7.3 MOSFET Threshold Voltage
Table of Contents -
Glossary -
Study Aids -
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®
In this section
- Threshold voltage calculation
- Threshold voltage adjustment
Reading: Neamen 10.1.5 pp 434-440
Required background: Band diagram and
flat band voltage of an
MOS capacitor
Next: 7.4 The body effect (substrate bias effect)
7.3.1 Threshold voltage calculation
The threshold voltage equals the sum of the flatband voltage, twice the
bulk potential and the voltage across the oxide due to the depletion layer charge, or:
(mcc8)
where the
flatband voltage, VFB,
is given by:
(mcc4)
with
(mcc3)
and
(mcc3a)
The threshold voltage dependence on the doping density is
illustrated with the figure below for both n-type and p-type MOS
structures with an aluminum gate metal.
threshol.xls - threshol.gif
Fig.7.3.1 Threshold voltage of n-type (upper curve)
and p-type (lower curve) MOSFETs versus substrate doping density.
The thresholds of both types of devices is slightly negative at low
doping densities and differs by 4 times the absolute value of the bulk potential. The
threshold of nMOS capacitors increases with doping while the threshold of
pMOS structures decreases with doping in the same way. A variation of the
flatband voltage due to oxide charge will cause both curves to move down if the
charge is positive and up if the charge is negative.
7.3.2 Threshold voltage adjustment
7.2
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® 7.4
© Bart J. Van Zeghbroeck, 1996, 1997