Fabrication and characterisation of AlGaN/GaN high electron mobolity transistors for power applications

The III-Nitrides were intensively studied during the last few years due to its tunable band gap range from 0.7 eV for InN to 6.2 eV for AlN. In comparison to other systems III-Nitrides have a much smaller lattice constant and therefore are mechanically stable materials with high breakdown fields. Thanks to these properties are good candidates for possible applications in the field of high-temperature, -power and -frequency electronics.The technological process developed in this work was derived from a HEMT fabrication process already established at our institute. The standard process was improved within the bound of this work by additional processes containing MOSHFET processing, surface passivation, air bridge technology, and field plate processing. HEMTs on undoped and doped layer structures were manufactured.Improved properties of intentionally doped structures in comparison to undoped ones were observed using Hall effect measurements. Results confirmed that layer structures were well grown with high mobility and high sheet carrier concentration in two-dimensional gas. Static (dc) measurements on unpassivated devices exhibited improved properties of HEMTs fabricated on doped layer structure, which is in agreement with Hall effect measurements. On the other side, the doped structures exhibited lower breakdown voltage connected with higher gate leakage current. The current collapse phenomenon was investigated using gate lag measurements for undoped and doped samples. The drain current in pulse mode was found to be dependent on the doping concentration of the barrier layer where with increased doping level the current collapse decreased. This collapsed behaviour was confirmed by output power measurements.Further improvement of static and large-signal properties was achieved using a 150 nm thick Si3N4 surface passivation layer. The positive influence on the surface states in between drain and source electrode using surface passivation resulted in a decrease of the current. Improvement of the static and pulse behaviour resulted to remarkably improved output power.Surface passivation was not the only investigated approach for improvements. A barrier to further output power increase is still the low breakdown voltage. Using the ATLAS simulation package the location of peak electric field in the under gate region was found and eliminated using field plate technology. This resulted in further output power increase with an excellent measured value of 12 W/mm in undoped and doped samples.In spite of these remarkable results, current collapse was still present in measured samples. Detailed study of our technological process, specifically the gate contact processing and pre-deposition surface cleaning, were found to be crucial regarding the current collapse. An extended HCl treatment prior to metal deposition and adjusted metal deposition resulted to samples with negligible, if any, current collapse down to GHz frequency range (ns pulses).Even though this work presents a major progress in GaN-based material system and processing there are still questions related especially to long-term reliability (in months and years), which have to be answered. The reliability also seems to be the last barrier before industrial production of AlGaN/GaN HEMTs. By now, industrial companies announced wholesale production for the year 2006 what presents the AlGaN/GaN material system as a future candidate for research and industry