Remarkable Reduction in I_G with an Explicit Investigation of the Leakage Conduction Mechanisms in a Dual Surface-Modified Al₂O₃/SiO₂ Stack Layer AlGaN/GaN MOS-HEMT

We demonstrated the performance of an Al₂O₃/SiO₂ stack layer AlGaN/GaN metal–oxide semiconductor (MOS) high-electron-mobility transistor (HEMT) combined with a dual surface treatment that used tetramethylammonium hydroxide (TMAH) and hydrochloric acid (HCl) with post-gate annealing (PGA) modulation at 400 °C for 10 min. A remarkable reduction in the reverse gate leakage current (I_G) up to $1.5\times {10}^{-12}\mathrm{A}/\mathrm{mm}$ (@ V_G = −12 V) was observed in the stack layer MOS-HEMT due to the combined treatment. The performance of the dual surface-treated MOS–HEMT was significantly improved, particularly in terms of hysteresis, gate leakage, and subthreshold characteristics, with optimized gate annealing treatment. In addition, an organized gate leakage conduction mechanism in the AlGaN/GaN MOS–HEMT with the Al₂O₃/SiO₂ stack gate dielectric layer was investigated before and after gate annealing treatment and compared with the conventional Schottky gate. The conduction mechanism in the reverse gate bias was Poole–Frankel emission for the Schottky-gate HEMT and the MOS–HEMT before annealing. The dominant conduction mechanism was ohmic/Poole-Frankel at low/medium forward bias. Meanwhile, gate leakage was governed by the hopping conduction mechanism in the MOS–HEMT without gate annealing modulation at a higher forward bias. After post-gate annealing (PGA) treatment, however, the leakage conduction mechanism was dominated by trap-assisted tunneling at the low to medium forward bias region and by Fowler–Nordheim tunneling at the higher forward bias region. Moreover, a decent product of maximum oscillation frequency and gate length (f_max × L_G) was found to reach 27.16 GHz∙µm for the stack layer MOS–HEMT with PGA modulation. The dual surface-treated Al₂O₃/SiO₂ stack layer MOS–HEMT with PGA modulation exhibited decent performance with an I_DMAX of 720 mA/mm, a peak extrinsic transconductance (G_MMAX) of 120 mS/mm, a threshold voltage (V_TH) of −4.8 V, a higher I_ON/I_OFF ratio of approximately $1.2\times {10}^9$, a subthreshold swing of 82 mV/dec, and a cutoff frequency(f_t)/maximum frequency of (f_max) of 7.5/13.58 GHz.