This article presents an analytical model of the barrier layer in GaN-MISHEMT that predicts a limitation of the channel charge density that is expected to reach an asymptotic value under high gate voltages, a feature that was not reported before. We find that this behavior is very sensitive to the device parameters such as the AlGaN barrier thickness, composition, and polarization-induced charge density. Explicit relationships for charge saturation in the quantum well (QW) are proposed and can be used to optimize GaN-MISHEMT architectures.