Sceadwian

Yes it's totally different than a resistor, but it still follows Ohm's law. PN junctions due to their molecular interactions cause otherwise conductive materials to exhibit 'virtual insulation' behaviors due to internally created electric fields which prevent electron flow between the two layers under various circumstances and to difference degrees. No they don't behave linearly, but at any given instant of time a semi conductor has definable ohmic state. Basically in a semi conductor the V and I parts of the circuit cause a variable R. It's dynamic, not fixed, but in the end it still follows ohms law, there's just a whole lot of other math tacked on with it.

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ericgibbs

hi sceadwian,
You are quite right, I am sure your explanation makes it clear to a student.

I was concerned that a student/newbie would think he could use a ohm meter
etc, to measure the 'resistance' of his semi's.

Once had a univ student on a mid summer work experience, he was trying to find the labs 'megger' [insulation tester].
Asked why, he said he wanted to check the gate input resistance of some FET's he was using, he had been
told it could be megohms!.

Regards
EricG

Sceadwian

That's a good way of turning a mosfet into a smoke generator, single shot of course =)

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kep

hi all
v=IR is always valid for any material for instance. that is
V(t) = r I(t) hold for any material : r is a function of time
in other words
V = r I

retro

Nice JavaScript Ohms Law Calculator

http://ourworld.compuserve.com/homep...en/ohmslaw.htm

DMW

Removed,
10 pages for formula and not a Thankyou

Ratchit

To the Ineffable All,
There seems to be some confusion about what Ohm's law is. The formula V=IR or V=IZ is NOT Ohm's law. It is the resistance or impedance formula. Ohm's law is a property of a material, not a method of calculating current,impedance, or voltage. Read what the physics books say about this.

"We stress that the relationship V=IR is not a statement of Ohm's law. A conductor obeys Ohm's law only if its V--I curve is linear, that is, if R is independent of V and I. The relationship R = V/I remains as the general definition of the resistance of a conductor whether or not the conductor obeys Ohm's law. ..... Ohm's law is a specific property of certain materials and is not a general law of electromagnetism, for example like Gauss's law."
The above snippet is from Physics, by Prof David Halliday, University of Pittsburgh & Prof Robert Resnick,Rensselaer Polytechnic Institute, 1967 , page 780.

And the following.
"Ohm's law states that for many materials (including most metals), the ratio of the current density and electric field is a constant, which is independent of the electric field producing the current.
Materials that obey Ohm's law, and hence demonstrate this linear behavior are said to be ohmic. The electrical behavior of most materials is quite linear for very small changes in the current. Experimentally, one finds that not all materials have this property. Materials that do not obey Ohm's law are said to be nonohmic. Ohm's law is not a fundamental law of nature, but an emperical relationship valid only for certain materials."
The above is from Physics for Scientists and Engineers, Raymond A Serway, James Madison University, Third edition, 1990, page 745.

There you have it. Ohm's law should not be confused with the always correct resistance or impedance formula. It is a property of a material, not a method of calculation. Materials like semiconductors with their bent V--I curves do not obey Ohm's law. Ratch