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Title: Exploitation of phemt in microwave power limiters
Author: Seng, Hing Weng
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2007
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This thesis describes the design and the study of implementing microwave power limiters with Gallium Arsenide (GaAs) pseudomorphic high electron mobility transistors (PHEMTs). Circuits were fabricated in microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) format, tested for performance at 2GHz and 10GHz respectively. The majority of microwave power limiters offered in the current market are based on the p-i-n or PIN diode for protection. Generally in the RF industries, very high power applications still utilize solid-state devices such as T-R tubes for power limiters due to their electrical robustness. However, with the ever growing urge for product improvements and the maturity of MMICs, the lack of integration of the PIN diodes with MMICs calls for an investigation into alternate approaches in power limiter design. Many switching applications in MMICs are implemented using pHEMTs in both series and shunt orientation. The Schottky diode is the favoured choice for microwave power detection purposes. The microwave power limiter approach suggested in this thesis utilizes the pHEMTs as a voltage controlled attenuator in conjunction with a Schottky diode based detector as the control voltage supplier. The MICs or hybrids have been designed using Avago's general purpose pHEMT ATF-35143 as the major element in the voltage controlled switch attenuator sub circuit. The voltage controlled attenuator is supplied with a control voltage from a detector or combination of detector and external bias, depending on the switch's configuration. Avago's Schottky diode HSMS-8101 was used as the detector diode in the power detecting sub circuit. The MMICs were designed using the FD05 O.251lm pHEMT process design kit from Filtronic Compound semiconductor PLC, Newton Aycliffe, Durham, United Kingdom. The pHEMT epi-layers were grown by molecular beam epitaxy on 150mm semi-insulating substrates, featuring O.51lm gate length and 50llm gate width. It is found that larger device provides lower loss but lower isolation. The Schottky diode used in fabricated MMICs is a single gate and 20llm wide device. In the design approach adopted here, the junction of the voltage controlled attenuator and detector is realized using aT-junction. Other methods used include the proximity coupler and capacitive power divider network. Each coupling method finds merit depending on the switch configuration of the attenuator. Eight types of MIC limiters were made, including the single series switch, double series switch, single shunt switch, double shunt switch, series-shunt switch, shunt-series switch, n-switch and tee-switch limiters. The lowest insertion loss achieved was -O.34dB while the isolation is 8dB, delivered by the single shunt switch, measured at 2GHz. The best isolation measured was 20dB, seen in the tee-switch and the double series switch limiters. From the fabricated MMIC limiters, 3.03dB insertion loss and 15dB isolation is provided by the single series switch limiter. While the double series switch limiter is seen to deliver 5.25dB insertion loss and 25dB isolation. Both MMIC limiters exhibit the same limiting threshold level at 10dBm on X-band.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available