I cannot see why, when talking about the resistance of semiconductors materials
its deemed necessary to explain in terms of semiconductor devices.
A piece of semiconductor material is passive, a semiconductor device ie a transistor/diode is formed
from the junction of two dissimilar semiconductor materials.
Conductors have a temperature/resistance coefficient and their resistance changes in a linear way with respect to temperature,
either in a negative or positive sense.
Manufactured alloys can be made that have close to zero temperature coefficients.
Manufactured compounds can be made that have a non linear resistance change with change in temperature,
again with a negative or positive sense.
Which ever type of material you choose, conductor or semi-conductor, it will obey Ohms Law at any given temperature.
The same rule applies to thermistor compounds.
...
Strictly speaking, semiconductors (like si diodes) do NOT obey Ohm's Law. This is
because the current through the element is not proportional to the voltage across it.
It's as simple as that. The voltage across a regular diode is around 0.7 volts and
as the current varies widely the voltage across it does not change that much.
It doesnt matter how small of a range you choose (a change of 1 amp, a change
of 0.1 amp, a change of 0.000001 amp) it still does NOT obey Ohm's Law.
An element that obeys Ohm's Law follows this equation exactly:
V=I*R, or similar.
A semiconductor diode does not follow this Law, therefore it does not obey
Ohm's Law...
EVERYTHING follows ohms law...
The materials effective resistance however is dynamically controlled
Mike, read my post again. I said effective resistance. The math doesn't prove the real world wrong. At any given instant period of time the equivalent V I R HAS to work out.
The dynamically controlled bit, at least as far as an ideal diode goes is determined by the equation you listed. Which can be turned into VIR curves that will EXACTLY match ohms law.
EVERYTHING follows ohms law, just because the voltage/current/temperature of a device produce effects that change the materials effective resistance doesn't mean it breaks the law, it just means it's an active device. Every device is in actuality (meaning the real world) an active device.
Simply put, at any given instant the values for ohms law are absolute, they can NOT be violated without breaking the first law of thermodynamics.
Semi conductor junctions follow this same exact law. The materials effective resistance however is dynamically controlled by the existing electro magnetic field. This does in NO way affirm that it breaks ohms law, only that that effective V/I or R are dynamically related to one another.
Ohms law does not state linearity. In its most basic form one might presume this. Ohms law is not violated if it R is raised to a degree of power. The basic law is still followed, only the result is different.Ohm's Law really requires proportionality, and you can not have that
without a straight line characteristic curve, and that curve must pass
through the origin (point 0,0).
Ohms law does not state linearity. In its most basic form one might presume this. Ohms law is not violated if it R is raised to a degree of power. The basic law is still followed, only the result is different.
But perhaps, if you included more of the quote you cited, we can make a different interpretation. More of your quoted source.Perhaps this quote from the University of Montana says it the best:
START QUOTE (University of Montana)
III. Large deviations from Ohm's Law: Some devices are useful because
they deviate so drastically from Ohm's Law. One of these is a diode.
END QUOTE
Pretty sloppy that the professor does not check his own writings. ow vs. flow. At any rate, I think his meaning is that the diode does not follow ohm's law due to the fact that it does not conduct until .6volt, and only conducts in one direction.Some devices are useful because they deviate so drastically from Ohm's
Law. One of these is a diode. It has the property that it will only allow current to ow in one direction, and
only if the voltage across it exceeds about 0.6 V.
I'm sure glad we've cleared this up for the OP
Lefty
After seeing contradictory posts on many forums , and getting differing opinions from my teachers , i'm completely lost with the ohms law....
I have a number of questions. Can someone please answer them for me?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?