In this work, three MISHEMT devices with different electric-field-dispersion layer (EDL) behave the same pristine electrical properties. EDL, which is low dielectric constant (low-k), can effectively disperse electric field, which enhances breakdown voltage and improves reliability in MISHEMT. In a comparison of devices with high-k and low-k EDL, the on-state current ( $\text{I}_{\mathrm{ on}}$ ) of the high-k EDL devices is more significantly reduced than low-k EDL devices after off-state stress. A model for the dispersion of electric fields by the EDL is proposed for this interesting phenomenon. The distribution of the electric field is verified by Silvaco electric field simulation. Finally, the breakdown voltages of the three devices were measured, confirming that the devices with a low-k EDL can increase their breakdown voltages by an additional 600 V, which is 250 % higher than that in the high-k EDL device.