully. Electrical characterization revealed, that the GdScO3 dielectric reduces the gate leakage currents by up to seven orders of magnitude. S-parameter measurements indicated the potential use of GdScO3 devices up to 60GHz, and Load-Pull measurements showed that the delivered output power at 7GHz can be more than doubled compared to conventional devices. In conclusion, GdScO3 was demonstrated to be a promising candidate for MISHFETs for high-power applications at high frequencies. The strengths in terms of gate leakage reduction, channel control increase, and increase of RF power performance not only outperforms conventional HFETs but also SiO2 MISHFETs. Besides the demonstration of the superior performance of GdScO3 MISHFETs, this study also identifies aspects for further optimization.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2009
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Heidelberger, Gero
- Contributors dc:contributor
-
- Lüth, Hans
Subjects
dc:subject × 16Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng