In this article, we present a closed-form solution for transcapacitances in GaN HEMTs derived from a design-oriented charge-based model of the AlGaAs/GaAs- and AlGaN/GaN-based HEMTs that were recently proposed. The analytical expressions are based on the concept of normalized current and are validated with the technology computer-aided design simulations. This represents an essential step toward the ac analysis of the circuits based on the HEMT devices.