Performance of AlGaN/GaN MISHFET using dual-purpose thin Al ₂O₃ layer for surface protection and gate insulator

In this work, we have investigated a role of a thin Al₂O ₃ layer in AlGaN/GaN MISHFET by characterizing the variation of the sheet resistance of the 2DEG channel layer. The Al₂O₃ layer, varying the thickness from 0 to 10 nm, was utilized as the gate insulator of the device as well as the surface protection layer during RTP for ohmic contact formation. After RTP, the 2DEG channel layer without the Al ₂O₃ layer was rapidly degraded by increasing the sheet resistance of the layer to 1360 Ω/□ from the sheet resistance of 400 Ω/□ of the as-grown sample. The degradation was still observed even when 1.5 nm-thick Al₂O₃ layer was used. However, the sheet resistances of the devices remained constant with slightly decreased value from that of the as-grown sample when the thickness is larger than 3 nm, which indicates that the 3 nm-thick Al₂O₃ layer well protects the AlGaN surface above the 2DEG channel during RTP. The slight decrease in sheet resistance is probably because some acceptor-like states existing at AlGaN surface become neutralized and hence the 2DEG density increases. The Al ₂O₃ layer was not removed for proceeding the fabrication of AlGaN/GaN MISHFET, but rather used as a gate dielectric, which simplifies the device fabrication eliminating the additional deposition steps for the gate dielectric. The threshold voltage of the device, investigated in this work, was increased to the negative direction with increasing the thickness of Al ₂O₃ layers while the transconductance was decreased. The best performances were obtained from the device with 8 nm-thick Al ₂O₃ layer, exhibiting very low gate leakage current of 10^-9 A/mm with subthreshold swing (SS) of 80 mV/dec and very high I_on/I_off ratio (>9 orders).