A detailed analysis of the capacitance of a subharmonic millimetre-wave mixer diode presented. The individual components contributing to the total capacitance are identified, with values derived for each component for a 183 GHz Schottky design. A new method is described to determine the shunt capacitance associated with the complicated geometry of the metal bonding pads for planar diodes. It is shown that this component can contribute up to 12 fF towards an overall device capacitance of 25 fF for the 183 GHz design. The inclusion of air bridging and back-etching techniques is shown to reduce this value by 70%, resulting in a lower capacitance, higher cutoff-frequency subharmonic diode device.