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Title: Characterisation of gallium nitride HFETs using non-linear measurement systems
Author: McGovern, Peter
ISNI:       0000 0004 2750 6757
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2007
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One of the weakest links in any communications system is the power amplifier, largely due to the necessary trade-off between linearity and efficiency. The challenge of power amplifier design is therefore to provide maximum power and efficiency while maintaining highly linear operation. Gallium Nitride HFET devices are of great interest in power amplifier applications and are potentially the best device technology to provide high power amplification at high frequencies. At the outset of this work, the development of GaN HFETs was in its early stages and the primary aim of this work is to help the development of the QinetiQ GaN program and to investigate the suitability of GaN HFETs for use as power amplifiers. This was achieved through the use of novel measurements using the custom RF non linear time domain measurement systems at Cardiff University. These QinetiQ devices exhibit the common problem of DC-RF dispersion, which results in much degraded RF performance relative to that predicted from DC measurements. It has been shown that the devices exhibit a direct correlation between dispersion and channel current, and that it is possible to overcome some of the unwanted side-effects of dispersion through suitable design, so that efficiency can be improved to acceptable levels, while maintaining high power operation. It has also been shown that there is a strong correlation between RF waveform measurements and pulsed l-V measurements, and that through careful choice of bias point, pulsed l-V measurements can be used to accurately predict the RF performance of a device. Through large-signal CW and two-tone characterisation, it has been determined that these devices operate in a very similar way to that predicted by simple theory and exhibit little AM-PM behaviour. Their CW operation is also found to be less complex than that of GaAs pHEMTs, and their two-tone operation correlates well with the CW analysis. They should therefore be ideally suited to power amplifier applications in terms of linearity as well as power and frequency performance. Finally, after the development of a new two-tone time domain measurement approach, it was determined that the devices suffer from intrinsic electrical memory effects due to their transit delay but are otherwise inherently free from memory within the measurement bandwidth - the range of measurable modulation frequencies is 200KHz-25MHz. All other memory effects detected are not caused by the device but are extrinsic electrical memory effects caused by the impedance environment that is presented to the device.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available