Research on a High-Threshold-Voltage AlGaN/GaN HEMT with P-GaN Cap and Recessed Gate in Combination with Graded AlGaN Barrier Layer

Due to the enhanced-mode (E-mode) operation, AlGaN/GaN high-electron-mobility transistors (HEMTs) are considered to be safer for circuit operation. In order to improve the threshold voltage (V_th) of the device, this work provides a hybrid gate structure HEMT by embedding a P-GaN cap on the etched graded AlGaN barrier layer. Through simulation calculations, the P-GaN cap (thickness of P-GaN = 50 nm, concentration of P-type = 2 × 10¹⁸ cm⁻³) and the aluminum (Al) composition (Al:0.3 → 0.24), in the graded AlGaN barrier layer were optimized. Although simulation calculations show that the optimized P-GaN layer can significantly increase the device’s V_th to 8.6 V and transconductance (g_m) to 94.7 mS/mm, the device exhibits a lower saturation current (I_sat). Therefore, to improve the output characteristics of the devices, the addition of an N-well in the GaN channel layer of such structures was proposed. It can increase the device’s source–drain current while maintaining a steady V_th. Compared with the HEMT structure/combined P-GaN cap with recessed gate and a graded AlGaN barrier layer, the device with the added N-well exhibits a significant improvement of 11.2% in the saturation current (I_sat = 718 mA/mm). The results demonstrate that HEMT structures combining recessed gates and P-GaN with N-well have promising applications in next-generation high-power devices.