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This paper experimentally verifies that for low bias voltages ≪2 kT/q, the noise of a saturated magnetic tunnel junction (MTJ) device is caused by thermally assisted barrier crossings (thermal noise), whereas for voltages ≫2 kT/q, it stems from field-assisted barrier crossings (shot noise). This can also be used as a criterion for an MTJ with a healthy, defect-free barrier. A new dual-channel ultralow-noise amplifier is described allowing noise measurements for very low resistance (large area) devices. A "barrier defect model" is developed and verified for noise, tunneling magnetoresistance ratio (TMR), and defective area, using healthy MTJ devices, which are given controlled defects by means of pulsed electrical overstress. It is shown that MTJ devices are very electrostatic-damage prone; narrow voltage pulses (0.5 ns) can damage the sensors at relatively low voltages (1.5 V). Relationships are shown between pulse voltage, pulse energy, duration, number of pulses, and the drop in sensor resistance. Finally, the sensor signal-to-noise, bandwidth, and electronic readout are discussed.