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

• New approach to make ohmic contacts to high Al-content AlGaN channel HFETs. • First demonstration of All-MOCVD grown AlGaN channel HFETs with a graded contact layer. • Demonstration of lowest contact resistance on AlGaN channel HFETs with Al composition above 50%. We report a gate recessed Al 0.7 Ga 0.3 N/Al 0.5 Ga 0.5 N 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 ρ c of 2.1 × 10−5 Ω·cm2 is demonstrated with current injection from the top of the Al 0.7 Ga 0.3 N barrier to the Al 0.5 Ga 0.5 N channel. The device with a gate length of 160 nm exhibits a drain current density at gate shorted to source (I D,SS) of 420 mA/mm, a cutoff frequency f T of 20 GHz, and a maximum oscillation frequency (f max) 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 (F BR) 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. [ABSTRACT FROM AUTHOR]