Can anyone explain Diffusion Capacitance of a PN juntion diode. All I know is that it comes into play when the diode is Forward biased and increases exponentially with increase in the voltage across the diode. But what is it that actually haapens. Why is the effect of a capacitor introduced? Thanks

It's one of the most important factor that we forget to consider when designing. So start with the basics of PN junction structure. I will describe briefly here, but I recommend reading Milliman and Halkias Integrated electronics. Besides there are two capacitance you must consider, junction and diffusion capacitancs. Diffusion Capacitance is prominent during Forward bias. So when you apply a Vf, there is flow of current. And the current flows at certain rate . So when this happens there is a certain amount of charge that is INISDE the diode. This shows there is certain amount of charge stored. As you keep increasing the Voltage, the charge stored at an Instant T is I(Vf) * T. It mean the charges stored at any instant increases as the voltage increases. Since the charges flows through diffusion mechanism, it's called diffusion capacitance, and it's deteremined by Fick's law.

thanks a lot WASSUP. Actually, I'm revising my electronics basics from Boylestad Nashelsky, and dont quite have Milliman handy. It's a great book though. Is Diffusion capacitance modeled as being connected parallel to an ideal diode? (From Boylestad again). What will be it's effect in the low frequency region? Am i correct in assuming that since at high frequencies, capacitive reactance will be extremely low, it also acts as a short circuit across the diode.

Phew u r making me go back in time , feeling nostalgic At low frequencies for sinusoidal input that is (omega*t << 1) Cd = 0.5 * g0 * t ! g0 is low frequency conductance. Well it can get little complex, refer to one of the diodes data sheet to get a feel of these things. If you are trying to design something then be specific with the application, I can help you out with more details.