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Study of enclosed gate P-Type MOS field effect transistors – characterization and variability

Drakos Vissarios

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Summary

A MOSFET (metal oxide semiconductor field effect transistor) is by far the most common transistor in digital circuits, as hundreds of thousands or millions of them may be included in a memory chip or microprocessor. Since they can be made with either p-type or n-type doped semiconductors, complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS (complementary metal oxide semiconductor) logic. The same transistors are used to implement analog circuitry, co-integrated with the digital circuits, to form systems-on-chip (SoCs).Nowadays, many integrated circuits are designed to operate in radiation environments such as aircraft, medical, space, nuclear applications and high energy physical experiments. Radiation total ionizing dose effects (TID) degrade the performance and reliability of MOSFETs, giving rise to positive charge trapping at the edges of shallow trench isolation (STI) corners, resulting to the formation of leakage current paths from drain to source diffusions.The use of transistors with enclosed gate layouts is the proposed solution for applications in radiation environments since due to their geometry and the absence of STI corners, leakage current paths along the edge of the active area are suppressed. However, enclosed gate MOS devices present drawbacks like: increased area and limitations in the choice of the W/L ratio that should be taken into consideration.In this thesis we study enclosed gate (circular) P-type MOSFETs. Specifically, we present a statistical analysis to the results of the electrical parameters (threshold voltage, electron mobility, slope factor, transconductance, voltage-current dependence) for seven different geometries of those specific MOSFETs in linear mode as well as saturation mode. Also, a detailed study of the three basic parameters (threshold voltage, mobility, slope factor) is implemented, leading to useful conclusions regarding their variability.

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