Enhancement‐mode AlGaN/GaN HEMTs with thin InGaN cap layer

AlGaN/GaN HEMTs with a thin InGaN cap layer has been proposed to implement the normally-off HEMTs. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised leading to the normally- off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm forthe device with a 1.9-μm-long gate. By etching-off the In0.2Ga0.8N cap layer at the access region using gate electrode as an etching mask, the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the parasitic source resistance. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 °C for 20 minutes in a N2 atmosphere. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)