TABLE OF CONTENTS The Schottky barrier diode has been the primary non-linear device for microwave mixers. Figure 7.4 shows its equivalent circuit. Also indicated are the key equations defining the non-linearities, C j and R j, which are responsible for the mixing operation. The parasitic series resistance, R s, has to be minimised, in order to achieve high diode cut-off frequency and low mixer conversion loss. This requires a low resistance current path within the diode structure, which is implemented by a highly doped active layer below the cathode.
In monolithic circuits, the fabrication process for Schottky diodes is generally not compatible with those for MESFETs, although fabrication processes incorporating both structures have been reported [35]. Consequently, implementing diodes using MESFET based processes usually involves some derivatives of the MESFET structure. For example, the gate finger can be utilised as the anode of a Schottky diode contact by connecting the drain and source fingers together to form the cathode. However, these diodes have high series resistance due to the absence of a highly doped active layer below the gate Schottky contact. This results in rather low diode cut-off frequencies, of around 200 300 GHz, compared to those realised with a planar diode process using a vertical structure, which can achieve cut-off frequencies of 1000 1500 GHz without recourse to sub-micron geometries. Thus, mixers realised using these suboptimum diodes have higher conversion losses than those fabricated using a diode-based fabrication process.
