Microtech 2011

MOSFET threshold voltage: definition, extraction, and applications (invited presentation)

M.B. Machado, O. Siebel, M.C. Schneider, C. Galup-Montoro
Federal University of Santa Catarina, BR

Keywords: threshold voltage, MOSFET characterization

Abstract:

The threshold voltage VT is a fundamental parameter in the characterization of MOS transistors and should be used, whatever is the adopted model for the transistor. The classical definition of threshold: phis = 2phiF +V which links the surface, the Fermi, and the channel potentials is indeed ‘surface-potential based’. VT represents a physical change in the phenomenon that prevails in the current flow through the device as it goes from weak to strong inversion. Since this transition is very gradual, no remarkable point can be directly identified as threshold voltage in the ID vs. VG characteristic. This is one of the reasons why different definitions of threshold voltage have been presented in the literature. Another reason is poor modeling, since to extract unambiguously VT it is essential to use a model that includes both the drift and diffusion transport mechanisms, because both phenomena are important near the threshold condition. In this work we apply the current-based threshold voltage definition (equality between the drift and diffusion components of drain current) to surface potential and charge based models and compare the main definitions of threshold. The paper summarizes a new procedure for the characterization of MOS transistors, which allows the direct determination of the threshold voltage and some other important electrical parameters with minimum influence of second order effects. The threshold voltage is determined at a constant gate-to-substrate voltage, at a low drain-to-source voltage and with transistor operation in the weak and moderate inversion regions. Under these operating conditions the effects of series resistances, mobility and slope factor variations, and channel length modulation are practically negligible, allowing a direct determination of the threshold voltage and of the DIBL effect. The new method will be compared with other ‘current-based’ procedures such as those based on the gm/ID characteristic and the constant current method. Finally, for several applications such as mismatch assessment, transistors aging, and thermal drift characterization, examples of ‘current-based’ VT extraction methods are given.
 

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