# Diffusion Capacitance in PN junction and forward bias diode resistance

1. ## Diffusion Capacitance in PN junction and forward bias diode resistance

What is the concept behind Diffusion Capacitance and on resistance of the diode ? Can we consider Diffusion Capacitance as a parallel plate capacitor ? It is easier to see the Depletion Capacitance and its calculation but diffusion Capacitance calculation is very confusing ?
Does the ON resistance comes from carrier scattering ?  Reply With Quote

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2. ## Re: Diffusion Capacitance in PN junction and forward bias diode resistance

I understand the concept for the resistance but I will rephrase my question for the Diffusion Capacitance. So Diffusion Capacitance comes into play in the forward bias because of the minority carriers in each of P and N region of the diode. As we change the forward bias across the PN junction, these minority carriers will change. Since the change in charge happening w.r.t. change in voltage and that results in Capacitance.....which is Diffusion -..... Can we think of this Capacitance as Parallel plate Capacitor with a very thin Dielectric (since the Depletion region is very small) and the charge on the plates which is changing is minority carriers ?

For Depletion Capacitance- with the change in reverse voltage, depletion width is changing- charge in P and N region is changing which again is similar to Parallel plate capacitance . Am I right ?  Reply With Quote

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3. ## Re: Diffusion Capacitance in PN junction and forward bias diode resistance

A way to define capacitance is opposition to instantaneous change in voltage. (Whereas inductance is opposition to instantaneous change in current level.)

If we were to create a thorough, excruciatingly detailed model for a diode, made up of all basic components which define it...
Then it might contain a capacitor as one aspect of its behavior.

However I doubt we can store a charge in it as we would a conventional parallel-plate capacitor. Instead I suppose it's dynamic, meaning it manifests momentarily. It's difficult to measure directly. The capacitive action is in the effect of slowing down a rise or fall in voltage.

I could be wrong.  Reply With Quote

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4. ## Re: Diffusion Capacitance in PN junction and forward bias diode resistance

You need to consider the capacitance and the on resistance independently (because their origins are distinct)

1. For junction capacitance, you need to consider two different cases (1) when forward biased and (2) under reverse bias. Why?

(1) when forward biased, the junction potential decreases, flow of majority carriers increases and the thickness of the diffusion layer increases. The diffusion layer is almost exponential shape (extends upto infinity) but we can use an effective thickness.

(2) under reverse bias, the junction potential increases, the thickness of the diffusion layer decreases and we have practically no majority carriers.

The current contribution from the minority carriers stay about the same in both cases.

The capacitance is formed due to majority carriers: high doping means thinner junctions.

Standard spice model of a diode has a const capacitor in parallel.

It is not difficult to measure the junction capacitance of a diode with simple tools.

Yes, most of the resistance for a forward biased diode is present in the junction (which is void of majority carriers formed by recombination of two different types of majority carriers).

I hope I am clear.  Reply With Quote

5. ## Re: Diffusion Capacitance in PN junction and forward bias diode resistance

Yes, most of the resistance for a forward biased diode is present in the junction (which is void of majority carriers formed by recombination of two different types of majority carriers).------ I do not follow this.

Depletion is important in Reverse bias and for Forward bias until the threshold voltage because after forward bias, the Depletion/Junction Capacitance decreases..
What about Diffusion Capacitance ?  Reply With Quote

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