In this work, we have investigated a role of a thin Al2O3 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 RIP for ohmic contact formation. After RIP, the 2DEG channel layer without the Al2O3 layer was rapidly degraded by increasing the sheet resistance of the layer to 1360 Omega/square from the sheet resistance of 400 Omega/square 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 Al2O3 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 Al2O3 layers while the transconductance was decreased. The best performances were obtained from the device with 8 nm-thick Al2O3 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/l(off) ratio (>9 orders). (C) 2014 Elsevier Ltd. All rights reserved.