GaN-based devices are seen as ideal candidates for power-switching applications. For the acceptance of GaN-based devices by module designers, obtaining enhancementmode (e-mode) behavior in GaN-based heterostructure field-effect transistors (HFETs) has long been in the focus. Although the gate-injection approach appears to be the most promising one to achieve e-mode devices, using a double heterostructure in conjunction with a gate insulator has still its advantages, such as steeper turn-ON characteristics and lower leakage currents. An analytical expression to predict the threshold voltage Vth for a given double heterostructure device has not yet been derived. Moreover, neither an evaluation of the tradeoff between Vth and ON-state resistance Rds,ON has been performed to date. This paper addresses these two aspects. We will show an analytical expression for a metal-insulator-semiconductor double HFET (MIS-DHFET), which in certain cases is also valid for the gateinjection transistor. On the basis of this, we will discuss the actual influence of the Al concentration in the backbarrier on Vth. We will further employ technology computer aided design (TCAD) device simulations to evaluate the impact on Rds,ON when using MIS-DHFETs. It will be shown that by implementing a double heterostructure in MIS devices, it is possible to suppress the typically observed negative Vth-oxide thickness relationship while maintaining a constant Rds,ON.