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Origin of Leakage Current in SOS MOSFETs
Student: Chee Cheah Lim
Supervisor: Y Yeow
Category: Engineering Thesis Project - Electrical
Over the last fifty years, semiconductor technology has improved tremendously. The minimum dimension in current production technology is 0.13um and it is going towards the nanometres. With downsizing of device dimension, Silicon-on-Sapphire (SOS)/ Silicon-on-Insulator (SOI) technology is being developed to reduce the parasitic capacitances of the devices so as to further improve the speed of the technology.
Peregrine Semiconductor Pty Ltd, Sydney manufactures CMOS integrated circuits using the SOS technology. This technology is applied to cutting edge areas like optoelectronic systems. However, the company is facing leakage current problem in the MOSFETs that they fabricate.
This thesis aims to narrow down the origin of the leakage current through experimental device characterization on test wafers provided by Peregrine Semiconductor. Measurements include both P and N-MOS transistors with different channel lengths fabricated on SOS silicon film grown under different conditions. The experiments centre on the measurement of DC drain leakage current in P and NMOS transistors on these wafers. Transient drain current was also measured on these transistors. In addition, recombination current characteristics of the SOS gated diode are also looked into to confirm the deductions arrived at from the transistors measurements.
Our results indicate that the SOS devices suffer from “instability” at the silicon-sapphire interface. The main problem appears to be the presence of high defect density at this interface resulting in electrically active centres which lead to high drain leakage currents. The different growth conditions were found to alleviate the leakage current problem in transistors of one channel type but aggravate that in the other channel type. The distribution of the defects is also found not to be uniform across the interface. It is concluded that a better and more controlled silicon film growth process is needed. To complement this, it is suggested that by increasing the film dopant concentration at the back surface (such as by ion implantation of film dopant type) it is possible to reduce the effectiveness of the defects as recombination/generation centres.