Low threshold voltage

[Hawth33]

I have recently fabricated multiple diodes onto a wafer use an NMOS process. As usual, not all of the diodes have the ideal threshold voltage (0.7 to 0.8 V). What are usually the causes of low threshold voltages? Thanks for your help in advance.

 

[kenambo]

Hi,

Fabrication side threshold voltage variation:

It depends upon technology.... and low threshold voltage means higher level of doping in your wafer...

it also depends upon some layers and thickness of layers and it changes according to the foundry..

Design side threshold voltage variation:

1) unequal voltages on the terminals...

2) IR drops due to layout...

3)mismatches in the device parameters and Process variations..

Thanks..

 

[vicky001.iith]

Thershold value calculation and its factors explained very well in wikipedia :- https://en.wikipedia.org/wiki/Threshold_voltage
Also you can check this one :-http://ecee.colorado.edu/~bart/book/book/chapter7/ch7_4.htm

 

[dick_freebird]

Is it really threshold voltage (MOS diode) or is it forward
voltage of an actual PN diode? There would be different
causes.

And of course every lateral diode has a (field or thin)
MOS gate sitting across it. This can easily compromise the
I-V if the oxide is charged, dirty or defective, as the MOS
action raises or lowers the surface "effective doping".
Damage can be simple charging short of outright failure,
from handling / probing and so on with inadequate ESD
controls, in-process charging (antenna), etc.