Novel Quaternary AlInGaN/GaN Heterostructure Field Effect Transistors on Sapphire Substrate

Undoped quaternary Al_xIn_0.02Ga_0.98-xN/GaN heterostructure field effect transistors (HFET) with different Al mole fractions were fabricated on sapphire substrate. An enhancement in two-dimensional electron gas density with increasing the Al mole fractions was observed. The properties of HFETs revealed that, with the Al incorporation, both maximum transconductance (g_mmax) and drain current (I_dmax) increased up to 138 mS/mm and 1230 mA/mm, respectively, for the device with 2 µm gate length. In addition, the threshold voltage strongly depended on Al mole fraction, by which normally-off HFET was demonstrated on a low Al-contained sample. No breakdown was observed for all the Al_xIn_0.02Ga_0.98-xN/GaN HFETs when the gate–drain reverse bias was applied up to 100 V. The results indicate that quaternary AlInGaN is a promising candidate for high power and high frequency device applications.