Separation technology is an indispensable step in the preparation of freestanding GaN substrate.In this paper,a large-area freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate,a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy (HVPE).The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer.It was found that the etching rate increased linearly with the applied voltage,while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation.Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel.Hence,a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem.Moreover,high temperature and ultrasonic oscillation were also found to increase the etching rate.Based on the results above,we succeeded in the liftoff of ～ 1.5 inch GaN layer.This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.