Characterization and Analysis of Surface Passivations and Gate Insulators for AlGaN/GaN Microwave HFETs

Doctoral thesis, 2012

The large bandgap of gallium nitride (GaN) and aluminum gallium nitride (AlGaN) offers an inherently high intrinsic breakdown field. When the materials are joined into the AlGaN/GaN heterostructure a 2-dimensional electron gas (2DEG) with a high electron density as well as high electron mobility is generated. The combination of high electron density with high mobility and high breakdown field results in excellent power handling capability at high frequencies. These inherently virtuous physical properties are utilized by the GaN-based heterostructure field effect transistor (HFET) technology. This thesis deals with developing appropriate fabrication processes for AlGaN/GaN HFETs by basic material characterization methods. The main focus is on the passivation of the heterostructure surface, which is notorious for its effect on device performance. Several examples of the effect of the passivation process on the electrical properties of the heterostructure are presented. For example, it is shown that passivating a heterostructure using a non optimized plasma based deposition method may redistribute more than 90\% of the channel electrons to a barrier accumulation layer. The use of metal-insulator-semiconducting-heterostructure (MISH) capacitor analysis for extracting interface states is described in detail. Based on a comparison of different passivation methods the low pressure chemical vapor deposition (LPCVD) silicon nitride (\SiNx{}) emerges as a suitable candidate for passivation of AlGaN/GaN heterostructures. In this thesis the effects of different LPCVD \SiNx{} deposition parameters are investigated. Furthermore, LPCVD \SiNx{} passivation will produce devices that are less sensitive to illumination. The illumination sensitivity is reduced to less than 3\% compared to 15\% to 130\% for other passivated and non passivated heterostructures. This thesis also reports on the fabrication and DC characterization of insulated gate HFETs. A 5\dunit{}nm thick layer of LPCVD silicon nitride is deposited below the gate which reduces the gate current by one to two orders of magnitude. This type of device is a promising candidate for reliable HFETs with applications in microwave and power electronics