High sensitivity AlGaN/GaN lateral field-effect rectifiers (L-FERs) were realised that can be monolithically integrated with AlGaN/GaN high electron mobility transistors (HEMTs) circuitry. F− ion plasma treatment was optimised to achieve strong nonlinearity of the L-FER at zero-bias. The L-FER treated by F− ion plasma at 120 W for 200 s exhibits a record level zero-biased curvature coefficient, γ, of 69 V−1 at room temperature. The direct-measured voltage sensitivity is as high as 6.75 mV/μW at 2 GHz. The high sensitivity and AlGaN/GaN HEMTs compatibility of the L-FER make them very promising for zero-bias square-law detector applications.