All MOCVD grown Al0.7Ga0.3N/Al0.5Ga0.5N HFET: An approach to make ohmic contacts to Al-rich AlGaN channel transistors

We report a gate recessed Al0.7Ga0.3N/Al0.5Ga0.5N heterostructure field effect transistor (HFET) with a graded contact cap layer grown by metal organic chemical vapor deposition (MOCVD) on AlN/Sapphire substrate. A low specific contact resistivity ρc of 2.1 × 10−5 Ω·cm2 is demonstrated with current injection from the top of the Al0.7Ga0.3N barrier to the Al0.5Ga0.5N channel. The device with a gate length of 160 nm exhibits a drain current density at gate shorted to source (ID,SS) of 420 mA/mm, a cutoff frequency fT of 20 GHz, and a maximum oscillation frequency (fmax) of 40 GHz. The same device has a three terminal off-state gate-to-drain breakdown voltage of 170 V, corresponding to an average breakdown field (FBR) of 2.8 MV/cm between the gate and drain, due to drain induced barrier lowering effect. Devices with a gate length of 1 µm demonstrate a gate to drain breakdown voltage of 195 V or an average breakdown field of 3.9 MV/cm. This work provides a way to make ohmic contacts to Al-rich AlGaN channel heterojunction transistors for high power and high frequency applications.