Modelling of Transient Effects in SOS MOSFET\'s

Student: Eng Kien Ho

Supervisor: Y Yeow

Category: Engineering Thesis Project - Electrical

This thesis is an investigation of the transient effects on SOS MOSFETs devices through a numerical simulation. The aim is to understand the turn on and turn off transients of SOS MOSFETs devices.

The 2-dimensional semiconductor simulator Medici from synopsis was used to study the transient effects. The model included the standard semiconductor drift and diffusion transport plus bulk carrier generation and recombination by Shockley-Read-Hall and front (Si-SiO2) interface and back (Si-Sapphire) interface recombination using surface recombination velocity. The device dopant profile used is a 0.5 micron N-MOSFET derived from process simulation done by Peregrine Semiconductor Sydney.

In the turn-on transient device where the gate voltage is switched from flat band to inversion, electrons concentration at the surface becomes much greater than the holes concentration. However hole concentration is found not to reduce instantaneously due to the fact that the substrate is floating. To balance the field exerted by gate bias, the inversion electron concentration increases above its final steady state level through supply from the source and drain. This leads to a drain current overshoot. The magnitude overshoot is dependent on and deviation of the carrier lifetime and surface recombination velocity.

In the turn-off transient device where the gate voltage is switched from inversion to flat band, holes concentration at the surface becomes much greater than the electron concentration. Nevertheless the holes concentration cannot be formed instantaneously, the electrons have to be decreased below the steady state. Therefore the drain current becomes lower during the turn off state.

Thesis Document (PDF)