Improved DC and RF Characteristics of GaN-Based Double-Channel HEMTs by Ultra-Thin AlN Back Barrier Layer

In order to improve the off-state and breakdown characteristics of double-channel GaN HEMTs, an ultra-thin barrier layer was chosen as the second barrier layer. The strongly polarized and ultra-thin AlN sub-barrier and the InAlN sub-barrier are great candidates. In this article, the two epitaxial structures, AlGaN/GaN/AlN/GaN (sub-AlN) HEMTs and AlGaN/GaN/InAlN/GaN (sub-InAlN) HEMTs, were compared to select a more suitable sub-barrier layer. Through TEM images of the InAlN barrier layer, the segregation of In components can be seen, which decreases the mobility of the second channel. Thus, the sub-AlN HEMTs have a higher output current density and transconductance than those of the sub-InAlN HEMTs. Because the high-quality AlN barrier layer shields the gate leakage current, a 294 V breakdown voltage was achieved by the sub-AlN HEMTs, which is higher than the 121 V of the sub-InAlN HEMTs. The current gain cut-off frequency (<i>f</i>_T) and maximum oscillation frequency (<i>f</i>_max) of the sub-AlN HEMTs are higher than that of the sub-InAlN HEMTs from low to high bias voltage. The power-added efficiency (PAE) and output power density (<i>P</i>_out) of the sub-AlN HEMTs are 57% and 11.3 W/mm at 3.6 GHz and 50 V of drain voltage (<i>V</i>_d), respectively. For the sub-InAlN HEMTs, the PAE and <i>P</i>_out are 41.4% and 8.69 W/mm, because of the worse drain lag ratio. Thus, the <i>P</i>_out of the sub-AlN HEMTs is higher than that of the sub-InAlN HEMTs.