A full 3D thermal simulation structure was built using Atar based on the GS66516T device from GaN Systems with individually controllable generation regions for each island transistor in an array. The thermal model was linked to electrical SPICE models in OptiSPICE to produce an electrothermal simulator to be used to investigate the device behaviour when cells in the array are deactivated. The individual simulations were all verified against models, previous simulations, and physical testing to ensure that the simulations behaved accordingly. The results of the electrothermal simulation verification showed that there was a less than 5% error between the measured and simulated values, providing an accurate simulation of the electrothermal behaviour of the GS66516T device. Electrothermal simulations were then run for various cases of random cells deactivated. From the simulations, data was gathered and interpreted to show the possibilities of the electrothermal simulations for determining device behaviour with cells deactivated.