Symmetric varactor frequency multipliers

Licentiate thesis, 2002

This thesis describes heterostructure barrier varactor (HBV) frequency multipliers for millimetre and sub-millimetre wave applications. The basic principles of HBVs are explained. Starting from basic varactor theory, an equivalent circuit model for HBVs is presented. The elements of the circuit model are explained and the current-voltage and capacitance-voltage characteristics are investigated. Special attention is paid to the series resistance, which is an important limiting factor for the conversion efficiency of HBV frequency multipliers. HBVs can be fabricated from various material systems. An overview of the heterostructures suitable for HBVs is presented, with special emphasis on GaAs/AlGaAs on GaAs and InGaAs/InAlAs on InP which are the most common HBV material systems. The fabrication processes for planar, whisker contacted and pillar geometry HBVs are described. A process where whisker contacted HBVs are fabricated on a copper substrate is presented in detail. By replacing the substrate with copper, the thermal properties of the HBVs are improved drastically. Self-heating reduces the conversion efficiency of HBV frequency multipliers. Therefore, it is important to reduce the thermal resistance of HBVs. Improved diode geometries and thermal models are presented. An advantage with HBVs is the possibility to tailor the device for different applications in terms of input frequency, input power level and multiplication factor. Material design considerations are discussed. HBV frequency multipliers are described, with emphasis on the influence of the capacitance-voltage characteristic on the conversion efficiency. Optimum impedances and losses caused by conduction current are investigated. An overview of HBV frequency multiplier topologies is presented, focusing on waveguide frequency triplers. Frequency multiplier measurements at room temperature and cooled temperatures are described. The final section presents a design example, where optimum impedances for a 500-GHz HBV quintupler are investigated with harmonic balance calculations.