Fabrication of High Frequency Nitride Semiconductors


High electron mobility transistors (HEMTs) are used for power management in high frequency products ranging from cell phones to radar equipment.

Problem Addressed

Their high breakdown field and electron velocity make HEMTs ideal devices for power amplification at high frequencies. However, in spite of recent progress in maximum operating frequency, the performance and electron velocity of these devices are still well below the theoretical predictions. The work on this device showed for the first time that the lower-than-expected frequency performance of HEMTs is mainly caused by a significant drop of the intrinsic small-signal transconductance (gm) with respect to the intrinsic DC gm. The proposed device combines vertical gate-recess, oxygen plasma treatment, and lateral gate-etch to fabricate AlGaN/GaN HEMTs with a record current-gain cutoff frequency (fT) of 162 GHz for a gate length (Lg) of 110 nm, and 225GHz for an Lg of 55 nm.


To mitigate the gm-collapse, the interface between the gate metal and the AlGaN surface needs to be passivated. Additionally, to maximize frequency performance the vertical and lateral dimensions of the gate have to be harmoniously scaled. To passivate the gate metal- AlGaN interface an ohmic contact metallurgy based on alloyed Si/Ge/Ti/Al/Ni/Au (2/2/20/100/25/50 nm)  was used and observed to reduce ohmic contact resistance by 55% and surface roughness of the contacts by 83% over conventional non-recessed Ti/Al/Ni/Au ohmic contacts. To scale the vertical and lateral dimensions of the gate, vertical gate-recess, oxygen plasma treatment, and then lateral gate-etch were used. First e-beam lithography is used to create the gate length. Then 10 nm of ALD-Al­2O3 sidewalls protects the gate length defined by e-beam lithography from the oxygen plasma treatment. After Ni-Au gate metal deposition, the bottom Ni layer is selectively etched to further reduce the gate length. 


  • Increases current-gain cutoff frequency
  • Increases electron velocity