Concept of "Crossover Point" and its Application on Threshold Voltage Definition for Undoped-Body Transistors

As the conventional MOSFET's scaling is approaching the limit imposed by short channel effects, Double Gate (DG) MOS transistors are appearing as the most feasible candidate in terms of technology in sub-45 nm technology nodes. As the short channel effect in DG transistor is controlled by the device geometry, undoped or lightly doped body is used to sustain the channel. There exits a disparity in threshold voltage calculation criteria of undoped-body symmetric double gate transistors which uses two definitions, one is potential based and the another is charge based definition. In this paper, a novel concept of "crossover point'' is introduced, which proves that the charge-based definition is more accurate than the potential based definition.The change in threshold voltage with body thickness variation for a fixed channel length is anomalous as predicted by potential based definition while it is monotonous for charge based definition.The threshold voltage is then extracted from drain currant versus gate voltage characteristics using linear extrapolation and "Third Derivative of Drain-Source Current'' method or simply "TD'' method. The trend of threshold voltage variation is found same in both the cases which support charge-based definition.

1 Introduction

In a continuous effort to increase current drive and better control short-channel effects, MOS transistors have evolved from classical bulk single-gate devices into multi-gate devices. As the dimensions of transistors are shrinking, the close proximity between the source and drain reduces the ability of the gate electrode to control the potential distribution and the flow of current in the channel region, and undesirable effects, called “short channel effects” starts plaguing MOSFETs. For all practical perposes, it seems impossible to scale the dimensions of classical “bulk” MOSFETs below 20nm. In a bulk device (fig. 1(a)), the electric field lines propagate through the depletion regions associated with the junctions. Their influence on the channel can be reduced by increasing the doping concentration in the channel region. In very small devices, unfortunately, the doping concentration becomes too high for proper device operation.