UC Santa Barbara

β-Ga2O3 is an ultra-wide bandgap semiconductor with an expected breakdown electric field (F_br) of 8 MV/cm, which, in addition to the availability of shallow donor species and large, melt-grown native substrates, makes it well-positioned as the material of choice for next generation power electronic devices. However, taking advantage of this high field strength in power diodes is intrinsically challenging, as device topologies that sufficiently suppress field-driven leakage current have typically involved high energy barriers that result in large turn-on voltage. This additional conduction loss counteracts the improved on-resistance enabled by a large breakdown field. Therefore, engineering a diode topology capable of reaching the high field strength of β-Ga2O3 with sufficiently low leakage current while maintaining low conduction losses is vital for enabling next generation power diodes.In this work, a novel diode with a Metal/TiO2/β-Ga2O3 anode structure is demonstrated to offer a better tradeoff of rectification and breakdown voltage versus turn-on voltage. The keys to this advancement are two characteristics of TiO¬2: its high permittivity of ~43ε_0 widens the conduction band energy barrier in the off-state, reducing tunneling current, and its negative conduction band offset relative to β-Ga2O3 does not create any additional energy barrier for on-state conduction. Initial demonstration of this diode shows more efficient on-state and off-state characteristics than a co-fabricated Schottky diode. Analysis of the transport mechanisms of the Metal/TiO2/β-Ga2O3 diode revealed thermionic emission as the dominant transport mechanism in forward and reverse bias. Based on the extracted conduction models, a Schottky barrier height of 1.1 eV was found as the minimum to match the rectification of commercial 1200 V 4H-SiC diodes; the corresponding on-state will have similar conduction losses as the SiC devices, indicating this device structure can enable β-Ga2O3 to match and potentially surpass the performance of incumbent semiconductors. In addition, a general analysis was conducted to explore the losses of unipolar diodes to offer a perspective on the potential benefits and limitations of ultra-wide bandgap materials compared to previous technologies. While the traditional figure of merit to compare materials for power electronic applications indicates a F_br^(-3) proportionality to on-resistance, it is shown that accounting for switching losses yields only a F_br^(-1) and F_br^(-2) reduction in total losses for hard and soft switching, respectively. Further, realizing this reduction requires area optimization that necessitates smaller devices area for a larger F_br material, leading to thermal limitations on the operating regimes where lower loss can be realized with a larger F_br. It is shown that a turn-on voltage of 2 V is prohibitive for β-Ga2O3 to exceed the performance of current 4H-SiC technology at typical voltage and frequency conditions. Overall, this work demonstrates a promising device topology for realizing the benefits of β-Ga2O3 in power diodes by leveraging the beneficial properties of a thin TiO2 layer in the anode. An analysis of total losses in unipolar diodes is also shown, indicating the potential benefits of ultra-wide bandgap materials like β-Ga2O3 but also showing the limitations on applications where those benefits can be realized.