# Kelvin Vs 4 point probe

Discussion in 'The Projects Forum' started by Peter2, May 13, 2008.

1. ### Peter2 Thread Starter New Member

May 9, 2008
4
0
Hello,
@ admin: I've posted this in the Homework Forum, but maybe it is in the wrong place there. Please delete it or place it somewhere else

@all: I'm new here and my english is also not very good, but I hope I can articulate my question anyway.
I have the agilent 4155C Semiconductor Analyzer. This one dosen't allow Kelvin probe measurements. the 4156 does. I know that the 4156 has two terminals for each SMU, one as force and one as sens I guess?
But why is it not possible to do a Kelvin probe with 2 SMU's from 4155 as current source and the other two as voltmeter?
In an Keithley Low level Measurement Handbook I read the four point probe (don't they actually mean Kelvin probe?) gets influenced by an AC current that flows from the LO terminal of the current source through the sample to the LO term of the voltmeter.
Then they explain how you can prevent this, using two electrometers and two buffers. (Figure 4-27 if anyone has the handbook, if not I will try to post the picture)

I also heard about the 4 point probe, but couldn't understand what's the difference between Kelvin and four point probe.
And if there is any, is it possible with the 4155C?
Thank you very much for your efforts,
Peter

2. ### John Luciani Active Member

Apr 3, 2007
477
0
Kelvin and four-point are the same. The purpose is to get two *independent*
points of contact to avoid voltage drops in your measurement lines.
You could do this with two *isolated* SMUs. Place the contacts
of the forcing SMU on one side of your DUT pin and the sensing contacts on the
other.

If the circle is the pin you would attach the force and sense as follows --

FORCE --|O|--- SENSE

(* jcl *)

3. ### Peter2 Thread Starter New Member

May 9, 2008
4
0
Hello Luciani,

I don't get together what you mean with
Force-----|o|---- sense
and the things I've read about 4 point or Kelvin:

Force---Sense---|o|---Sense---force

Making it the way you mean I may have no "sink" and the current will flow through my sense lead. So It will be a normal 2 contact measurement or what do I understand wrong?

4. ### Peter2 Thread Starter New Member

May 9, 2008
4
0
Hello Luciani,

now i had the possibility to take a look at a Kelvin-Probeholder.
Now I understand what you mean.
Thanks a lot
Peter