We report the effect of relatively weak (classical) magnetic fields on the gate current in heterostructure field‐effect transistors. With a three‐terminal device using the measured dependences, we deduce an average electron velocity and the average concentration of the two‐dimensional (2‐d) electron gas in the channel. In addition, our results clearly show the onset of velocity saturation in high electric field. The value of the effective saturation velocity in the channel is of the order of 105m/s, in agreement with the values deduced from device transconductance.