Development and analysis of low resistance ohmic contact to n-AlGaN/GaN HEMT

High quality ohmic contacts were realized in order to obtain an AlGaN/GaN high electron mobility transistor (HEMT) on different substrates (Si, Al2O3, SiC). A metallization scheme based on Ti/Al/Ni/Au was used. The ohmic contacts were obtained using an optimized rapid thermal annealing (RTA) at temperature as high as 900 °C for 30 s in a N2 atmosphere up to the two dimensional electron gas (2DEG). The piezoelectric stress, measured by microRaman spectroscopy, reveals to be dependent on the distance between the transmission line model (TLM) patterns on Si substrate grown by molecular beam epitaxy (MBE). This should be related to the non-linearity of the total resistance (Rt) measured on the largest spacing between two electrodes. After a long thermal treatment at 500 °C for 2000 h, the ohmic contact shows a very stable behaviour.