A device and a method for measuring small voltages and potentials on biological, chemical and other samples. The device comprises at least one field effect transistor including a source, a drain, a gate that is in contact with the sample and insulated by a gate dielectric from the conducting channel of the field effect transistor, means for applying a voltage between the source and the drain, and means for applying a bias voltage to the gate. The gate dielectric includes at least one attachment site in the interior thereof, which is able to trap charge carriers from the channel and, conversely, to release these to the channel. Such an attachment site superimposes the current flowing through the channel, and thus through the transistor, with a telegraph modulation signal by continuously statistically exchanging charge carriers with the channel, the characteristic time constants of the telegraph modulation signal being usable as a measuring signal for the potential or voltage of the sample instead of the previously used average time value of the drain current. In this way, a larger portion of the measured information is evaluated with respect to the required result, which considerably improves the sensitivity and accuracy of the measurement.