A Ka-Band GaAs MESFET monolithic downconverter
The objective of the work of this thesis is to design, fabricate, and characterise a GaAs MESFET based monolithic microwave integrated circuit (MMIC) downconverter which operates at Ka-band frequency (35GHz). In the course of the project active and passive elements required for the MMIC were designed, fabricated, characterised and their equivalent circuit models extracted. Fabrication processes for passives, actives and MMIC were realised using mainly electron beam lithography (EBL) techniques. The main findings of this thesis were: Devices - Influence of gate recess offset on MESFETs The MESFETs were patterned by EBL and gate recessing was accomplished by selective dry etching. The influence of the gate recess offset on the small signal AC equivalent circuit, DC device characteristics, overall high frequency device performance, and low frequency noise behaviour of 0.2 m gate length GaAs MESFETs implemented in the low noise amplifier (LNA) circuit design in this thesis was investigated. Numerical simulations of the AC small signal equivalent circuit dependence were carried out in order to help understand the effects observed. Good qualitative agreement between measured and simulated response was obtained. - Schottky diodes The performance of Schottky-contact diodes used in the MMIC mixer were studied as a function of their geometry and processing conditions. Passives - CPW losses Losses in coplanar interconnect topologies (coplanar waveguide and slotline) using different metallisation processes were investigated. - CPW to slotline transitions A range of coplanar waveguide to slotline transitions required for the MMIC mixer were studied. Broadband performance with insertion loss of < 0.5dB per transition was observed. Transmission line models of the structures have been implemented to enable circuit performance to be predicted and designed to suit the application frequency. The effect of parasitic modes on transitions performance was also investigated.