Charge Control in Schottky-Type p-GaN Gate HEMTs With Partially and Fully Depleted p-GaN Conditions

The Schotty-type p-GaN gate high-electron-mobility transistors (HEMT) feature a unique gate structure. A comprehensive understanding of the charge control mechanism in the p-GaN gate region is a fundamental step for the optimization of this technology. In this work, a physics-based analytical model is presented which takes into consideration all the capacitive effects from gate metal deep into the GaN buffer. According to our analysis, the p-GaN layer can be either partially depleted by the metal/p-GaN Schottky junction or fully depleted, depending on the doping concentration and thickness of the p-GaN layer. Our model accurately captures the charge control properties under both conditions and is validated against TCAD numerical simulations. For a certain p-GaN thickness, a lightly doped p-GaN leads to a full-depletion condition, such that the acceptor concentration directly affects the band diagram at AlGaN/GaN interface. The ${V}_{\text {th}}$ of the HEMT increases quickly with acceptor concentration in p-GaN. With sufficiently high acceptor concentration in p-GaN, the device reaches the partial-depletion condition, the acceptor concentration loses its influence over the band diagram at the location of the AlGaN/GaN interface, since the Fermi-level at the AlGaN surface is pinned near the valence band of p-GaN. The ${V}_{\text {th}}$ starts to decrease with acceptor concentration, but at a relatively slow rate. The maximum ${V}_{\text {th}}$ is obtained near the boundary between partial-and full-depletion conditions. In consideration of the process margin, the device designed with a partially depleted p-GaN is preferable, since it ameliorated the ${V}_{\text {th}}$ sensibility against acceptor concentration.