Applications of GaN HFETs in UV detection and Power electronics

Gallium nitride (GaN) has some unique material properties including direct band gap, ability to form a heterostructure resulting in two dimensional electron gas (2DEG) formation and a wide band gap (3.4eV) to offer high breakdown voltage. Such material properties make GaN extremely attractive for optoelectronics and power electronics applications. In this thesis, GaN HFETs applications as an Ultraviolet light detector and for power electronics sector are explored.
In comparison to other GaN based UV detectors, the AlGaN/GaN HFET is found to be ultra sensitive to UV illumination. A very high dc responsivity (~4.3×107A/W) value is reported and gain mechanisms in the devices are shown to be due to a photo voltage effect in both the AlGaN barrier layer and the GaN buffer region. Understanding of the gain mechanisms from this work will help optimise the design of the future UV photo detectors.
For power electronics applications, GaN HFETs grown on a Si substrate are characterized. To reduce buffer leakage both Iron (Fe) and Carbon (C)-doped structures are considered. The vertical leakage mechanism is identified as a Poole Frenkel emission process for both the Fe and C-doped structures. A novel method to reduce the gate leakage current in GaN HFETs is established by using surface chemical treatments. Sulfuric acid works by oxidizing the surface which has a strong passivating effect on the gate leakage current. The surface leakage mechanism is explained by a combination of Mott hopping and Poole Frenkel models.
The fluorine ion implant technique is used in GaN HFETs for the development of enhancement mode transistors required in power switching applications. The requirement for a +3V threshold voltage in the power electronics sector is met by combining the fluorine implant with a deposited dielectric layer under the gate. More efficient fluorine incorporation is observed in AlInN/GaN HFETs compared to conventional AlGaN/GaN HFETs. The recipe for fluorine implant in AlInN/GaN HFETs is also optimized to maintain high channel conductivity and transconductance.