Magnetoresistive switch effect (MRS effect) devices containing two gold (Au) electrodes with a gap less than 2 µm were successfully fabricated on semi-insulting GaAs substrates by wet etching method. Huge MRS effect was observed. Magnetoresistance (MR) ratio reached 1,000,000% under the magnetic filed of 1.5 T when the devices were operated just above the threshold voltage. The magnetic field sensitivity at small magnetic fields was significantly improved. MR ratio of more than 1000% was achieved at 0.03 T. A relative high MR ratio of 100,000% under the magnetic filed of 1.5 T was also achieved when the devices operating before the threshold voltage.