We study the response of a field-effect transistor to focused terahertz radiation. The radiation-induced plasma oscillations are rectified due to nonlinear properties of the transistor channel, thus leading to change of source-to-drain voltage delta V. We demonstrate that delta V dramatically depends both on the precise focus position of the incoming beam and on the current in the channel. The observed voltage response contains contributions of different signs that we attribute to radiation induced signals between source-and-gate and gate-and-drain electrodes of the device. Published by Elsevier Ltd.