Design Study of Enhancement-Mode β-(Al_xGa_1-x)₂O₃/Ga₂O₃ HEMT for Multi-kV Power Electronic Applications

β-Ga₂O₃ (GaO) is a potential next generation material for power electronics because of superior material properties to those of currently used semiconductors. Its estimated critical electric field (E_cr) of 8 MV/cm based on an ultra-wide bandgap (E_g) of 4.8eV improves the tradeoff between the breakdown voltage (V_br) and on-resistance (R_ON) of power devices [1]. The (Al_xGa_1-x)₂O₃/Ga₂O₃ (AlGaO/GaO) heterostructure provides a pristine epitaxial interface for forming a highly conductive two-dimensional electron gas (2DEG). Modulation-doped AlGaO/GaO high electron mobility transistors, which are depletion-mode devices, have been widely studied for radio-frequency applications. For power-switching applications, enhancement-mode (E-mode) or normally off transistors are desired for fail-safe circuitry. In this work, the design space and electrical performance of lateral E-mode AlGaO/GaO power transistors was explored with commercial TCAD software. Normally off operation was realized by eliminating modulation doping. The drain access region—which acts as the drift region in a lateral transistor—is doped at different concentrations (N_DD) to tailor the off-state electric field strength, while the contacts and source access region are highly doped to minimize R_ON. These varieties of doping profiles can be engineered in actual devices via ion implantation in conjunction with a self-aligned process. Simulated devices assumed a (100) surface orientation since this orientation has demonstrated the highest Al incorporation up to 52% [2] and the largest conduction band offset (ΔE_c) with GaO [3], which provide for low gate leakage and high 2DEG concentration. Anisotropic material properties including electron velocity [4] and relative permittivity were incorporated in the model. In these devices, the Schottky barrier height (φ_b) on AlGaO needs to be significantly larger than ΔE_c to realize a large positive threshold voltage (V_T), but φ_b is typically limited by Fermi level pinning to less than 1.5 eV for x &lt; 0.2 [5]; in addition, the finite background electron concentration in the GaO channel layer (typically 10¹⁵ to 10¹⁶cm^-3) also limits the maximum allowable GaO thickness (T_ch) to ensure full depletion under the gate, which in turn limits the conductance in the drain access region. Incorporation of a p-type material in the gate stack effectively increases V_T by an amount determined by the p-type material's E_g and its ΔE_c with AlGaO, thereby enabling larger T_ch for lower drain access resistance. At a gate-to-drain length (L_gd) of 10µm, which is capable of blocking 8kV for an idealized uniform electric field in the drift region, field-plated devices with a 20-nm-thick (Al_0.5Ga_0.5)₂O₃ and an N_DD of 1×10¹⁷cm^-3 showed a V_T of 2.5V, a maximum drain current density of 280mA/mm at a gate voltage of 10V, and a V_br of ~4kV (peak field in GaO reaching the critical value of 8MV/cm). The R_ON of ~ 15mΩcm² for this configuration was about one order of magnitude higher than the theoretical minimum [6], but can be improved by further optimizing field management. Lower values of N_DD increased V_br while bringing penalties on R_ON as expected. The design strategies studied for the proposed E-mode device including the use of AlGaO/GaO HEMT structures to enhance 2DEG transport properties, selective-area doping as a degree of freedom to engineer R_ON and V_br, p-type gating layer to increase V_T, and field plating to reduce field crowding will promise high-breakdown and low-loss GaO-based power transistors.<br/><br/>1. M. Higashiwaki<i> et al.</i>, Appl. Phys. Lett. <b>100</b> (1), 013504 (2012).<br/>2. A. F. M. Anhar Uddin Bhuiyan<i> et al.</i>, Crystal Growth & Design <b>20</b> (10), 6722 (2020).<br/>3. A. F. M. Anhar Uddin Bhuiyan<i> et al.</i>, J. Vac. Sci. Technol. A <b>39</b> (6), 063207 (2021).<br/>4. K. Ghosh<i> et al.</i>, J. Appl. Phys. <b>122</b> (3), 035702 (2017).<br/>5. E. Ahmadi<i> et al.</i>, Semicond. Sci. Technol. <b>32</b> (3), 035004 (2017).<br/>6. B. J. Baliga, Semicond. Sci. Technol. <b>28</b>, 074011 (2013).