Observations on substrate characterisation through Coplanar Transmission Line Impedance measurements

In the course of developing a GaAs Schottky diode membrane technology for millimeter wave applications (viz. THz mixers and multipliers) it was found that subtle changes and variations introduced into the membrane structure can significantly affect the circuit performance. We describe how these effects manifest themselves and, using S-parameter measurements on a variety of substrates, show how they can be explained in terms of conductive or charge layers that are observable through Coplanar Transmission Line Impedance measurements. A modified transmission line model is given.