Equation defined device modelling of floating gate M.O.S.F.E.Ts.

Cullinan, Michael (2015) Equation defined device modelling of floating gate M.O.S.F.E.Ts. Doctoral thesis, London Metropolitan University.


This research work covers the development of a novel compact model for a Floating Gate nMOSFET that will be added to the QUCS library. QUCS (Quite Universal Circuit Simulator) is a GPL simulation software package that was created in 2006 and is continuing to develop and evolve, and there is already a substantial library of components, devices and circuits. Fundamentally, a floating gate device is an analogue device, even though modern technology uses it mainly as a non-volatile memory element there are numerous uses for it as an analogue device.

A study has been carried out with regards to the principles of the physical phenomenon of charge transfer through silicon dioxide to a floating gate. The study has concentrated on the physical properties of the fabricated device and the principles of charge transfer through an oxide layer by Fowler–Nordheim principles. The EKV2.6 MOSFET was used as the fundamental device for the model that has been adapted by the addition of the floating gate. An equivalent circuit of an FGMOSFET was developed and analysed theoretically. This was then formulated into the QUCs environment and created as a compact model. Simulations were carried out and the results analysed to compare with the theoretical expectations and previous research works.

It is well documented that the creation of equivalent circuits for floating gate devices is complicated by the fact that the floating gate is isolated as a node and as such cannot be directly analysed by simulators.

For the equivalent circuit created, circuit analysis was achieved by the introduction of high value resistances connected in parallel with the capacitive elements that are representative of the incursion of the floating gate that is intrinsic to a floating gate device. The resistance elements were of such value that the time constants were of the order of 10000s and did not interfere with the simulation. The equations from the analysis were formulated and the anticipated responses were shown. The analytical equations developed were then used within the QUCS environment with explicit use of EDDs (Equation Defined Devices) to create a novel model of a FGMOSFET.

Simulations of the model created were carried out with a range of voltage pulses being applied to the tunnelling terminal to affect a charge transfer to the floating gate by means of Fowler-Nordheim principles. The changes of the charge stored on the floating gate were clearly shown by the measured anticipated associated shift in the Threshold voltage.

Simulated results have been compared with previous research and development work and the new model is considered effective. Also because of the ability of the QUCS software to allow the variation of the multiplicity of the parameters associated with the fabrication process, it is considered to be adaptable to a range of modern floating gate device structures and materials.

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