# Ohmic Resistance in diodes

CaptainMarvel1899
Well I was just wondering how ohmic resistance works in a diode . I mean let's take forward bias mode.In forward bias mode electrons are diffused from the n type region to the p type region.The depletion layer still exists but is very narrow.So according to Drude model of ohmic resistance electrons bump into atoms of the resistor and lose all their momentum during the movement of them opposite of the battery voltage.But in a pn junction diode those atoms are basicly negatively charged because the boron atoms in the depletion layer are negatively charged . So how does this work?

Gold Member
Ohmic Resistance is not universal. It applies in Metals but you can’t expect to find it in all (non-linear) materials.

CaptainMarvel1899
Im a sure drude model is correct for all ohmic resistances and a diode has ohmic resistance even in forward bias.

Staff Emeritus
Im a sure drude model is correct for all ohmic resistances and a diode has ohmic resistance even in forward bias.
If you're sure, then what is the point of this thread?

Dale
CaptainMarvel1899
The B- ions are negatively charged so I ask if this will affect somehow.

Gold Member
Once the depletion region is closed the diode behaves similar to a conductor. That is why you must put a current limiting resistor in the circuit.

Gold Member
That is why a diode is different than a regular ohmic device. The dopants are charge carriers making the material conductive not resistive.

CaptainMarvel1899
Drude model also is correct for conductors.The depletion region isn't disappeared it get very narrow.

Staff Emeritus
Well I was just wondering how ohmic resistance works in a diode . I mean let's take forward bias mode.In forward bias mode electrons are diffused from the n type region to the p type region.The depletion layer still exists but is very narrow.So according to Drude model of ohmic resistance electrons bump into atoms of the resistor and lose all their momentum during the movement of them opposite of the battery voltage.But in a pn junction diode those atoms are basicly negatively charged because the boron atoms in the depletion layer are negatively charged . So how does this work?

The Drude model predates quantum mechanics and the understanding of how particles behave at the atomic and subatomic scale. What you really want to look at is the free electron model. In this model, there are relatively few ion-electron collisions. The mean free path of an electron is influenced more by lattice defects, impurities, and thermal fluctuations and how these influence the electric potential and band structure of the local area.

CaptainMarvel1899
When I was a student 60 years ago we were taught the Drude model but let's look at this free electron model.

Nothing is true.Everything is permitted
Evie Frye

Gold Member
That is why a diode is different than a regular ohmic device. The dopants are charge carriers making the material conductive not resistive.
The term 'Ohmic' refers to a constant ration between PD and Current - in my experience. A diode doesn't behave like that. If you insist, you can measure the PD and the Current for one value of Current and you could assign a value of R (V/I) but that R will not be constant so it doesn't mean much. The R, measured this way will drop rapidly, once the 'knee' voltage of around 0.7V is exceeded.