High-voltage switching applications require normally-off or enhancement-mode devices because safety is the primary concern. Although AlGaN/GaN HEMTs are capable of delivering breakdown voltage and on-resistance beyond the material limits of other semiconductors used for high-voltage switching applications, these devices are typically normally-on or depletion-mode devices due to the large polarization-induced charge in AlGaN/GaN heterostructures. This dissertation focuses on development of enhancement-mode AlGaN/GaN HEMTs for high-voltage switching applications.

The barrier created by the gate between the source and drain-side access regions was identified as an important design parameter for minimizing off-state leakage and operation at high temperatures. The design space of gate-recessed AlGaN/GaN enhancement-mode HEMTs showed that in order to obtain enhancement-mode devices with sufficiently low on-resistances, very thin AlGaN barriers are needed below the gate and AlN interlayers cannot be used to improve device performance. Although fabricated devices showed positive threshold voltages, devices showed significant off-state leakage due to a low source-to-drain barrier height.

The effects of fluorine-based plasma treatment on AlGaN/GaN HEMTs were systematically studied and a self-aligned low-damage treatment for enhancement-mode operation was developed. With the integration of self-aligned integrated field-plates, true enhancement-mode AlGaN/GaN HEMTs with low off-state leakage were developed. Although these devices exhibited record breakdown voltage of 1400 V and good switching characteristic, negative shift of threshold voltage at moderately high temperatures were observed.

The design space of AlGaN/GaN HEMTs with a full-depleted p-GaN layer below the gate showed that it is possible to get a relatively low on-resistance with a reasonably thick AlGaN layer. In addition, AlN interlayers can be used to further improve device performances in this approach. Although enhancement-mode devices with low off-state leakage were developed, DC-to-RF dispersion was observed in these devices even after passivation. The cause of this dispersion is yet unclear and this issue needs to be resolved. Although AlGaN/GaN HEMTs have the potential to deliver low on-resistance and high breakdown voltages, several challenges remain for high-voltage switching applications.