Microwave characterisation and parameters extraction of silicon-on-sapphire (SOS) MOSFET's for device and circuit modelling

Bertling, Karl (2012). Microwave characterisation and parameters extraction of silicon-on-sapphire (SOS) MOSFET's for device and circuit modelling PhD Thesis, School of Information Technol and Elec Engineering, The University of Queensland.

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Author Bertling, Karl
Thesis Title Microwave characterisation and parameters extraction of silicon-on-sapphire (SOS) MOSFET's for device and circuit modelling
School, Centre or Institute School of Information Technol and Elec Engineering
Institution The University of Queensland
Publication date 2012
Thesis type PhD Thesis
Supervisor Aleksandar Rakic
Yew T. Yeow
Total pages 120
Total colour pages 19
Total black and white pages 111
Language eng
Subjects 090604 Microelectronics and Integrated Circuits
Formatted abstract
The present integrated circuit technology based on bulk silicon CMOS devices is reaching its limit in terms of device dimensions. Several new technologies are currently being investi-gated. Silicon-on-sapphire (SOS) is one of these technologies. In the past decade it has been extensively investigated and found to be well suited for radio-frequency (RF) operation. By adopting and adapting the established fabrication processes of the bulk CMOS industry, the thin silicon film (< 100 nm) SOS CMOS became a mature RF CMOS technology. This thesis presents an investigation into several aspects of SOS MOSFETs using test structures from the SOS industry.

Traditionally RF SOS MOSFETs are characterised by microwave frequency measure-ments using special RF test transistors. The work reported covers RF measurement using microwave Vector Network Analyser (VNA) and low frequency (LF) characterisation using an inductance-capacitance-resistance (LCR) meter. It was shown that the two measurement methods produced almost identical RF equivalent circuit elements. From these RF circuit elements it is possible to calculate the the cut-off frequency, fT, and the maximum oscillation frequency, fMAX of the device. These values also agree well with the values extracted from direct S-parameter measurement. This is attributed to the fact that the transistor is quasi-static when operated in the active regime. Non-quasistatic phenomenon was only observed when the device is in the cut-off regime and this again was observed in both RF and LF measurements. The non quasi-static phenomenon was attributed to the slow generation of majority carriers when the thin silicon lm was pulsed from inversion to accumulation.

The interface between the thin silicon lm and the sapphire substrate has signicant in uence on the performance of the SOS MOSFET. A special MOS capacitor test structure was used to investigate this back interface. This test structure allowed for LCR meter measurement of the eective quasi-static C-V curve of the MOS capacitors at 100 kHz. The experimental result was compared with computer simulation to provide insight into the surface potential of this back interface and hence deduce information regarding the dopant level and the trapped charge at the interface. Device modelling using a 2-D simulator was used to understand the eect of back interface trapped charge and thickness of the thin silicon film on the threshold voltage of the transistor.

With the thin silicon film (<100 nm) and good quality back interface, the SOS technology can be used to produce what is referred to as a depletion channel transistor. In this transistor the silicon lm forming the channel is of the same dopant type as that of the source and drain material. Due to higher mobility of carriers in this transistor (compared against the conventional inversion channel transistor) it is expected to have a higher fT. Such a transistor was fabricated, its RF equivalent circuit extracted from LCR meter measurement and its fT determined. It was found that this depletion channel transistor is indeed superior to the corresponding conventional inversion channel MOSFET with the same channel length.
Keyword MOSFET

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Created: Thu, 30 Aug 2012, 11:47:01 EST by Karl Bertling on behalf of Scholarly Communication and Digitisation Service