MTL Annual Research Report 2011

GaN-based high electron mobility transistors (HEMTs) have great potential for high power/frequency applications due to their outstanding combination of large breakdown voltage and high electron velocity. Among the different possible nitride structures, InAlN/GaN heterostructures have attracted much attention recently because they enable an extremely high charge density with a thin barrier thickness ^[1] . With the use of these advantages, outstanding progress in the frequency performance of InAlN/GaN transistors has been recently achieved. Sun et al. reported a 55-nm gate length device with f_T of 205 GHz (f_max = 191 GHz) ^[2] and Lee et al. demonstrated a 30-nm gate length device with f_T of 245 GHz ^[3] .

In this study, we used an AlGaN back-barrier in InAlN/GaN HEMT structures for the first time and studied its impact on the DC and RF characteristics of these devices ^[4] . A maximum drain current of 1.49 A/mm is obtained at V_gs=2 V in the device with the back-barrier, about 27 % lower than that of the standard device (2.05 A/mm at V_gs=2 V). The smaller drain current in the device with the back-barrier mainly results from the lower sheet charge density and subsequent higher threshold voltage. However, the output conductance is significantly smaller in the device with the back-barrier, which shows an effective suppression of the  short-channel effects. In addition, in sub-100-nm-gate-length transistors, the back-barrier makes it possible to maintain a drain-induced barrier lowering (DIBL) near 50-60 mV/V while preventing the degradation of the subthreshold swing (SS). Thanks to the reduced short-channel effects, 65-nm-gate-length devices with a back-barrier showed an f_T of 210 GHz, which is higher than that of the standard device with the same gate length (195 GHz). Moreover, in a sub-30-nm-gate-length device with AlGaN back-barrier, an f_T of 270 GHz, the highest f_T ever reported in GaN transistors, was achieved.