7.3 MOSFET Threshold Voltage

Table of Contents - Glossary - Study Aids - ¬? ®

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:

where the flatband voltage, V_FB, is given by:

with

and

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 ¬? ® 7.4