Performance Comparison of Au-Based and Au-Free AlGaN/GaN HEMT on Silicon

We report on the electrical performance comparison of Au-based and Au-free AlGaN/GaN high electron mobility transistors (HEMTs) on silicon (Si). The chemical composition of both types of contacts, i.e., Ti/Al/Ti/W (Au-free) and Ti/Al/Ni/Au (Au-based) are studied using transmission electron microscope (TEM). For the former, the top W layer is found to be restricted in its interaction with the lower metal layers, leading to a continuous W cap at the contact surface which results in a
$20\times $
improvement in the surface roughness for the Au-free contacts. From transistor measurements, devices with Au-free contacts are found to exhibit +0.4 V shift in the threshold voltage and a
$10\times $
increase in the gate leakage which is attributed to the strain associated with the sputtered W-capping layer of the Ni/W gate-metal stack and plasma induced damage caused at the barrier surface due to high-power sputter deposition of gate contact. Lowest
${R}_{C}$
value of
$0.4~\Omega \cdot $
mm obtained for the Au-free Ohmic contacts is comparable to the
${R}_{C}$
value of
$0.38~\Omega \cdot $
mm obtained for the Au-based Ohmic contacts used in this work.