Stray or parasitic capacitance

[sakhawat]

i have tried a lot but menacingly failed to understand how the parasitic capacitance exists in a pnp or npn transistor or at any diode??

 

[yungman]

Well, there is a boundary between the P and N junction. Capacitance mostly become important when the junction is reverse biased and is not conducting. When reversed biased, a depletion layer is form at the junction and the thickness increase with the reverse voltage.

Guess how does the reverse biased junction with depletion layer in between? Remember the P and N material is conductive. Sooooo...You have two conductor plate one on each side. In the middle there is a layer that is not conductive ( depletion layer). What does this look like? Two conductive plate with an insulator in between?....A capacitor!

People usually don't care much on the forward biased PN junction as it is conducting. Putting a cap across a conductive wire give you just a conductive wire! At least that is true in the first approximation.

 

[sakhawat]

Well, there is a boundary between the P and N junction. Capacitance mostly become important when the junction is reverse biased and is not conducting. When reversed biased, a depletion layer is form at the junction and the thickness increase with the reverse voltage.

Guess how does the reverse biased junction with depletion layer in between? Remember the P and N material is conductive. Sooooo...You have two conductor plate one on each side. In the middle there is a layer that is not conductive ( depletion layer). What does this look like? Two conductive plate with an insulator in between?....A capacitor!

Thanks for this part cause its really have given me a clear assumption of what should be the design of a capacitor in the pn junction diode haha ... and i also got the picture of the stray capacitance that exists in a minimal form in the forward bias . Many many thanks ...