InAlN/GaN heterostructure high electron mobility transistors : technology, properties and characterization

Abstract

Si and GaAs are still playing an important role in power electronic devices even so their performance is approaching the limits due to intrinsic material parameters, such as the bandgap energy.<br />Nowadays, the demand for high-power, high frequencies, robustness at high temperatures and heavy radiation has forced the development of wide bandgap semiconductors such as GaN and SiC for electronic devices.<br />Indeed, the large bandgap, the saturation electron velocity, the thermal conductivity and the electrical breakdown make III-N-based transistors suitable for devices which can be operated at high voltages and large drive currents in the high frequency and high tempearture regime.<br />Since the first observation in 1992 of a two-dimensional electron gas (2DEG) in an AlGaN/GaN heterostructure, constant improvements in terms of material growth, technology and reliability have been achieved.<br />However,because of the lattice mismatch between AlGaN and GaN, stress induced defects are responsible for the reduction of the lifetime in such AlGaN/GaN devices, particularly for high Al contents.<br />On the other hand, the only way to increase the sheet carrier density in the channel and thus the maximum DC output drain current is by increasing the Al content in the AlGaN barriers. This decreases the crystallographic quality and thus increases the amount of defects.<br />Since InAlN can be grown lattice matched to GaN, the use of InAlN instead of AlGaN for the barrier would lead to further improvements in terms of both device reliability and sheet carrier density.<br />In this work, the impact of the technology on the performance of InAlN/GaN-based High-Electron-Mobility-Transistors (HEMTs) is investigated.<br />InAlN/GaN-based HEMTs can have superior properties in comparison to AlGaN/GaN-based HEMTs primarily because of the substantially higher carrier density in the 2DEG. But such devices suffer from high gate leakage currents which reduce the reliability and the efficiency. In order to reduce the gate leakage currents, Metal-Oxide-Semiconductor-HEMTs (MOS-HEMTs) structures with high-k dielectrics instead of the common Schottky Barrier-HEMT (SB-HEMT) have been developed and optimized within this thesis.<br />Moreover, high quality ohmic contacts are required to maximize the microwave output power. Here, a low resistance ohmic contact annealed at 600 °C is presented.<br />Another effect limiting the GaN-based HEMT is the electron trapping.<br />Charges in the channel can be captured by traps located either on the surface or in the bulk GaN. These are responsible of the so called "current collapse", which limits the performance of the devices even at low frequencies.<br />I present the fabrication and the pulsed drain current-voltage characteristic of InAlN/GaN MOS-HEMTs with gate insulation and surface passivation by using high-k dielectrics like ZrO2, HfO2 and Al2O3.<br /> Due to the high breakdown electric field of GaN, a high voltage can be applied without damaging the device. But one of the phenomena which is observed at high electric fields is the impact ionization which causes voltage breakdown in devices limiting high power applications.<br />The mechanism of the off-state breakdown in novel unpassivated InAlN/GaN HEMTs with different gate-drain distance is analysed.<br />In conclusion, a first reliability study is presented in order to confirm that the use of InAlN material lattice matched to GaN has some advantages over strained AlGaN on GaN, leading to higher quality Nitride-based transistors.