Metal oxide thin film transistors for CMOS applications

Abstract Title: Metal oxide thin film transistors for CMOS applications Author: N.C.A. van Fraassen CMOS technology based on oxide thin film transistors (TFTs) is essential to reduce the power consumption and increase the complexity of low-cost (flexible) processors. These processors have the potential to create ultralow-cost (~1 pence) chips that can turn everyday objects into smart-objects. This thesis presents research on the development of all-oxide CMOS technology. N-type amorphous indium-silicon-oxide (a-ISO) and p-type tin monoxide (SnO) TFTs were fabricated and studied in detail. Both these oxides are excellent candidates for low-cost, low-power, flexible CMOS technology which is essential to reduce the power consumption of flexible processors. All oxide CMOS inverters were created by connecting n-type a-ISO and p-type SnO TFTs. By carefully tuning the geometric aspect ratio of the inverter, a rail-to-rail voltage swing was demonstrated for supply voltages as low as 1 V. We investigated how changing the width-to-length ratio (W/L) of p-type SnO TFTs affects the characteristics of the all-oxide CMOS inverter. Typically, W/L of the lower mobility p-type TFT (n-type for organics) is scaled up (inversely with mobility) to match the higher on-current of the n-type (p-type); this is also common for silicon CMOS technology. In this work it is shown that this method is unsuitable for transistors where not only the on-current, but also the off-current, scales with W/L - including flexible p-type metal-oxide and n-type organic TFTs. The concept of an optimal geometric aspect ratio is introduced that can be applied universally to silicon, metal-oxide and organic complementary inverters. This ratio determines the W/L of the p-type (n-type) transistor that maximises the inverter efficiency represented by the average switching current divided by the static currents. Notably, this work shows that reducing W/L of metal-oxide p-type TFTs increases the inverter efficiency, while reducing the area compared to simply scaling up W/L inversely with mobility. A high inverter efficiency is critical to reduce static power consumption and increase the gate density of flexible processors. Lastly, we investigated a novel memristor-transistor inverter, where the p-type TFT in the standard CMOS configuration is replaced by a memristor. We looked in detail at the fabrication method and inverter design of the memristor-transistor combination. The required switching characteristics of the memristor are investigated by modelling a current- and voltage-controlled ‘reset’ as well as a voltage-swept ‘set’. The results show it is critical that the memristor can be set by sweeping the input voltage across a small range in the reverse direction. To achieve this, precise control and excellent repeatability of the memristor set (and reset) voltage are required.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Van Fraassen, Niels

Advisor dc:contributor.advisor

• Flewitt, Andrew J

• TFT
• CMOS
• metal oxide
• flexible electronics
• transistor
• inverter
• SnO
• ISO
• IGZO
• thin film transistor

Licence

• https://creativecommons.org/licenses/by-sa/4.0/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.92165

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/344743