Okay so here I'll lay a few mathematical examples of what's going on and you can tell me what you think about it:
- On the LEFT we've got a normal DC LED operation at 2V/20mA which makes a total power consumption of 40mW (P=U.I) So far so good.
- Then on the right we have an AC LED operation at F=10Ghz. At such a speed the LED would remain still as if it were powered by DC. This is where it gets interesting. The same LED as the green one is magically lit into full power with just 10mV of AC. While the total power consumption according to the Power formula P=Urms^2/Z is around 100uW. Yes uW! That's like 400 times less power requirement than with DC.
Now let's get to the why - Both LED's have a 32pF junction capacitance. When it comes to DC that's not a problem. But with AC, especially at such high frequencies things work differently. The capacitance of the junction has a reactive resistance of around 0.5ohm at 10Ghz. This means that the LED is now acting more like a capacitor than a diode. Usually the diode's crystal wouldn't get energised at 10mV but since the capacitance is dominant now, rather than the PN junction, that makes the diode's resistance very low at such frequencies even to 10mV and the current through it actually becomes enough (20mA) to energise the LED and make it lit even with 400 times less power requirement. From 40mW to 100uW.
Reference points to reasearch:
- Semiconductor Junction capacitance
- Reactance of Capacitance on AC
