In this paper, we present a comparative study on small-signal modeling of AlN/GaN/AlGaN double hetero-structure high electron mobility transistors (HEMTs) grown on silicon (Si) and silicon carbide (SiC) substrate. The traditional small signal equivalent circuit model is modified to take into account the transmission loss mechanism of coplanar waveguide (CPW) line which cannot be neglected at high frequencies. CPWs and HEMTs-on-AlN/GaN/AlGaN epitaxial layers are fabricated on both the Si and SiC substrates. S-parameter measurements at room temperature are performed over the frequency range from 0.5 GHz to 40 GHz. Transmission loss of CPW lines are modeled with a distributed transmission line (TL) network and an equivalent circuit model is included in the small-signal transistor model topology. Measurements and simulations are compared and found to be in good agreement. (C) 2015 Elsevier Ltd. All rights reserved.