Advanced III-Nitride Technology for mm-Wave Applications

Doktorsavhandling, 2019

Within wireless communication, there is a continuously growing need for more bandwidth due to an increasing number of users and data intense services. The development within sensor systems such as radars, is largely driven by the need for increased detection range and robustness. In such systems, power amplification and generation at high frequency are of importance. High-electron mobility transistors based on gallium nitride (GaN HEMTs) offer efficient generation of high output power at high frequency. This is enabled by the unique characteristics of GaN and its heterostructures, with a large breakdown field, related to the wide bandgap, and excellent electron transport properties. Due to this, it is today used in high-performing radar, telecommunications, as well as power electronic systems. Despite substantial progress over the last decade, the GaN HEMT is still the subject of intense research to reach its full potential.  

Recent development within epitaxy has significantly improved the quality of III-nitride semiconductors, and enabled indium aluminum nitride (InAlN) and InAlGaN as alternatives to AlGaN in the conventional AlGaN/GaN heterostructure. The higher polarization charge in these materials allows for considerable downscaling of the barrier layer thickness with a sustained high sheet carrier density.  This has opened new possibilities for optimization of the high frequency performance.   

In this work, HEMTs with downscaled InAl(Ga)N barrier layers have been developed with the goal to optimize the devices for power amplification in the mm-wave range. Electron trapping and short-channel effects have been addressed in the design of the epi and in the optimization of the process modules. Different surface passivation layers and deposition methods have been evaluated to mitigate electron trapping at the surface. The output power density of a HEMT increased from 1.7 to 4.1 W/mm after passivation with a SiNx layer. The deposition method for Al2O3 passivation layers showed to have a profound impact on the electron trapping. A layer deposited by plasma-assisted atomic layer deposition (ALD) exhibited superior passivation of the surface traps as compared to the layer deposited by thermal ALD, resulting in an output power at 3 GHz of 3.3, and 1.9 W/mm, respectively. The effect of the channel layer thickness (50 – 150 nm) in InAlGaN/AlN/GaN HEMTs with and AlGaN back barrier demonstrated a trade-off between short-channel effects and deep-level electron trapping in the back barrier. The maximum output power was 5.3 W/mm at 30 GHz, obtained for a GaN layer thickness of 100 nm.

To further enhance the high frequency performance, the ohmic contacts were optimized by the development of a Ta-based, Au free, metal scheme. Competitive contact resistance of < 0.2 Ωmm was achieved on both AlGaN/GaN and InAlN heterostructures with a Ta/Al/Ta metal stack. The contacts are annealed at a low temperature (550 – 575 ºC) compared to more conventional contact schemes, resulting in a smooth morphology and good edge acuity. 

The implementation of microwave monolithic integrated circuits (MMICs) based on III-nitride HEMTs facilitate the use of III-nitride HEMTs in a system where frequency and compactness are key requirements. Thin film resistors (TFRs) are one of the passive components required in MMICs. In this work, a low-resistance titanium nitride (TiN) TFR was developed as a complement to the higher resistance tantalum nitride (TaN) TFR and mesa resistor in the in-house MMIC process. The developed TiN TFR exhibits a sheet resistance of 10 Ω/□, compared to 50 and 200-300 Ω/□ of the TaN TFR and semiconductor resistor, respectively. The critical dissipated power in the TFR showed a correlation to the footprint area, indicating that Joule-heating was the main cause of failure. TiN- and TaN films exhibit different signs of the thermal coefficient of resistance. This feature was used to demonstrate a temperature compensated TFR (TCR = -60 ppm ºC) with application in MMICs operating in a wide temperature range.