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Help me in understanding capacitances in transistors.

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iVenky

iVenky

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I just couldn't understand the reason for having capacitances in transistors. It is due to some stored charges but I still don't get it. I need a very clear explanation.


Thanks in advance.
 

Again Hi my friend!
How are you?
It is simple . you have two type of semiconductor ( p and n) . and the are between them is not conductor ( considered open circuit ) . it is like a capacitor . because at capacitors , we have 2 conductor and one open circuit ( dielectric ) . the are between pn , considered dielectric.
Best Wishes
Goldsmith
 

I think you are talking about parasitic capacitance in MOSFET. These capacitance are not desirable but they are associated with MOSFET structure itself. A capacitor is formed when two conducting surfaces are separated by some dielectric material. In case of MOSFET PN junction forms parasitic capacitors. When a PN junction is formed, electrons from N region are transferred to P region and holes from P region are transfer to N region by diffusion leaving a region where there is no free charge. This region is called space charge region. This space charge region acts as dielectric between two conducting regions (N and P). In MOSFET there are lot of pn junctions like drain-bulk, source-bulk etc.

Pn-junction-equilibrium.png


Capacitance in MOFSFET:
**broken link removed**

PN juction
 

A simple answer.
any thing/material or even the earth has a capasitance.
any two materials or more that are near or far from each other have capacitance between them.
even the air between makes them a capasitor.
if they are joined between them and they are in total vacume there is a small capacitance between them.
if two materials are different joined together with or without a third material between them had cap...even the same material like silicon but one is p and one is n( p...n..thats for another post) has a "stray cap.. that cap value decides if a pentium will work at 100khz or 3ghz.

thats it a simple answer ha..ha..:lol:
 
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