  We study gated field effect transistors (FETs) with an eccentric Corbino-disk
geometry, such that the drain spans its circumference while the off-center
inner ring acts as a source. An AC THz potential difference is applied between
source and gate while a static source-drain voltage, rectified by the
nonlinearities of FET electrons, is measured. When a magnetic field is applied
perpendicular to the device, a strong resonance appears at the cyclotron
frequency. The strength of the resonance can be tuned by changing the
eccentricity of the disk. We show that there is an optimum value of the
eccentricity that maximizes the responsivity of the FET.
