A new approach to RF compact modeling with a "sub-surface model" for the nonlinear substrate current/charge (rather than the conventional linear R/C sub-circuit) is proposed for GaN-HEMTs, which is also extended to the source/drain (S/D) access regions as ungated HEMTs. By comparison with numerical device S-parameters, preliminary results show that the composite sub-circuit model demonstrates good matching with scalable dc-biases as well as gate and S/D lengths and body doping concentrations. Model verification with experimental devices (105nm and 40nm) is also given. It proves to be a useful approach to RF circuit design and optimization with physical and scalable predictability.