A digital filter for a frequency synthesizer may include an analog-to-digital converter, which is responsive to a control voltage at an input thereof, and a digital-to-analog converter, which has an input responsive to a signal generated at an output of the ADC. The body includes a body radially inner surface arcuate at least circumferentially, and a body radially outer surface from which a plurality of hooks extend generally radially outwardly. An interface calibrating unit is coupled to the second and third interfaces and a non-volatile calibrating parameter memory is arranged in the interface calibrating unit or coupled to the calibrating unit. In a planar gate device the stripes extend around the periphery of the channel-accommodating region leaving the region near the gate in a direction perpendicular with respect to the gate electrotes.