Hall sensors are fabricated from GaAs/AlGaAs based heterostructures. The sensors are characterized for sensitivity using A.C and D.C. excitation techniques w.r.to a known magnetic field in the range of 0-100Oe. Sensitivities of the order of 1000V/AT are obtained with a low variation of 0.15% in the temperature range of 30°C-100°C. A.C excitation (30Hz to 1 kHz) for the sensor current and phase-sensitive detection of the Hall Voltage are used to eliminate thermo-emfs and its drift with a consequent improvement in data acquisition rates of >50% over that of D.C excitation. This improves scan rates in applications such as magnetic flux leakage (MFL) measurements. The sensitivity is independent of the frequency of excitation as expected; however, the zero-field Hall voltage 'offset' value is found to have small frequency dependence.