University of Twente Student Theses


Investigation of the band gap widening effect in thin silicon double gate MOSFETs

Steen, J.L.P.J. van der (2006) Investigation of the band gap widening effect in thin silicon double gate MOSFETs.

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Abstract:The topic of this project is the analysis of the subthreshold current in thin silicon Double Gate (DG) MOSFETs, with a particular focus on observing the so-called effective band gap widening effect using electrical measurements. DG MOSFETs with a very thin silicon layer are expected to be promising candidates for the next generation CMOS technology: moving from single to double gate gives better control over the channel region, which improves the short channel performance and gives rise to an ideal subthreshold slope. As for the drain current, the charge carriers have to traverse an energy barrier when owing from source to drain. For low gate bias, the height of the barrier is mainly determined by the band gap, making particularly the subtreshold current suitable for observing changes in the effective band gap. As the silicon layer thickness (i.e. the channel region) is shrunk to values below about 15 nm, quantum mechanical effects are expected to show up in the device characteristics. One of those effects is the widening of the effective band gap, which originates from splitting of both the conduction and valence band into subbands, due to confinement of the charge carriers in the very thin silicon layer between the Si=SiO2 interfaces. The effective band gap widening effect is investigated using temperature dependent IV and CV measurements. Essentially, both measurements rely on the strong temperature dependence of diffusion, which constitutes the dominant part of carrier transport in the subthreshold regime. Generally speaking, by comparing the characteristics of a device with a 'thick' silicon layer to the characteristics of a thin device (i.e. in which carrier confinement is important), the effective band gap widening effect should become apparent. The results obtained from the IV measurements indicate that an increase in effective band gap occurs for decreasing silicon layer thickness. For a detailed quantitative analysis however, the measurements should be repeated on a larger number of devices, with thinner layers, fabricated in a more accurate process. The CV measurements show that the device behavior in depletion and weak inversion is dominated by the implanted p+ and n+ regions adjacent to the intrinsic channel, being more pronounced for shorter devices. This should be investigated further with additional simulations, particularly concerning the temperature dependence.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:54 computer science
Programme:Computer Science MSc (60300)
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