Plate materials in capacitors

[Aladdin123]

hello every one

 

I was wondering about an issue i thought about while attending a masters thesis discussion earlier

Does the material of the plate (not the filling dielectric) affect capacitance (even if in the fringing level) ?

Check the attached picture , for a better explanation

[Joatmon]

In theory it should not make a difference. The capacitance is determined by the dielectric, the area of the plates and the distance between them. The plate material is not considered a factor.

I can only wonder if you think there may be some small difference if ,in bonding the silicon to certain dielectrics, there may be some degree of merging which effectively reduces the distance between the plates a small amount. I've never heard of such a thing.

 

http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml

[Aladdin123]

What about the electric field distribution , especially of the fringing capacitance part ?

 

 

 

isnt having a higher conductivity something that will upset the electric field distribution, since more charges will accumulate on the metal than on the silicon ?

Edited July 28, 2012 by Aladdin123

[ewmon]

Do these have the EXACT same capacitance?

I would hesitate to say yes not only due to field distribution but also differences in frequency response and whatever phenomenon might occur at the aluminum-silicon junction.

[Joatmon]

I would hesitate to say yes not only due to field distribution but also differences in frequency response and whatever phenomenon might occur at the aluminum-silicon junction.

 

On the basis that two different scenarios are being considered then I agree that one feels that there should be some small difference. I think we all understand the basics of capacitance but I'll chuck my two pennyworth in anyway. The general theory states that since the dielectric is an insulator no electrons enter or leave it in the charging/discharging process. The material of the plates that cause the changes in the dielectric neither, therefore, add nor subtract to the general consideration of effect on the dielectric. I am ignoring any effect on the time taken for charge/discharge due to plate resistance which is usually considered negligible. I quote from a web page concerning ceramic capacitors and any differences regarding accuracy and frequency response are accounted for by consideration of the dielectric alone.

 

"A ceramic capacitor is a fixed capacitor with the ceramic material acting as the dielectric. It is constructed of two or more alternating layers of ceramic and ametal layer acting as the electrodes. The composition of the ceramic material defines the electrical behavior and therefor the application of the capacitors which are divided into two stability classes:" ( Made bold by me)

 

• Class 1 ceramic capacitors with high stability and low losses for resonant circuit application
• Class 2 ceramic capacitors with high volumetric efficiency for buffer, by-pass and coupling applications."

http://en.wikipedia....ramic_capacitor

 

An interesting point (which means I don't know the answer ) is what difference would it make if the plates were different polarity silicon (n-type and p-type).

Edited July 29, 2012 by Joatmon

[Aladdin123]

Alright Guys thanks

I think I got what i need

[Enthalpy]

The only difficulty is that silicon can make bizarre contacts with the wire and can be depleted - including at its surface under some circumstances - in which case it will act as an insulator with Er~12 instead of a conductor. Then, the silicon area would bring a smaller capacitance than a metal.

 

If the frequency is high enough, silicon's resistance (which depends fundamentally on its doping, over ten magnitudes) can add in series with the capacitance, and be itself shunted by silicon's internal capacitance. Then the resulting capacitance would again be smaller, and you could define it only if telling what equivalent circuit you choose, basically series or parallel.

 

About the field : the DC field cannot be defined with so little information, because it depends on silicon's doping, and - much worse - on the cleanliness of its surface, which is essentially unknown and poorly understood. But capacitance is normally defined for AC fields, which superimpose to the DC field, and don't depend on these complications if all dopings permit silicon to act as a resonable conductor.

[Ronald Hyde]

Actually silicon is used as an insulator in some capacitors, just small ones used at radio frequencies. They're called 'varactors' because as a reverse voltage is applied to the 'plates' the depletion of the junction causes a decrease in capacitance. If you have an FM walkie-talkie set it may use them to modulate the frequency.

Edited August 11, 2012 by Ronald Hyde