Threshold voltage in a metal oxide semiconductor field-effect transistor (better known as a MOSFET) is usually defined as the gate voltage at which an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. A MOSFET is said to be turned ON or be in the ON state beyond the threshold voltage.
With the scaling of MOSFET channel lengths, researchers are looking at new transistor designs. Among them is the gate-all-around nanowire MOSFET. Due to quantum mechanical confinement in both the transverse directions, an inversion channel is formed at the center of the device. This phenomenon is called volume inversion. Threshold voltage for the simulated nanowire device in the accompanying image is ~0.45V.
For a basic understanding of the threshold voltage in MOSFET in general, refer to
* introductory presentation : ECE 612 Lecture 13: Threshold Voltage and MOSFET Capacitances presented by M. Lundstrom
* several methods of extracting a threshold voltage: Dieter K. Schroder's book "Semiconductor material and device characterization", pp 222-230
Further reading material can be found in the following papers* Hamdy Abd El Hamid; Benjamin Iniguez; Jaume Roig Guitart; , "Analytical Model of the Threshold Voltage and Subthreshold Swing of Undoped Cylindrical Gate-All-Around-Based MOSFETs," Electron Devices, IEEE Transactions on , vol.54, no.3, pp.572-579, March 2007
* Ray, B.; Mahapatra, S.; , "Modeling and Analysis of Body Potential of Cylindrical Gate-All-Around Nanowire Transistor," Electron Devices, IEEE Transactions on , vol.55, no.9, pp.2409-2416, Sept. 2008
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