【International Papers】Double drift layers to minimize on-resistance in 2.7-kV β-Ga₂O₃ (001) vertical trench Schottky barrier diodes

      Researchers from the University of Bristol have published a dissertation titled " Double drift layers to minimize on-resistance in 2.7-kV β-Ga₂O₃ (001) vertical trench Schottky barrier diodes " in Applied Physics Letters.

Background

      Ultra-wide bandgap beta-gallium oxide (β-Ga₂O₃) has attracted significant attention for high-voltage power electronics, owing to its high critical electric field that supports kilovolt-class operation. Vertical Ga₂O₃ power devices typically employ a few-micrometer-thick drift layer on conductive substrates, across which most of the voltage is dropped when the device is in the off-state. These drift layers are lightly or unintentionally doped (UID) to allow for a wide depletion extension, reducing the electric field and enabling high breakdown voltages (V_br). However, in two-terminal devices, such as vertical Ga₂O₃ planar Schottky barrier diodes (SBDs), the electric field peaks at the Schottky metal/Ga₂O₃ interface, leading to thermionic emission-driven reverse leakage currents that severely limit breakdown voltage and thus off-state device performance. Trench Schottky barrier diodes (TSBDs) overcome this limitation by leveraging the reduced surface field (RESURF) effect. This improvement, however, comes at the expense of specific on-resistance (R_on,sp), as trench etching and dielectric shielding of the trench sidewalls/bottom reduce the effective area of the Schottky contact to the Ga₂O₃ semiconductor material [the device schematic is shown in Fig. 1(a)]. In this work, we introduce a double drift layer (DDL) TSBD design that relaxes this trade-off between R_on,sp and V_br by selectively increasing the conductivity of fin regions through Si ion implantation, which reduces R_on,sp while maintaining a low doping density in the drift region and thus not having a negative impact on V_br. The DDL architecture minimizes R_on,sp by 46% compared to the conventional single-UID drift layer TSBD without compromising V_br, resulting in ∼2× improvement in the Baliga's figure of merit (BFOM). We also show that the RESURF effect is more pronounced in DDL devices, with Baliga's figure of merit (BFOM) increasing by ∼160× from planar to trench SBDs, compared to only a ∼7× improvement observed for similar devices with a single UID-drift layer.

Abstract

      Improved specific on-resistance (R_on,sp) in beta-gallium oxide (β-Ga₂O₃) trench Schottky barrier diodes (TSBDs) is achieved without the usual trade-off of reduced breakdown voltage, enabled through the use of a double drift layer (DDL) TSBD architecture; this incorporates a Si-doped drift layer atop an unintentionally doped (UID) drift region. DDL TSBDs show a 46% reduction in R_on,sp compared to single-UID drift layer TSBDs, while maintaining a similar V_br of 2.6–2.7 kV. As a result, the DDL TSBD design enhances Baliga's figure-of-merit from 481 MW/cm² for conventional TSBDs to 959 MW/cm², marking a twofold improvement. Silvaco TCAD simulations further confirm that both structures exhibit similar dominant peak electric fields at the trench corners, consistent with their comparable breakdown voltages. While reducing fin width in TSBDs enhances V_br because of an improved reduced surface field effect, we demonstrate that this effect is significantly amplified in DDL TSBDs, enabling a substantial shift in the conventional R_on,sp–V_br trajectory.

Conclusion

      In summary, we demonstrated a double drift layer (DDL) architecture that redefines the balance between specific on-resistance (R_on,sp) and breakdown voltage (V_br) in trench Schottky barrier diodes (TSBDs). By selectively doping the top of the fin regions, the DDL TSBDs achieve a much lower R_on,sp compared to conventional single drift layer TSBDs, while maintaining a high V_br. In contrast to single drift layer devices, where V_br improvements come with a substantial penalty in R_on,sp, the DDL design shifts the R_on,sp–V_br trajectory, achieving a 2× higher BFOM by efficiently managing the breakdown-limiting electric fields. This demonstrates that fin doping, rather than geometry alone, can be used to engineer the performance of vertical Ga₂O₃ TSBDs.