#junction capacitance

So in trying to answer why germanium diodes are used for crystal sets, I came across the concept of junction capacitance - that is - a diode acting as a capacitor.  I thought I'd explore this more so that I wouldn't make the next post a ginormous one.

Remember all the hoopla about:

Forward bias: Putting the power source such that charge is able to flow through the diode

Reverse bias: Putting the power source such that charge is not able to flow through the diode

Well, we're going to focus on reverse biasing today when talking about junction capacitance.  As you remember capacitance is two conductors separated by an insulator/dielectric.  Well, when you reverse bias:

You end up with a depletion region which if you think about it is like a dielectric.  Does this then look familiar? 

Capacitor = 2 conductors with a dielectric in between? 

Diode in reverse bias mode = 2 semiconductors with a depletion region in between?

So there is capacitance that occurs!  But what does this mean?  Essentially, if the junction capacitance is too large, it will reduce the effectiveness of the circuit at high frequencies; the whole waveform will pass through the diode-turned-capacitor instead of being demodulated. 

Note: As the frequency of a current increases, the capacitor passes more charge across the capacitive plates in a given time resulting in a greater overall current flow through the capacitor appearing as if the internal resistance of the capacitor has decreased:

http://tymkrs.tumblr.com/post/3383866342/11-5-ham-lesson-o-de-day

Why does any of this matter to AM radio?  Because AM radio frequencies are quite high.  And therefore the diode cannot have a large junction capacitance otherwise it will allow a modulated signal through instead of just the audio signal.  Thus this is something that we still need to consider.

@atdiy/@tymkrs

http://electronicsprogramming.com/uncategorized/junction-capacitance/